answers from the questions from the assigned chapters should be in detail and only from the assigned chapters, NOT from external sources, NO plagiarism!!!!!!!!
detail No Plagiarism
Chapter 9
1
. What is the relationship between the hypothalamus, the pituitary, and the ovary during the follicular phase? How does this compare or contrast with the relationship between these same structures during the luteal phase?
2. In detail, describe the physiological process of the corpus luteum formation.
3. How does the dominant follicle inhibit the growth of additional antral follicles?
Chapter 11
1. Describe the sequence of spermatogenesis in mammals. You may want to use a drawing to accompany your discussion.
2. In regards to a spermatid, what is differentiation? What are the phases and what physiological mechanism(s) occur during each?
3. Describe the cycle of seminiferous epithelium.
Chapter 14
1. What are the four types of placenta accoroding to chorionic villi distribution? For each, please describe the distribution pattern and give an example of a species in which it would be found.
2. What are the three types of placenta based on number of placental layers separating fetal and maternal blood supply? For each, please include the number of layers, any unique characterisitics (if applicable), and an example of a species in which it would be found.
3. In detail, expalin how does fetal cortisol initiates parturition.
1
The Puerperium & Lactation
Parturition
Fetal Attachment & Gestation
Early Embryogenesis &
Maternal Recognition of Pregnancy
Ovulation & Fertilization
Cyclicity
Regulation of
Reproduction
Tract Function
Puberty
Prenatal
Development
.. “‘
\
, ..
Spermatogenesis
Regulation of
Reproduction
Tract Function
Puberty
Prenatal
Development
Take Home Message
The luteal phase consists of three major processes. They are: 1) luteinization (th e
transformation of follicular cells into luteal cells after ovulation), 2) synthesis and secretion
(growth and development of the co1p11s luteum accompanied by increasing quantities of
progesterone) aml3) luteolysis (destruction of the c01pus luteum) accompanied by rapidly
declining blood progesterone that results in a subsequent follicular phase. Regression of
the corpus luteum is brought about by prostaglandin F1a that is synthesized and secreted
by the uterine endometrium in most mammals and by the ovmy in women. The negative
feedback exerted by progesterone on the hypothalamus is removed and the f emale enters a
new follicular phase because the pulse frequency and amplitude ofGnRH increases thus
allowing FSH and LH to increase. In women, luteolysis causes the initiation ofmenstma-
tion that is follo wed by another follicular phase.
The luteal phase lasts fro m the time of ovula-
tion untilluteolysis of the corpus luteurn (CL) near
the end of the estrous cycle. It includes metestrus
and diestrus (See Figure 9- 1 ). The dominant ovar ian
hormone during the luteal phase is progesterone.
The luteal phase consists of:
•luteinization (formation ofthe CL)
• synthesis and secretion of large
quantities of progesterone
• luteolysis
When the fo ll icle ruptures at ovulation, blood
vessels within the foll icul ar wall also rupture . This
vascular breakage results in a structure with a ” bloody”
clot-li ke appearance . Th is structure is ca ll ed the
corpus hemorrhagicum because of its hemorrhagic
(bl oody) appearance when viewed fro m the surface
of the ovary. Corpora hemo!Thagica can be observed
from the time of ovulation until about day 1 to 3 of the
estrous cycle (See Figures 9-3 through 9-6). Imme-
diately after ovulation, corpora hemo1Thagica appear
as small, pimple-like struch1res on the surface of the
ovmy. At about day 3 to 5, the corpus luteum (CL)
begins to increase in size and lose its hemorrhagic ap-
pearance. It increases in mass until the m iddle of the
cycle, when its size is maximal and coinc ides with the
maximum secretion of progesterone duri ng diestrus.
Near the end of the luteal phase, luteolysis occurs and
the CL loses its functional integrity and decreases in
size. Luteolysis results in an irreversible struchrral
degradation of the corpus luteum. A regressed corpus
luteum will become a corp us albi cans (white body).
Figure 9-1. The Luteal Phase
Luteal Phase
METESTRUS
p,,
p roduct io n
0 I 2 3 4 5 6 7 8 9 10 I I 12 13 14 15 16 17 18 19 20 2 1 0
Day of the estrous cycle
The luteal phase beg ins immediately afte r
ovulation. During the early luteal phase, the
corpus luteum (CL) develops (luteinization )
and progesterone increases. Duri ng the mid-
luteal phase (diestrus) the corpus luteum is
fully functional and progesterone (P 4 ) plateaus.
During the last 2-3 days of the luteal phase ,
destru ction of the corpus luteum occurs (lute-
olysis) and the luteal phase terminates. Fol-
lowing luteol ysis , pro estrus is initiated.
V
et
B
oo
ks
.ir
The Puerperium & Lactation
Parturition
Fetal Attachment & Gestation
Early Embryogenesis &
Maternal Recognition of Pregnancy
Ovulation & Fertilization
Cyclicity
Regulation of
Reproduction
Tract Function
Puberty
Prenatal
Development
.. “‘
\
, ..
Spermatogenesis
Regulation of
Reproduction
Tract Function
Puberty
Prenatal
Development
Take Home Message
The luteal phase consists of three major processes. They are: 1) luteinization (th e
transformation of follicular cells into luteal cells after ovulation), 2) synthesis and secretion
(growth and development of the co1p11s luteum accompanied by increasing quantities of
progesterone) aml3) luteolysis (destruction of the c01pus luteum) accompanied by rapidly
declining blood progesterone that results in a subsequent follicular phase. Regression of
the corpus luteum is brought about by prostaglandin F1a that is synthesized and secreted
by the uterine endometrium in most mammals and by the ovmy in women. The negative
feedback exerted by progesterone on the hypothalamus is removed and the f emale enters a
new follicular phase because the pulse frequency and amplitude ofGnRH increases thus
allowing FSH and LH to increase. In women, luteolysis causes the initiation ofmenstma-
tion that is follo wed by another follicular phase.
The luteal phase lasts fro m the time of ovula-
tion untilluteolysis of the corpus luteurn (CL) near
the end of the estrous cycle. It includes metestrus
and diestrus (See Figure 9- 1 ). The dominant ovarian
hormone during the luteal phase is progesterone.
The luteal phase consists of:
•luteinization (formation ofthe CL)
• synthesis and secretion of large
quantities of progesterone
• luteolysis
When the fo ll icle ruptures at ovulation, blood
vessels within the foll icul ar wall also rupture . This
vascular breakage results in a structure with a ” bloody”
clot-li ke appearance . Th is structure is ca ll ed the
corpus hemorrhagicum because of its hemorrhagic
(bl oody) appearance when viewed fro m the surface
of the ovary. Corpora hemo!Thagica can be observed
from the time of ovulation until about day 1 to 3 of the
estrous cycle (See Figures 9-3 through 9-6). Imme-
diately after ovulation, corpora hemo1Thagica appear
as small, pimple-like struch1res on the surface of the
ovmy. At about day 3 to 5, the corpus luteum (CL)
begins to increase in size and lose its hemorrhagic ap-
pearance. It increases in mass until the m iddle of the
cycle, when its size is maximal and coinc ides with the
maximum secretion of progesterone duri ng diestrus.
Near the end of the luteal phase, luteolysis occurs and
the CL loses its functional integr ity and decreases in
size. Luteolysis results in an irreversible struchrral
degradation of the corpus luteum. A regressed corpus
luteum will become a corp us albi cans (white body).
Figure 9-1. The Luteal Phase
Luteal Phase
METESTRUS
p,,
p roduct io n
0 I 2 3 4 5 6 7 8 9 10 I I 12 13 14 15 16 17 18 19 20 2 1 0
Day of the estrous cycle
The luteal phase beg ins immediately afte r
ovulation. During the early luteal phase, the
corpus luteum (CL) develops (luteinization )
and progesterone increases. Duri ng the mid-
luteal phase (diestrus) the corpus luteum is
fully functional and progesterone (P 4 ) plateaus.
During the last 2-3 days of the luteal phase ,
destru ction of the corpus luteum occurs (lute-
olysis) and the luteal phase terminates. Fol-
lowing luteol ysis , pro estrus is initiated.
V
et
B
oo
ks
.ir
182 The Luteal Phase
In general , a corpus albicans can be observed for a
substantial period oftime (several estrous cycles) after
luteolysis. The corpus albicans appears as a white scar-
like structure because the connective tissue remains
after the glandular tissue disappears.
The corpus lutemn originates from an
ovulatory follicle.
After ovulation the theca interna and the
granulosa! cells of the follicle undergo a dramatic
transfom1ation known as luteinization. Luteinization
is the process whereby cells of the ovulatory follicle
are transformed into luteal tissue. This transforma-
tion is governed by LH. Shortly before ovulation the
basement membrane of the follicle undergoes partial
disintegration and the physical separation of the thecal
and granulosa! cells disappears (See Figure 9-2). Dur-
ing ovulation, follicular fluid leaks from the follicle. At
the same time, the wall of the follicle collapses fann-
ing many folds (See Figure 9-2). These folds begin to
interdigitate, allowing thecal cells and the granulosa!
cells to mix, thus forming a gland consisting of con-
nective tissue cells, thecal cells and granulosa! cells.
In general, the cells of thecal origin and the cells of
granulosa! origin mix unifom1ly with one another (See
Figure 9-2). An exception to this is found in the corpora
Jutea of the woman and other primates, where thecal
and granulosa! cells are clustered into distinct “islets”.
It is easy to distinguish microscopically between luteal
cells that originate from the granulosa! cells and those
that originate from the thecal cells. Large luteal cells
are derived from granulosa! cells while small luteal
cells are derived from thecal cells. Portions of the
basement membrane that separated the thecal cells
from the granulosa! cells remain and constitute the
connective tissue network of the corpus Juteum (See
Figure 9-2).
Luteal tissue consists of large and
small luteal cells:
• large cells originate from the
granulosal cells
• small cells originate from the cells
oftlze theca interna
Large luteal cells (sometimes called granu-
losal-lutein cells) va1y in diameter from 20-70 microm-
eters (!lm), depending on species. In some species
(nuninants ), there are a large number of dense secretory
granules close to the plasma membrane. These secre-
tory granules contain oxytocin in the corpus luteum
of the cycle and are believed to contain relaxin in the
corpus luteum of pregnancy.
Small luteal cells (sometimes called thecal-
lutein cells) are less than 20 in diameter, have an
irregular shape and possess numerous lipid droplets in
their cytoplasm. They do not contain secretory gran-
ules as do the large luteal cells. Both small and large
luteal cells are steroidogenic (possessing the ability to
produce steroids), in this case progesterone.
In general, the corpus luteum increases in size
until about midway through the luteal phase (See Fig-
ures 9-3 through 9-6). For example, a skilled examiner
can almost always determine whether a corpus luteum
is present or absent in cows. In mares , it is almost
impossible to ascertain the presence or absence of the
corpus luteum because it does not protrude from the
surface of the ovary.
In the cow, palpation cannot accurately predict
the functional status of the corpus luteum. In four
separate studies, cows were transrectally palpated by
experienced diagnosticians. Corpora lutea were classi-
fied as func tional (secreting high quantities of proges-
terone) or nonfunctional (regressing or secreting low
levels of progesterone) by the diagnosticians. Using
measurements of blood proge sterone as the indi cator
of corpus luteum function, it was found that 25% to
39% of cows classified as having a functional corpus
luteum were not secreting high quantities of progester-
one. Furthermore, 15% to 21% of cows classified as
having a nonfunctional corpus luteum had high blood
progesterone. Clearly, the use of transrectal palpation
to assess the functional status of the corpus luteum has
limitations. From a practical reproductive manage-
ment perspective, this problem lim its the effectiveness
of treating animals with luteolytic agents to induce
estrus and ovulation. In other words, administering
luteolytic agents (prostaglandin F 2a) on the basis of
transrectal palpation of the ovaries alone will provide
suboptimal results.
The use of real-time ultrasonography has
proven effective for the examination of corpora lutea,
as well as ovarian follicles . In cattle, progesterone
concentration in blood is correlated with the diameter of
the corpus luteum as measured by ultrasonography.
L arge luteal cells rarely multiply after ovula-
tion. Therefore, the total number of granulosa! cells
” donated” by the follicle detem1ines the number of
large luteal cells in the newly-formed CL. Luteal
function may in-part be related to the vigor (as judged
by the number of granulosa! cells) of the follicle prior
to ovulation. In the ewe (and presumably other spe-
cies), an increase in corpus luteum size and weight is
due to a threefo ld increase in volume of large luteal
cells coupled with a fivefold increase in the number
The Luteal Phase 183
Figure 9-2. Formation of the Corpus Luteum
Preovulatory Follicle
The preovulatory follicle consists of granu-
losa! cells that line the antrum. The base-
ment membrane, separating the granulosa!
cells from the cells of the theca interna
begins to deteriorate prior to ovulation be-
cause of the action of collagenase . Com-
plete separation between the granulosa!
cells and the th eca intern a no longer exists
and the cells can begin to intermingle.
Corpus Hemorrhagicum (CH)
During ovulation, many small blood vessels
rupture causing local hemorrhage. This
hemorrhage appears as a blood clot on
the surface of the ovary that sometimes
penetrates into the center of the foll icle
after ovulation (See Figures 9-3, 1 A and
9-4, 1A and B). Followi ng evacua-
of the follicular fluid and oocyte, the
foll icle collapses into folds. The cells of
the theca interna and the granulosa begin
to mix. The basement mem brane fo rms
the connective tissue substructure of the
corpus luteum.
Functional Corpus Luteum (CL)
The corpus luteum is now a mixture of large
luteal cells, LLC (formerly granulosa! cells)
and many small luteal cells, SLC (fo rmerly
thecal cells). In some cases, there is a
remna nt of the follicu lar antrum th at forms
a small cavity in the center of the corpus
luteum (See Figures 9-3, 3B and 9-4, 2B;
9-6, 3B).
V
et
B
oo
ks
.ir
182 The Luteal Phase
In general , a corpus albicans can be observed for a
substantial period oftime (several estrous cycles) after
luteolysis. The corpus albicans appears as a white scar-
like structure because the connective tissue remains
after the glandular tissue disappears.
The corpus lutemn originates from an
ovulatory follicle.
After ovulation the theca interna and the
granulosa! cells of the follicle undergo a dramatic
transfom1ation known as luteinization. Luteinization
is the process whereby cells of the ovulatory follicle
are transformed into luteal tissue. This transforma-
tion is governed by LH. Shortly before ovulation the
basement membrane of the follicle undergoes partial
disintegration and the physical separation of the thecal
and granulosa! cells disappears (See Figure 9-2). Dur-
ing ovulation, follicular fluid leaks from the follicle. At
the same time, the wall of the follicle collapses fann-
ing many folds (See Figure 9-2). These folds begin to
interdigitate, allowing thecal cells and the granulosa!
cells to mix, thus forming a gland consisting of con-
nective tissue cells, thecal cells and granulosa! cells.
In general, the cells of thecal origin and the cells of
granulosa! origin mix unifom1ly with one another (See
Figure 9-2). An exception to this is found in the corpora
Jutea of the woman and other primates, where thecal
and granulosa! cells are clustered into distinct “islets”.
It is easy to distinguish microscopically between luteal
cells that originate from the granulosa! cells and those
that originate from the thecal cells. Large luteal cells
are derived from granulosa! cells while small luteal
cells are derived from thecal cells. Portions of the
basement membrane that separated the thecal cells
from the granulosa! cells remain and constitute the
connective tissue network of the corpus Juteum (See
Figure 9-2).
Luteal tissue consists of large and
small luteal cells:
• large cells originate from the
granulosal cells
• small cells originate from the cells
oftlze theca interna
Large luteal cells (sometimes called granu-
losal-lutein cells) va1y in diameter from 20-70 microm-
eters (!lm), depending on species. In some species
(nuninants ), there are a large number of dense secretory
granules close to the plasma membrane. These secre-
tory granules contain oxytocin in the corpus luteum
of the cycle and are believed to contain relaxin in the
corpus luteum of pregnancy.
Small luteal cells (sometimes called thecal-
lutein cells) are less than 20 in diameter, have an
irregular shape and possess numerous lipid droplets in
their cytoplasm. They do not contain secretory gran-
ules as do the large luteal cells. Both small and large
luteal cells are steroidogenic (possessing the ability to
produce steroids), in this case progesterone.
In general, the corpus luteum increases in size
until about midway through the luteal phase (See Fig-
ures 9-3 through 9-6). For example, a skilled examiner
can almost always determine whether a corpus luteum
is present or absent in cows. In mares , it is almost
impossible to ascertain the presence or absence of the
corpus luteum because it does not protrude from the
surface of the ovary.
In the cow, palpation cannot accurately predict
the functional status of the corpus luteum. In four
separate studies, cows were transrectally palpated by
experienced diagnosticians. Corpora lutea were classi-
fied as func tional (secreting high quantities of proges-
terone) or nonfunctional (regressing or secreting low
levels of progesterone) by the diagnosticians. Using
measurements of blood proge sterone as the indi cator
of corpus luteum function, it was found that 25% to
39% of cows classified as having a functional corpus
luteum were not secreting high quantities of progester-
one. Furthermore, 15% to 21% of cows classified as
having a nonfunctional corpus luteum had high blood
progesterone. Clearly, the use of transrectal palpation
to assess the functional status of the corpus luteum has
limitations. From a practical reproductive manage-
ment perspective, this problem lim its the effectiveness
of treating animals with luteolytic agents to induce
estrus and ovulation. In other words, administering
luteolytic agents (prostaglandin F 2a) on the basis of
transrectal palpation of the ovaries alone will provide
suboptimal results.
The use of real-time ultrasonography has
proven effective for the examination of corpora lutea,
as well as ovarian follicles . In cattle, progesterone
concentration in blood is correlated with the diameter of
the corpus luteum as measured by ultrasonography.
L arge luteal cells rarely multiply after ovula-
tion. Therefore, the total number of granulosa! cells
” donated” by the follicle detem1ines the number of
large luteal cells in the newly-formed CL. Luteal
function may in-part be related to the vigor (as judged
by the number of granulosa! cells) of the follicle prior
to ovulation. In the ewe (and presumably other spe-
cies), an increase in corpus luteum size and weight is
due to a threefo ld increase in volume of large luteal
cells coupled with a fivefold increase in the number
The Luteal Phase 183
Figure 9-2. Formation of the Corpus Luteum
Preovulatory Follicle
The preovulatory follicle consists of granu-
losa! cells that line the antrum. The base-
ment membrane, separating the granulosa!
cells from the cells of the theca interna
begins to deteriorate prior to ovulation be-
cause of the action of collagenase . Com-
plete separation between the granulosa!
cells and the th eca intern a no longer exists
and the cells can begin to intermingle.
Corpus Hemorrhagicum (CH)
During ovulation, many small blood vessels
rupture causing local hemorrhage. This
hemorrhage appears as a blood clot on
the surface of the ovary that sometimes
penetrates into the center of the foll icle
after ovulation (See Figures 9-3, 1 A and
9-4, 1A and B). Followi ng evacua-
of the follicular fluid and oocyte, the
foll icle collapses into folds. The cells of
the theca interna and the granulosa begin
to mix. The basement mem brane fo rms
the connective tissue substructure of the
corpus luteum.
Functional Corpus Luteum (CL)
The corpus luteum is now a mixture of large
luteal cells, LLC (formerly granulosa! cells)
and many small luteal cells, SLC (fo rmerly
thecal cells). In some cases, there is a
remna nt of the follicu lar antrum th at forms
a small cavity in the center of the corpus
luteum (See Figures 9-3, 3B and 9-4, 2B;
9-6, 3B).
V
et
B
oo
ks
.ir
F
igure 9-3.
Luteal A
natom
y in R
elation to P
rogesterone S
ecretion D
uring the E
strous C
ycle in the C
ow
3A
2A
The area designated by the circle repre-
sents a developing corpus luteum
.
28
T
he corpus luteum
has been sliced in
half.
N
otice the increase in size w
hen
com
pared to that show
n in 1 B.
E
arly M
etestrus
–
Late M
etestrus
D
iestrus
E
10
– b.O
c::::
-“‘CCV
0
c::::
0
0
-:….
alcv
en
1A
C
ircled area is a corpus hem
or-
rhagicum
.
N
otice the bloody ap-
o..
pearance at the apex.
1
8
The corpus hem
orrhagicum
has
been sliced in half.
N
otice the
rem
nant of the follicular lum
en that
is filled w
ith a blood clot (arrow
).
8 6 4 2
0 +
O
vulation
5
10
D
ay of C
ycle (C
ow
)
IS
A
large corpus luteum
(circle) at peak
progesterone secretion.
3
8
A
large m
ass of orange tissue can be
seen w
hen the C
L is sliced in half. The
orange color reflects the high content of
P
-carotene. The central cavity (arrow
) is a
rem
nant of the follicular antrum
. A
central
cavity does not exist in every C
L.
21
+
O
vulation
The circle indicates the approxim
ate
area ofthe regressing corpus luteum
.
4
8
The corpus luteum
has changed
in color and in size.
The secre-
tory com
pon
ent of th
e tissue has
decreased significantly as a result
of luteolysis. A
rrow
designates re-
gressing C
L from
a previous cycle.
Figure 9-4. Luteal A
natom
y in R
elation to P
rogesterone S
ecretion D
uring the E
strous C
ycle in the E
w
e
2A
C
ircles A
and B
indicate deve
loping cor-
pora lut ea
.
28
C
orpus luteum
B
has been sliced in half.
N
otice the developing luteal tissue (circle)
that surro
unds a sm
all cavity (arrow
) that
is the rem
nant of the
follicular antrum
.
N
otice that the hem
orrhagic appearance
is no longer present.
E
arly M
etestrus
– -E
– b.O
c::::
-“‘CCV
0
c::::
0
0
-:….
m
c
v
cv
C
ircles indicate corpora hem
or-
b.O
h
.
0
r ag1ca
.
:….
1
8
C
ircled area show
s the corpus
hem
orrhagicum
sliced in half.
The clot is indicated by the ar-
row
.
0..
Late M
etestrus
D
iestrus
3
0 t
O
vulation
2
4
6
8
10
12
D
ay of C
ycle (Ew
e)
14
3A
A
corpus luteum
(circle) during the peak
luteal phase.
3
8
Th
e luteal tissue (sliced in half) is a rela-
tively large m
ass of secretory tissue.
16 1
O
vulation
Th
e circle indicates the
surface of a reg
ressing
co
rpus luteum
.
48
T
h
e co
rpu
s luteum
has
becom
e pale and th
e se-
cretory tissue m
ass has
decreased in size.
0
)
::.. :;! m r-c: m
-Q
)
en m
:;! m
r-c: Cil’
Q
)
-m
0
)
0
1
VetBooks.ir
F
igure 9-3.
Luteal A
natom
y in R
elation to P
rogesterone S
ecretion D
uring the E
strous C
ycle in the C
ow
3A
2A
The area designated by the circle repre-
sents a developing corpus luteum
.
28
T
he corpus luteum
has been sliced in
half.
N
otice the increase in size w
hen
com
pared to that show
n in 1 B.
E
arly M
etestrus
–
Late M
etestrus
D
iestrus
E
10
– b.O
c::::
-“‘CCV
0
c::::
0
0
-:….
alcv
en
1A
C
ircled area is a corpus hem
or-
rhagicum
.
N
otice the bloody ap-
o..
pearance at the apex.
1
8
The corpus hem
orrhagicum
has
been sliced in half.
N
otice the
rem
nant of the follicular lum
en that
is filled w
ith a blood clot (arrow
).
8 6 4 2
0 +
O
vulation
5
10
D
ay of C
ycle (C
ow
)
IS
A
large corpus luteum
(circle) at peak
progesterone secretion.
3
8
A
large m
ass of orange tissue can be
seen w
hen the C
L is sliced in half. The
orange color reflects the high content of
P
-carotene. The central cavity (arrow
) is a
rem
nant of the follicular antrum
. A
central
cavity does not exist in every C
L.
21
+
O
vulation
The circle indicates the approxim
ate
area ofthe regressing corpus luteum
.
4
8
The corpus luteum
has changed
in color and in size.
The secre-
tory com
pon
ent of th
e tissue has
decreased significantly as a result
of luteolysis. A
rrow
designates re-
gressing C
L from
a previous cycle.
Figure 9-4. Luteal A
natom
y in R
elation to P
rogesterone S
ecretion D
uring the E
strous C
ycle in the E
w
e
2A
C
ircles A
and B
indicate deve
loping cor-
pora lut ea
.
28
C
orpus luteum
B
has been sliced in half.
N
otice the developing luteal tissue (circle)
that surro
unds a sm
all cavity (arrow
) that
is the rem
nant of the
follicular antrum
.
N
otice that the hem
orrhagic appearance
is no longer present.
E
arly M
etestrus
– -E
– b.O
c::::
-“‘CCV
0
c::::
0
0
-:….
m
c
v
cv
C
ircles indicate corpora hem
or-
b.O
h
.
0
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.
:….
1
8
C
ircled area show
s the corpus
hem
orrhagicum
sliced in half.
The clot is indicated by the ar-
row
.
0..
Late M
etestrus
D
iestrus
3
0 t
O
vulation
2
4
6
8
10
12
D
ay of C
ycle (Ew
e)
14
3A
A
corpus luteum
(circle) during the peak
luteal phase.
3
8
Th
e luteal tissue (sliced in half) is a rela-
tively large m
ass of secretory tissue.
16 1
O
vulation
Th
e circle indicates the
surface of a reg
ressing
co
rpus luteum
.
48
T
h
e co
rpu
s luteum
has
becom
e pale and th
e se-
cretory tissue m
ass has
decreased in size.
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–
–
F
igure 9-5. Luteal A
natom
y in R
elation to P
rogesterone S
ecretion D
uring the E
strous C
ycle in the S
ow
1
A
and 1B
D
eveloping corpora lutea betw
een days 3
and 6.
B
ecause of variation in length of
the cycle and the tim
e of ovulation relative
to the stage of the cycle, precise age of
these corpora lutea is difficult to estab-
lish
. N
otice that all structures still have a
hem
orrhagic appearance and som
e have
a visible stigm
a (arrow
s) indicating the
point at w
hich ovulation occurred.
– -E
— 1:).0 c ._.
-ccu
0
c
0
0
–
:
I
.
.
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C
C
1
)
…, en C1) 1:).0 0 :I..
I:L
40
30
20
10
0 i
Ovulation E
arly to Late M
etestrus
D
iestrus
5
I 0
I 5
D
ay of C
ycle (Sow
)
2
A
a
n
d
2B
N
um
bers designate six corpora lutea dur-
ing high secretory activity. C
orpora lutea
4, 5 and 6 have been sliced in half. N
otice
that corpus luteum
5 has an antrum
. A
lso,
notice that P
4 is m
uch higher in the sow
than in the cow
, ew
e and m
are.
2
1
i
Ovulation
P
roestrus
3
A
a
n
d
3B
R
egressing corpora lute
a
.
N
otice the pale color. The in-
terval from
luteolysis to estrus
is long er than rum
inants.
Figure 9-6.
Luteal A
natom
y in R
elation to P
rogesterone S
ecretion D
uring the E
strous C
ycle in the M
are
2
A
A
rea desig
nated by th
e arrow
is th
e
developing corpus luteum
. N
ote: th
e C
L
does not protrude from
the surface and
is not yellow
as in oth
er species.
2B
The corpus luteum
seen in 2A
has been
sliced in half. A
rrow
indicates a follicle
sliced in half.
E
arly M
etestrus
–E
10
– b.O
c
-“CC1
J
0
c
0
0
–
:r…
1
A
a::IC
L
J
…….
U
l
C1J
T
he corp
us hem
orrhag
icum
is
w
ithin the circle.
It is not highly
visible from
the exterior as in
other species. 1B
A
rrow
indicates the hem
orrhag
ic
tissue w
ithin the w
all of the new
ly
ovulated follicle th
at has been
sliced in half.
b.O
0 :r…
D
.
8 6 4 2
0
Late M
etestrus
D
iestrus
5
10
O
vulation
D
ay of C
ycle (M
are)
15
3
A
a
n
d
3B
The structures are sliced in half to expose the
inner tissue m
ass. Tw
o distinct types of corpora
lutea can be seen during the peak luteal phase.
In som
e, there is a heterogeneous m
ass of tissue
w
ithout a central cavity (3A
), w
hile in others a
hom
ogenous m
ass of tissue w
ith a central cavity
(arrow
) exists (3B). Both types ae norm
al and
secrete adequate quant ities of progesterone.
In alm
ost all cases, corpora lutea in the m
are
are “buried” w
ithin the ovarian cortex and are
not palpable per rectum
. Ultrasonography easily
identifies a corpus luteum
in the m
are
2
1
t
O
vulation
P
roestrus
Tw
o exam
ples of reg
ressing
co
rpo
ra lutea
.
S
pecim
ens
w
ere sliced in half. N
otice that
the size has decreased. A
rrow
in 4B
indicates a residual blood
cl ot w
ithin the corpus luteum
.
0
0
(j’)
:;! (!) r-t: (i) Q) -Q) (/) (!) :;! (!) r-t: iD Q) -Q) (/) (!) 00 ……..
VetBooks.ir
–
–
F
igure 9-5. Luteal A
natom
y in R
elation to P
rogesterone S
ecretion D
uring the E
strous C
ycle in the S
ow
1
A
and 1B
D
eveloping corpora lutea betw
een days 3
and 6.
B
ecause of variation in length of
the cycle and the tim
e of ovulation relative
to the stage of the cycle, precise age of
these corpora lutea is difficult to estab-
lish
. N
otice that all structures still have a
hem
orrhagic appearance and som
e have
a visible stigm
a (arrow
s) indicating the
point at w
hich ovulation occurred.
– -E
— 1:).0 c ._.
-ccu
0
c
0
0
–
:
I
.
.
£
C
C
1
)
…, en C1) 1:).0 0 :I..
I:L
40
30
20
10
0 i
Ovulation E
arly to Late M
etestrus
D
iestrus
5
I 0
I 5
D
ay of C
ycle (Sow
)
2
A
a
n
d
2B
N
um
bers designate six corpora lutea dur-
ing high secretory activity. C
orpora lutea
4, 5 and 6 have been sliced in half. N
otice
that corpus luteum
5 has an antrum
. A
lso,
notice that P
4 is m
uch higher in the sow
than in the cow
, ew
e and m
are.
2
1
i
Ovulation
P
roestrus
3
A
a
n
d
3B
R
egressing corpora lute
a
.
N
otice the pale color. The in-
terval from
luteolysis to estrus
is long er than rum
inants.
Figure 9-6.
Luteal A
natom
y in R
elation to P
rogesterone S
ecretion D
uring the E
strous C
ycle in the M
are
2
A
A
rea desig
nated by th
e arrow
is th
e
developing corpus luteum
. N
ote: th
e C
L
does not protrude from
the surface and
is not yellow
as in oth
er species.
2B
The corpus luteum
seen in 2A
has been
sliced in half. A
rrow
indicates a follicle
sliced in half.
E
arly M
etestrus
–E
10
– b.O
c
-“CC1
J
0
c
0
0
–
:r…
1
A
a::IC
L
J
…….
U
l
C1J
T
he corp
us hem
orrhag
icum
is
w
ithin the circle.
It is not highly
visible from
the exterior as in
other species. 1B
A
rrow
indicates the hem
orrhag
ic
tissue w
ithin the w
all of the new
ly
ovulated follicle th
at has been
sliced in half.
b.O
0 :r…
D
.
8 6 4 2
0
Late M
etestrus
D
iestrus
5
10
O
vulation
D
ay of C
ycle (M
are)
15
3
A
a
n
d
3B
The structures are sliced in half to expose the
inner tissue m
ass. Tw
o distinct types of corpora
lutea can be seen during the peak luteal phase.
In som
e, there is a heterogeneous m
ass of tissue
w
ithout a central cavity (3A
), w
hile in others a
hom
ogenous m
ass of tissue w
ith a central cavity
(arrow
) exists (3B). Both types ae norm
al and
secrete adequate quant ities of progesterone.
In alm
ost all cases, corpora lutea in the m
are
are “buried” w
ithin the ovarian cortex and are
not palpable per rectum
. Ultrasonography easily
identifies a corpus luteum
in the m
are
2
1
t
O
vulation
P
roestrus
Tw
o exam
ples of reg
ressing
co
rpo
ra lutea
.
S
pecim
ens
w
ere sliced in half. N
otice that
the size has decreased. A
rrow
in 4B
indicates a residual blood
cl ot w
ithin the corpus luteum
.
0
0
(j’)
:;! (!) r-t: (i) Q) -Q) (/) (!) :;! (!) r-t: iD Q) -Q) (/) (!) 00 ……..
VetBooks.ir
[]]
I
188 The Luteal Phase
of small luteal cells. Thus, large luteal cells undergo
hypertrophy (increase in size), while small luteal cells
undergo hyperplasia (increase in cell numbers) as the
CL develops. In addition to changes in steroidogenic
cells, non-steroidogenic cells (fibroblasts, capillary
cells and eosinophils) increase in number during the
estrous cycle. The net effect of these cellular changes
is a marked enlargement ofthe corpus luteum.
The “vigor” of the corpus luteum
probably depends on:
• the number of luteal cells
• the degree to which the CL
becomes vascularized
The functional capability (ability to secrete pro-
gesterone) of the newly developed corpus luteum may
also depend on the degree of vascularity in the cellular
layers of the follicle. The ability of the corpus luteum
to vascularize may relate to its ability to synthesize
and deliver hom1ones. As presented in the previous
chapter, follicular fluid contains angiogenic factors.
The degree to which these angiogenic factors promote
vascularization of the corpus luteum is probably related
to the quantity of angiogenic factors present in the fol-
licular tissue.
Insufficient luteal function (poor progesterone
synthesis and secretion) is believed to be a possible
contributor to reproductive failure in mammals. A
corpus luteum secreting suboptimal concentrations
of progesterone probably results in the inability of
the dam’s uterus to suppmi development of the early
embryo.
The primary target organs for progesterone
are the hypothalamus, the uterus and the mammary
gland (See Figure 9-7). The uterus has two target
components: 1) the glandular endometrium and 2)
the muscular myometrium. Progesterone stimulates
maximal secretion by the endometrial glands. Secre-
tory products from the endometrial glands contribute
to an environment that supports the development
of the “free-floating” conceptus after it enters the
uterine lumen. An important inhibitory role of pro-
gesterone is to reduce the motility (contractions) of
the myometrium. Such a role causes a “uterine quies-
cence” effect on the myometrium in the cow, pig and
ewe. In the mare, myometrial motility is not inhibited
to the same degree so that the conceptus is transported
around the uterus but not expelled. Myometrial inhi-
bition is thought to be important because it provides
“calming” conditions for attachment of the conceph1s
to the uterine endometrium. In the mare, the conceptus
is transported about in the uterine lumen by contrac-
tions of the myometrium. This phenomenon will be
discussed in more detail in Chapter 13. Progesterone
causes final alveolar development of the mammary
gland during pregnancy, thereby allowing initiation
of lactation.
Progesterone Synthesis Requires
Cholesterol and LH
The presence of basal (tonic) LH and cho-
lesterol is necessary for progesterone to be secreted
by luteal cells. The mechanism whereby LH causes
secretion of progesterone in luteal cells is illustrated in
Figure 9-8. In order to fillly understand progesterone
synthesis, you should carefully read the explanation of
each step in the boxes provided in Figure 9-8.
Progesterone is of major importance in the en-
docrine control of reproduction because it exerts a strong
negative feedbacl\: on the hypothalamus (See Figure
9-7). Elevated progesterone reduces the pulse frequency
of GnRH by the tonic GnRH center in the hypothala-
mus. However, the amplih1de of the LH pulses is still
relatively high. Such a pattern of LH secretion along
with tonic FSH secretion allows follicles to develop
during the luteal phase. These follicles do not reach
preovulatory stah1s until progesterone decreases and
the frequency ofLH pulses increases. H igh progester-
one therefore prevents development of steroidogenic
preovulatory follicles, secretion of estradiol, behavioral
estrus and the p reovulatory surge of GnRH and LH.
Progesterone is an inhibitor because it:
• reduces GnRH pulse frequency
• prevents behavioral estrus
• stops the preovulat01y LH surge
• reduces myometrial tone (except in
the mare)
Progesterone almost totally inhibits estrual
behavior. In general, females under the influence of
progesterone do not display estrus and will not copulate
with the male. However, as pointed out in Chapter 7,
progesterone exerts a positive priming effect on the
brain to enhance the behavioral effects of estradiol after
progesterone is reduced. For example, if females are
ovariectomized (removal of ovaries) and treated with
estradiol, they will display behavioral characteristics of
estrus. These traits will be amplified in both intensity
and duration if cows are treated with progesterone for
about 5 to 7 days before they receive estradiol.
Lysis of the Corpus Luteum
Must Occur Before the Female Can Enter
the Follicular Phase
Luteolysis is the loss of progesterone secre-
tion by the CL followed by loss of luteal tissue m ass .
It occurs dming a one-to-three day period at the end
of the luteal phase. Luteolysis is a process whereby
the corpus luteum undergoes irreversible degeneration
characterized by a dramatic drop in blood concentra-
tions of progesterone (See Figures 9- 1,9-3 through 9-6).
The hormone inducing luteolysis is PGF2a secreted by
The Luteal Phase 189
the uterine endometrium. Communication between the
and the ute1ine endometrium is necessary
m order to bnng about successfhl luteolysis. The utems
f1mctions as an endocrine organ and is responsible for
secreting PGF2a that causes luteolysis. Ifluteolysis does
not occur, the animal remains in a sustained luteal phase
because progesterone inhibits gonadotropin secreti on
(See Figure 9-7). The importance of the uterus in con-
trolling the life-span of the corpus luteum is illustrated
in Figure 9-9. In mammals , other than p rimates, com-
plete removal ofthe utems (uterectomy) after ovulation
causes the corpus luteum to be maintained j ust as if the
Figure 9-7. Progesterone (P4 ) has Many Physiological Effects
Corpus luteum
( ovory)
P4 produced by the CL exerts a nega-
tive (-) feedback on the GnRH neu-
rons of the hypothalamus. Therefore,
GnRH, LH and FSH are suppressed
and little estrogen is secreted. Pro-
gesterone is thought to decrease the
number of GnRH receptors on the
anterior pituitary.
U t e rine t issu e
(ute ru s)
P4 promotes alveo-
lar developme nt
in t he mamma ry
gla nd , es pecially
during pregnancy.
G la ndular se crctlo ti s
P 4 exerts a strong positive ( +) influ-
ence on the endometrium of the
uterus. Under the influence of P4,
the uterine glands secrete materials
into the uterine lumen. Progester-
one inhibits the myometrium and
th us reduces its con tractility and
tone.
V
et
B
oo
ks
.ir
[]]
I
188 The Luteal Phase
of small luteal cells. Thus, large luteal cells undergo
hypertrophy (increase in size), while small luteal cells
undergo hyperplasia (increase in cell numbers) as the
CL develops. In addition to changes in steroidogenic
cells, non-steroidogenic cells (fibroblasts, capillary
cells and eosinophils) increase in number during the
estrous cycle. The net effect of these cellular changes
is a marked enlargement ofthe corpus luteum.
The “vigor” of the corpus luteum
probably depends on:
• the number of luteal cells
• the degree to which the CL
becomes vascularized
The functional capability (ability to secrete pro-
gesterone) of the newly developed corpus luteum may
also depend on the degree of vascularity in the cellular
layers of the follicle. The ability of the corpus luteum
to vascularize may relate to its ability to synthesize
and deliver hom1ones. As presented in the previous
chapter, follicular fluid contains angiogenic factors.
The degree to which these angiogenic factors promote
vascularization of the corpus luteum is probably related
to the quantity of angiogenic factors present in the fol-
licular tissue.
Insufficient luteal function (poor progesterone
synthesis and secretion) is believed to be a possible
contributor to reproductive failure in mammals. A
corpus luteum secreting suboptimal concentrations
of progesterone probably results in the inability of
the dam’s uterus to suppmi development of the early
embryo.
The primary target organs for progesterone
are the hypothalamus, the uterus and the mammary
gland (See Figure 9-7). The uterus has two target
components: 1) the glandular endometrium and 2)
the muscular myometrium. Progesterone stimulates
maximal secretion by the endometrial glands. Secre-
tory products from the endometrial glands contribute
to an environment that supports the development
of the “free-floating” conceptus after it enters the
uterine lumen. An important inhibitory role of pro-
gesterone is to reduce the motility (contractions) of
the myometrium. Such a role causes a “uterine quies-
cence” effect on the myometrium in the cow, pig and
ewe. In the mare, myometrial motility is not inhibited
to the same degree so that the conceptus is transported
around the uterus but not expelled. Myometrial inhi-
bition is thought to be important because it provides
“calming” conditions for attachment of the conceph1s
to the uterine endometrium. In the mare, the conceptus
is transported about in the uterine lumen by contrac-
tions of the myometrium. This phenomenon will be
discussed in more detail in Chapter 13. Progesterone
causes final alveolar development of the mammary
gland during pregnancy, thereby allowing initiation
of lactation.
Progesterone Synthesis Requires
Cholesterol and LH
The presence of basal (tonic) LH and cho-
lesterol is necessary for progesterone to be secreted
by luteal cells. The mechanism whereby LH causes
secretion of progesterone in luteal cells is illustrated in
Figure 9-8. In order to fillly understand progesterone
synthesis, you should carefully read the explanation of
each step in the boxes provided in Figure 9-8.
Progesterone is of major importance in the en-
docrine control of reproduction because it exerts a strong
negative feedbacl\: on the hypothalamus (See Figure
9-7). Elevated progesterone reduces the pulse frequency
of GnRH by the tonic GnRH center in the hypothala-
mus. However, the amplih1de of the LH pulses is still
relatively high. Such a pattern of LH secretion along
with tonic FSH secretion allows follicles to develop
during the luteal phase. These follicles do not reach
preovulatory stah1s until progesterone decreases and
the frequency ofLH pulses increases. H igh progester-
one therefore prevents development of steroidogenic
preovulatory follicles, secretion of estradiol, behavioral
estrus and the p reovulatory surge of GnRH and LH.
Progesterone is an inhibitor because it:
• reduces GnRH pulse frequency
• prevents behavioral estrus
• stops the preovulat01y LH surge
• reduces myometrial tone (except in
the mare)
Progesterone almost totally inhibits estrual
behavior. In general, females under the influence of
progesterone do not display estrus and will not copulate
with the male. However, as pointed out in Chapter 7,
progesterone exerts a positive priming effect on the
brain to enhance the behavioral effects of estradiol after
progesterone is reduced. For example, if females are
ovariectomized (removal of ovaries) and treated with
estradiol, they will display behavioral characteristics of
estrus. These traits will be amplified in both intensity
and duration if cows are treated with progesterone for
about 5 to 7 days before they receive estradiol.
Lysis of the Corpus Luteum
Must Occur Before the Female Can Enter
the Follicular Phase
Luteolysis is the loss of progesterone secre-
tion by the CL followed by loss of luteal tissue m ass .
It occurs dming a one-to-three day period at the end
of the luteal phase. Luteolysis is a process whereby
the corpus luteum undergoes irreversible degeneration
characterized by a dramatic drop in blood concentra-
tions of progesterone (See Figures 9- 1,9-3 through 9-6).
The hormone inducing luteolysis is PGF2a secreted by
The Luteal Phase 189
the uterine endometrium. Communication between the
and the ute1ine endometrium is necessary
m order to bnng about successfhl luteolysis. The utems
f1mctions as an endocrine organ and is responsible for
secreting PGF2a that causes luteolysis. Ifluteolysis does
not occur, the animal remains in a sustained luteal phase
because progesterone inhibits gonadotropin secreti on
(See Figure 9-7). The importance of the uterus in con-
trolling the life-span of the corpus luteum is illustrated
in Figure 9-9. In mammals , other than p rimates, com-
plete removal ofthe utems (uterectomy) after ovulation
causes the corpus luteum to be maintained j ust as if the
Figure 9-7. Progesterone (P4 ) has Many Physiological Effects
Corpus luteum
( ovory)
P4 produced by the CL exerts a nega-
tive (-) feedback on the GnRH neu-
rons of the hypothalamus. Therefore,
GnRH, LH and FSH are suppressed
and little estrogen is secreted. Pro-
gesterone is thought to decrease the
number of GnRH receptors on the
anterior pituitary.
U t e rine t issu e
(ute ru s)
P4 promotes alveo-
lar developme nt
in t he mamma ry
gla nd , es pecially
during pregnancy.
G la ndular se crctlo ti s
P 4 exerts a strong positive ( +) influ-
ence on the endometrium of the
uterus. Under the influence of P4,
the uterine glands secrete materials
into the uterine lumen. Progester-
one inhibits the myometrium and
th us reduces its con tractility and
tone.
V
et
B
oo
ks
.ir
I
190 The Luteal Phase
Figure 9-8. Mechanism of Progesterone Synthesis by Luteal Cells
Esterified cholesterol is delivered to
the luteal cell primarily by way of
low and high density lipoprotein
(LDL and HDL). The blood-borne
lipoprotein-cholesterol complex
binds to specific receptors on the
outside of the plasma membrane.
The LDL-cholesterol complex binds
to specific receptors on the outside
of the plasma membrane. The
LDL-cholesterol receptor complex is
internalized and cholesterol Is
released from the receptor complex
in the fo rm of cholesterol esters.
After LDL-cholesterol is removed,
the receptor is “recycled” and
becomes available to transport
another LDL-cholesteroi complex.
LH binds to specific LH
receptors (LHR) on the
plasma membrane.
Pregnenolone leaves the
mitochondria and is
converted enzymatically to
progesterone (PROG) by
the smooth endoplasmic
reticulum. Progesterone
leaves the cell and enters
the blood, where it travels
to target tissues.
0 ‘ CHOL \… ‘ ESTERASE …
‘, • m
‘ ‘ ‘ \
‘,0®
” ‘ ‘ \ I ………. ,
The LH receptor complex
activates a G-proteln (G) that
activates membrane-bound
adenylate cyclase (AC}.
Mitochondrial enzymes are
res po nsible fo r converting
cholest erol to p regneno-
lone (PREG).
Cyclic AMP activates
prot ein kinase enzymes.
Protein kinases (a) acceler-
ate LDL-cholesterol
receptor internalizat ion,
(b) activat e cholesterol-
esterase that cleaves
ch olesterol fro m its ester
and (c) promote entry of
cholesterol into mitochon-
dria.
Ad enylate cyclase prom otes
t he con version of ATP to
cyclic AM P (cAMP}, t he
second messenger.
female were pregnant. For example, in ewes with an
intact uterus the life-span of the corpus luteum is identi-
cal to that seen in the nonnal cycle ( 17 days). However,
when the entire uterus is removed (total uterectomy), the
life-span of the corpus luteum is prolonged for months
and is similar to a nonnal gestation period (148 days).
Clearly, removal of the entire uterus extends the life-
span of the corpus luteum dramatically.
The uterus is required for successful
luteolysis in many species.
When partial uterectomy is perfor med, a less
dramatic effect can be seen. For example, when the
uterine hom ipsilatera l (on the same side) to the corpus
luteum is removed, the life-span of the corpus luteum
is almost twice as long (about 35 days) as the nonnal
cycle. In contrast, when the contral ateral (opposite
side) uterine horn is removed, there is little, if any, ef-
fect on th e life-span of the corpus luteum. The response
to complete and partial uterectomy is summarized in
Figure 9-9. Several important findings have emerged
from the classic exper iments illustrated in Figure 9-9.
First, the uterus is requ ired for lysis of the corpus
luteum. Therefore, the uterus secretes a substance(s)
that causes luteolysis. Second, removal of the utem s
ipsilateral to the corpus luteum increases the life-span
of the corpus luteum, while removal of the uterine hom
contralateral to the corpus luteum does not. A local
effect of the utems directly upon the ipsilateral ovary
containing the corpus luteum is obvious. A local effect
can be further supported by the fact that when the ovary
is transplanted into the neck of the female, but the uterus
remains intact, the corpus luteum life-span is prolonged
by many weeks. Co llectively, what these experime nts
have told us is: I ) the uterus is responsible fo r luteo lysis
and 2) the uterus must be near the ovary.
You should now understand from the above
discussion that the utems is required for luteolysis.
Clearly then, the uterus must secrete a substance that
causes destruction of the corpus luteum. After years
of intensive and heavi ly focused research, it has been
conclus ive ly demonstrated that prostag land in F2a is
the luteolysin in domestic animals. Prostaglandin F2a
is also the luteolytic agent in p rimates but is secreted
by the corpus luteum. Among domestic animals, the
uterectomized bitch cycles normally and has a luteal
phase of nom1al duration suggesting that the uterus has
little or no influence upon luteal fimction in canids.
The Luteal Phase 191
A vascular countercurren t transport
system ensures that PGF2a will reach the
ovary in sufficient quantities to cause
luteolysis in the ewe, cow and sow.
How does PGF2a get fro m the uterus to the
ovary, where it causes luteolysis? Prostaglandin F2a
from the utems is transported to the ipsilateral ovary
through a vascular countercurrent exchange mecha-
nism. A countercurrent exchange system involves two
closely associated blood vessels in whi ch bl ood from
one vessel fl ows in the oppos ite direction to that of the
adjacent vessel. Low molecular weight substances
in high concentrations in one vessel diffuse into the
adjacent vessel, w here they are in low concentrations.
The PGF2a secreted by the endometr ium enters the
uterine vei n and the uterine lymph vessels, at relatively
high concentrations. The ovari an artery lies in close
association with the utero-ovarian vein (See Figure
9- 1 0). By countercuiTent exchange, transfer ofPGF2 a
is accomplished by a prostaglandin transport protein
that faci litates movement ofPGF2u across the wall of
the uterine vein into the blood of the ovarian artery.
This spec ial anatomical relationship ensures that a high
prop ortion of the PGF2a secreted by the uterus will be
transpor ted directly to the ovary and the corpus luteum
without dilution by the systemic circ ulation. This
mechanism is particularly important because much of
PGF2a is denatured during one circul atory pass through
the pulmonary system in the ewe and the cow (around
90%). In the sow, only about 40% of the PGF2u is de-
natured in the pulmonary circulation. By entering the
ovarian arte1y, PGF 2a can exert its lytic effect directly on
the corpus luteum. The counterctment transport system
is present in the cow, sow and ewe, but not in the mare.
The mare does not metabolize PGF2a as rapidly as other
species, so the need for a local transport specialization
is not as important in the m are. In addition, the mare
CL is thought to be m ore sensitive to PGF2a than the
CL of the cow, sow and ewe.
Exogenous PGF2a causes luteolysis during
about 60% of the cycle in most species. For example,
it exerts its most potent effect after day six of the cycle
and will almost always cause luteolysis if administered
after this time in the cow. In contrast, PGFza has a
negligib le effect during the first two to fou r days after
ovulation. In the pig, the corpus luteum does not be-
come responsive to the luteolytic action of a single dose
of PGF2a until day 12 to 14 of the cycle. Prostaglandin
Fza and its analogs are used widely to cause regression
of the corpus luteum and thus synchronize estrus and
ovulation, to induce abortion and sometimes to induce
parhtr ition.
V
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.ir
I
190 The Luteal Phase
Figure 9-8. Mechanism of Progesterone Synthesis by Luteal Cells
Esterified cholesterol is delivered to
the luteal cell primarily by way of
low and high density lipoprotein
(LDL and HDL). The blood-borne
lipoprotein-cholesterol complex
binds to specific receptors on the
outside of the plasma membrane.
The LDL-cholesterol complex binds
to specific receptors on the outside
of the plasma membrane. The
LDL-cholesterol receptor complex is
internalized and cholesterol Is
released from the receptor complex
in the fo rm of cholesterol esters.
After LDL-cholesterol is removed,
the receptor is “recycled” and
becomes available to transport
another LDL-cholesteroi complex.
LH binds to specific LH
receptors (LHR) on the
plasma membrane.
Pregnenolone leaves the
mitochondria and is
converted enzymatically to
progesterone (PROG) by
the smooth endoplasmic
reticulum. Progesterone
leaves the cell and enters
the blood, where it travels
to target tissues.
0 ‘ CHOL \… ‘ ESTERASE …
‘, • m
‘ ‘ ‘ \
‘,0®
” ‘ ‘ \ I ………. ,
The LH receptor complex
activates a G-proteln (G) that
activates membrane-bound
adenylate cyclase (AC}.
Mitochondrial enzymes are
res po nsible fo r converting
cholest erol to p regneno-
lone (PREG).
Cyclic AMP activates
prot ein kinase enzymes.
Protein kinases (a) acceler-
ate LDL-cholesterol
receptor internalizat ion,
(b) activat e cholesterol-
esterase that cleaves
ch olesterol fro m its ester
and (c) promote entry of
cholesterol into mitochon-
dria.
Ad enylate cyclase prom otes
t he con version of ATP to
cyclic AM P (cAMP}, t he
second messenger.
female were pregnant. For example, in ewes with an
intact uterus the life-span of the corpus luteum is identi-
cal to that seen in the nonnal cycle ( 17 days). However,
when the entire uterus is removed (total uterectomy), the
life-span of the corpus luteum is prolonged for months
and is similar to a nonnal gestation period (148 days).
Clearly, removal of the entire uterus extends the life-
span of the corpus luteum dramatically.
The uterus is required for successful
luteolysis in many species.
When partial uterectomy is perfor med, a less
dramatic effect can be seen. For example, when the
uterine hom ipsilatera l (on the same side) to the corpus
luteum is removed, the life-span of the corpus luteum
is almost twice as long (about 35 days) as the nonnal
cycle. In contrast, when the contral ateral (opposite
side) uterine horn is removed, there is little, if any, ef-
fect on th e life-span of the corpus luteum. The response
to complete and partial uterectomy is summarized in
Figure 9-9. Several important findings have emerged
from the classic exper iments illustrated in Figure 9-9.
First, the uterus is requ ired for lysis of the corpus
luteum. Therefore, the uterus secretes a substance(s)
that causes luteolysis. Second, removal of the utem s
ipsilateral to the corpus luteum increases the life-span
of the corpus luteum, while removal of the uterine hom
contralateral to the corpus luteum does not. A local
effect of the utems directly upon the ipsilateral ovary
containing the corpus luteum is obvious. A local effect
can be further supported by the fact that when the ovary
is transplanted into the neck of the female, but the uterus
remains intact, the corpus luteum life-span is prolonged
by many weeks. Co llectively, what these experime nts
have told us is: I ) the uterus is responsible fo r luteo lysis
and 2) the uterus must be near the ovary.
You should now understand from the above
discussion that the utems is required for luteolysis.
Clearly then, the uterus must secrete a substance that
causes destruction of the corpus luteum. After years
of intensive and heavi ly focused research, it has been
conclus ive ly demonstrated that prostag land in F2a is
the luteolysin in domestic animals. Prostaglandin F2a
is also the luteolytic agent in p rimates but is secreted
by the corpus luteum. Among domestic animals, the
uterectomized bitch cycles normally and has a luteal
phase of nom1al duration suggesting that the uterus has
little or no influence upon luteal fimction in canids.
The Luteal Phase 191
A vascular countercurren t transport
system ensures that PGF2a will reach the
ovary in sufficient quantities to cause
luteolysis in the ewe, cow and sow.
How does PGF2a get fro m the uterus to the
ovary, where it causes luteolysis? Prostaglandin F2a
from the utems is transported to the ipsilateral ovary
through a vascular countercurrent exchange mecha-
nism. A countercurrent exchange system involves two
closely associated blood vessels in whi ch bl ood from
one vessel fl ows in the oppos ite direction to that of the
adjacent vessel. Low molecular weight substances
in high concentrations in one vessel diffuse into the
adjacent vessel, w here they are in low concentrations.
The PGF2a secreted by the endometr ium enters the
uterine vei n and the uterine lymph vessels, at relatively
high concentrations. The ovari an artery lies in close
association with the utero-ovarian vein (See Figure
9- 1 0). By countercuiTent exchange, transfer ofPGF2 a
is accomplished by a prostaglandin transport protein
that faci litates movement ofPGF2u across the wall of
the uterine vein into the blood of the ovarian artery.
This spec ial anatomical relationship ensures that a high
prop ortion of the PGF2a secreted by the uterus will be
transpor ted directly to the ovary and the corpus luteum
without dilution by the systemic circ ulation. This
mechanism is particularly important because much of
PGF2a is denatured during one circul atory pass through
the pulmonary system in the ewe and the cow (around
90%). In the sow, only about 40% of the PGF2u is de-
natured in the pulmonary circulation. By entering the
ovarian arte1y, PGF 2a can exert its lytic effect directly on
the corpus luteum. The counterctment transport system
is present in the cow, sow and ewe, but not in the mare.
The mare does not metabolize PGF2a as rapidly as other
species, so the need for a local transport specialization
is not as important in the m are. In addition, the mare
CL is thought to be m ore sensitive to PGF2a than the
CL of the cow, sow and ewe.
Exogenous PGF2a causes luteolysis during
about 60% of the cycle in most species. For example,
it exerts its most potent effect after day six of the cycle
and will almost always cause luteolysis if administered
after this time in the cow. In contrast, PGFza has a
negligib le effect during the first two to fou r days after
ovulation. In the pig, the corpus luteum does not be-
come responsive to the luteolytic action of a single dose
of PGF2a until day 12 to 14 of the cycle. Prostaglandin
Fza and its analogs are used widely to cause regression
of the corpus luteum and thus synchronize estrus and
ovulation, to induce abortion and sometimes to induce
parhtr ition.
V
et
B
oo
ks
.ir
I
192 The Luteal Phase
Figure 9-9. Effect of Uterectomy upon Estrous Cycle Duration in the Ewe
Intact uterus
G1 CL
\ n ‘”•’
Ovary
In the intact uterus, the CL lifespan is the
same as in a normal cycle (15-17 d).
Partial uterectomy
(Contralateral to CL)
..
‘. . ‘ ‘.
” . ‘
‘,
‘,
. ‘
… _,’ \
‘, … ;
Ovary · . – G1 CL r /) ., ••.
A partial uterectomy contralateral to the CL
will yield a lifespan similar to a normal
cycle (15-17 d).
The requirements for luteolysis (in
subprimate mammals) are:
• presence of oxytocin receptors on
endometrial cells
• presence of a critical level of oJ..ytocin
• PGF
2
a. synthesis by the endometrium
Total uterectorny
.. — … _
” ” . . . ‘ . ‘ ..
‘ ‘ ‘ ” .. _ .. ‘ \
Ovary
\.-?
…
‘
‘ ‘ ” •:
•’ ‘ . ‘ .
” ‘ ‘
/ /
With a total uterectomy, the CL lifespan is
similar to a normal gestation length (148 d).
Partial uterectomy
(Ipsilateral to CL)
:
:
Ovary
” ” ” ‘ ” ”
A partial uterectomy ipsilateral to the CL
will cause the CL to have a lifespan longer
than normal (35 d).
What stimulates the secretion ofPGF2a during
the late luteal phase? In addition to progesterone, large
luteal cells synthesize and secrete oxytocin. In fact, in
the cow and the ewe the corpus luteum contains very
large quantities of oxytocin. Luteal oxytoci n is s tored
in secretory granules analogous to those observed in
the nerve terminals of the posterior pih1itary gland.
When oxytocin is injected into ewes near the end of the
luteal phase, PGF2a appears in the circulating blood in
response to these injections.
During the first-half of the luteal phase, pros-
taglandin secretion by the endometrium of the uterus
is almost nonexistent. However, during the late luteal
phase, secretion ofPGF2a. begins to occur in pulses (See
Figure 9-11 ). The pulses increase in frequency and
amplitude as the end of the luteal phase approaches.
The Luteal Phase 193
Figure 9-10. The Utero-Ovarian Vascular Countercurrent Transport System
To
Schematic illustration of
the countercu rrent trans-
port system in the cow,
sow and ewe. A portion
of uterine PG Fza is trans-
ported directly from the
ute ro-ovaria n ve in into
the ovarian artery where
it has a direct lytic effect
on the corpus luteum.
In the two photographs, a blue latex
medium was injected into the utero-
ov arian v ein (UOV) a nd a red latex
medium into the ovaria n artery (OA).
The latex was allowed to polymerize and
solidify. The tissue was then dissolved
with repeated treatme nts of saturated
sodium hydroxide followed by wash ings
with water until all of the tissue was
removed (Fro m Cody et al. 1999. Bioi.
Reprod. 60(Suppl 1 ): 90). The dashed
lines in the photo at left approximate the
boundaries of the uterine horns (UH)
and the ovary (0 ). Th e uterus secretes
prostaglandin Fza that enters the venous
drainage at high concentrations. In
the photo below PGFza diffuses from
the utero-ovarian vein into the ovarian
artery and is tra nsported directly to the
ovary (artery-arrows) where it causes
luteolys is.
V
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I
192 The Luteal Phase
Figure 9-9. Effect of Uterectomy upon Estrous Cycle Duration in the Ewe
Intact uterus
G1 CL
\ n ‘”•’
Ovary
In the intact uterus, the CL lifespan is the
same as in a normal cycle (15-17 d).
Partial uterectomy
(Contralateral to CL)
..
‘. . ‘ ‘.
” . ‘
‘,
‘,
. ‘
… _,’ \
‘, … ;
Ovary · . – G1 CL r /) ., ••.
A partial uterectomy contralateral to the CL
will yield a lifespan similar to a normal
cycle (15-17 d).
The requirements for luteolysis (in
subprimate mammals) are:
• presence of oxytocin receptors on
endometrial cells
• presence of a critical level of oJ..ytocin
• PGF
2
a. synthesis by the endometrium
Total uterectorny
.. — … _
” ” . . . ‘ . ‘ ..
‘ ‘ ‘ ” .. _ .. ‘ \
Ovary
\.-?
…
‘
‘ ‘ ” •:
•’ ‘ . ‘ .
” ‘ ‘
/ /
With a total uterectomy, the CL lifespan is
similar to a normal gestation length (148 d).
Partial uterectomy
(Ipsilateral to CL)
:
:
Ovary
” ” ” ‘ ” ”
A partial uterectomy ipsilateral to the CL
will cause the CL to have a lifespan longer
than normal (35 d).
What stimulates the secretion ofPGF2a during
the late luteal phase? In addition to progesterone, large
luteal cells synthesize and secrete oxytocin. In fact, in
the cow and the ewe the corpus luteum contains very
large quantities of oxytocin. Luteal oxytoci n is s tored
in secretory granules analogous to those observed in
the nerve terminals of the posterior pih1itary gland.
When oxytocin is injected into ewes near the end of the
luteal phase, PGF2a appears in the circulating blood in
response to these injections.
During the first-half of the luteal phase, pros-
taglandin secretion by the endometrium of the uterus
is almost nonexistent. However, during the late luteal
phase, secretion ofPGF2a. begins to occur in pulses (See
Figure 9-11 ). The pulses increase in frequency and
amplitude as the end of the luteal phase approaches.
The Luteal Phase 193
Figure 9-10. The Utero-Ovarian Vascular Countercurrent Transport System
To
Schematic illustration of
the countercu rrent trans-
port system in the cow,
sow and ewe. A portion
of uterine PG Fza is trans-
ported directly from the
ute ro-ovaria n ve in into
the ovarian artery where
it has a direct lytic effect
on the corpus luteum.
In the two photographs, a blue latex
medium was injected into the utero-
ov arian v ein (UOV) a nd a red latex
medium into the ovaria n artery (OA).
The latex was allowed to polymerize and
solidify. The tissue was then dissolved
with repeated treatme nts of saturated
sodium hydroxide followed by wash ings
with water until all of the tissue was
removed (Fro m Cody et al. 1999. Bioi.
Reprod. 60(Suppl 1 ): 90). The dashed
lines in the photo at left approximate the
boundaries of the uterine horns (UH)
and the ovary (0 ). Th e uterus secretes
prostaglandin Fza that enters the venous
drainage at high concentrations. In
the photo below PGFza diffuses from
the utero-ovarian vein into the ovarian
artery and is tra nsported directly to the
ovary (artery-arrows) where it causes
luteolys is.
V
et
B
oo
ks
.ir
I
I
194 The Luteal Phase
Figure 9-11. Changes in PGF Metabolite (PGF-M) During
Late Diestrus and Proestrus
PGF-M (brown line) is
an accurate estimate
of PGF2a· As the graph
shows, PGF2a is low.
The amplitude and fre-
quency of episodes of
PGF2a secretion increase
at about day 16. Abou t
5 pulses of PGFza in a 24
hour period are required
to cause luteolysis and a
dramatic drop in P4.
Episod ic secretion of
PGF 2a remains high
for about 2 days after
luteolysis.
P4 Luteolysis
350 8 ::::-
::::- 300
E -b(l c. 250 -I:
I
200 u.
C)
a.
‘”0 ISO 0
..2
1:0
7 E b:o c -6 Cl)
l
c
5 0 ,_ Cl)
4 Ill Cl)
b(l
0
3
,_
c.
‘”0
2 0 0
100 1:0
50
13 14 IS 16 17 18 19 20
Day of estrous cycle
A critical number of PGF 2a pulses within a given time-
span are required to induce complete luteolysis. The
exact number of pulses required has not been defined
for all species. However, based on data from the ewe,
about five pulses in a 24 hour period are required to
induce complete luteolysis. Pulsatile release ofPGF2a is
apparently not required under conditions of exogenous
PGF2a administration. For example, one injection of
PGF2a is sufficient to cause luteolysis.
The exact stimulus that initiates PGF2a secre-
tion is controversial. One school of thought maintains
that the uterus must be exposed to elevated progester-
one for a period of days before it can synthesize and
secrete PGF2a in sufficient quantities to cause luteolysis.
During the first half of the estrous cycle, progesterone
prevents secretion ofPGF2a by blocking the fonnation
of oxytocin receptors in the uterus . After I 0 to 12 days
progesterone loses its ability to block formation of
oxytocin receptors, although it is not known how this
occurs. During the late luteal phase injections of exog-
enous oxytocin cause secretion of PGF2a by the uterus.
Injections ofPGF2a during the late luteal phase lead to
a rapid release of ovarian oxytocin. Thus, oxytocin
and PGF2a stimulate each other in a positive feedback
manner. In the ewe, oxytocin episodes precede PGF2a
episodes.
It should be emphasized that our understand-
ing of the precise luteolytic mechanis m is not com-
plete. Progesterone is believed to play a maj or role in
regulating the timing of PGF 2a secretion . For example,
as progesterone increases during the luteal phase ,
progesterone receptors decrease in the endometrium.
The decrease in progesterone receptor numbers in
the endometrium is fo llowed by episodes of PGF2a
secretion by the endometrium later in the cycle. The
exact interaction between progesterone concentrations,
progesterone receptors, oxytocin secretion and PGFza
secretion n eeds further clarification.
The Luteal Phase 195
Figure 9-12. Proposed Steps Resulting in the Loss of Progesterone
Secretion from Luteal Cells
/
PGF2a binds to specific
receptors on the plasma
membrane of the luteal
cells.
The PGF2a recept or
complex is believed to
open ca++ cha nn els so
that Ca++ influ x occurs.
High intracell ular Ca++
is thought to ca use
apoptotic effects.
Luteoly sis results in:
• :e
… .
• cessation of progesterone seaetion
• structural regression to form a c01pus
albicans
• removal of negative feedback by pro-
gesterone upon GnRH secretion
resulting in a new follicular phase
The intracellular mechanis ms that cause
luteolysis have been the subject of intense resear ch
during the last 20 years. One of the original theories
to explain the demise of the corpus luteum was that
PGF2a caused reduction in blood flow to the corpus
luteum by causing vasoconstriction (contraction) of
arterioles supplying the luteal tissue. While blood flow
to the corpus luteum does decrease during luteolysis,
blood flow to the corpus luteum is still 5 to 20 times
greater than to the sun ounding ovarian tissue. Thus,
ischemia (reduced blood flo w) as a primary mode for
luteolysis seems unlikely. It is known that capillaries
in the corpus luteum undergo degeneration during
luteo lysis. It is possible that this cap illary degenera-
tion is more responsible for reducing blood flow than
vasoconstriction associated w ith PGF2u . Neverthe-
less, a degree of circulatory disruption is associated
Luteal Cell
T h e PGF2a rece pto r
comp lex also activates
protein kin ase-C (PK-C)
wh ich inh ib its proges-
terone synthesis.
with the luteo lytic process. However, it is unlikely that
disruption to the luteal vasculature can totally account
for luteolys is.
A second line of thinking is the theory that PGF2a
binds to specific receptors on large luteal cells a nd trig-
gers a cascade of events resulting in the death of these
cells and thus , cessation of steroidogenesis. These events
are presented in Figure 9-1 2 .
The Immune System May Be Involved
in Regression of the Corpus Luteum
It is well -known that immune cells are present
in the corpus luteum at the time of luteo lysis . These
cells are capable of performing phagocytosis of lutea l
cells. P hagocytic cells increase prior to the onset of lu-
teo lysis . Lymphocytes secrete cytoldnes. C ytokines are
non-antibody proteins secre ted by a variety of immune
cells that activate macrophages that then phagocytize
damaged dead luteal ce lls and cellular debris . Examples
of cytokines are interferons , interleukins and tu mor
necrosis factors (TNF). Cytokines have been shown to
cause luteal cell death in vitro. They a lso inhibit proges-
terone synthes is by luteal cells. While the mechanism
involving the role s of cytokines in luteolysis is far from
clear, it appears that normal morphologic and fu ncti onal
integrity of the corpus luteum can be reduced when
cytokines are present.
V
et
B
oo
ks
.ir
I
I
194 The Luteal Phase
Figure 9-11. Changes in PGF Metabolite (PGF-M) During
Late Diestrus and Proestrus
PGF-M (brown line) is
an accurate estimate
of PGF2a· As the graph
shows, PGF2a is low.
The amplitude and fre-
quency of episodes of
PGF2a secretion increase
at about day 16. Abou t
5 pulses of PGFza in a 24
hour period are required
to cause luteolysis and a
dramatic drop in P4.
Episod ic secretion of
PGF 2a remains high
for about 2 days after
luteolysis.
P4 Luteolysis
350 8 ::::-
::::- 300
E -b(l c. 250 -I:
I
200 u.
C)
a.
‘”0 ISO 0
..2
1:0
7 E b:o c -6 Cl)
l
c
5 0 ,_ Cl)
4 Ill Cl)
b(l
0
3
,_
c.
‘”0
2 0 0
100 1:0
50
13 14 IS 16 17 18 19 20
Day of estrous cycle
A critical number of PGF 2a pulses within a given time-
span are required to induce complete luteolysis. The
exact number of pulses required has not been defined
for all species. However, based on data from the ewe,
about five pulses in a 24 hour period are required to
induce complete luteolysis. Pulsatile release ofPGF2a is
apparently not required under conditions of exogenous
PGF2a administration. For example, one injection of
PGF2a is sufficient to cause luteolysis.
The exact stimulus that initiates PGF2a secre-
tion is controversial. One school of thought maintains
that the uterus must be exposed to elevated progester-
one for a period of days before it can synthesize and
secrete PGF2a in sufficient quantities to cause luteolysis.
During the first half of the estrous cycle, progesterone
prevents secretion ofPGF2a by blocking the fonnation
of oxytocin receptors in the uterus . After I 0 to 12 days
progesterone loses its ability to block formation of
oxytocin receptors, although it is not known how this
occurs. During the late luteal phase injections of exog-
enous oxytocin cause secretion of PGF2a by the uterus.
Injections ofPGF2a during the late luteal phase lead to
a rapid release of ovarian oxytocin. Thus, oxytocin
and PGF2a stimulate each other in a positive feedback
manner. In the ewe, oxytocin episodes precede PGF2a
episodes.
It should be emphasized that our understand-
ing of the precise luteolytic mechanis m is not com-
plete. Progesterone is believed to play a maj or role in
regulating the timing of PGF 2a secretion . For example,
as progesterone increases during the luteal phase ,
progesterone receptors decrease in the endometrium.
The decrease in progesterone receptor numbers in
the endometrium is fo llowed by episodes of PGF2a
secretion by the endometrium later in the cycle. The
exact interaction between progesterone concentrations,
progesterone receptors, oxytocin secretion and PGFza
secretion n eeds further clarification.
The Luteal Phase 195
Figure 9-12. Proposed Steps Resulting in the Loss of Progesterone
Secretion from Luteal Cells
/
PGF2a binds to specific
receptors on the plasma
membrane of the luteal
cells.
The PGF2a recept or
complex is believed to
open ca++ cha nn els so
that Ca++ influ x occurs.
High intracell ular Ca++
is thought to ca use
apoptotic effects.
Luteoly sis results in:
• :e
… .
• cessation of progesterone seaetion
• structural regression to form a c01pus
albicans
• removal of negative feedback by pro-
gesterone upon GnRH secretion
resulting in a new follicular phase
The intracellular mechanis ms that cause
luteolysis have been the subject of intense resear ch
during the last 20 years. One of the original theories
to explain the demise of the corpus luteum was that
PGF2a caused reduction in blood flow to the corpus
luteum by causing vasoconstriction (contraction) of
arterioles supplying the luteal tissue. While blood flow
to the corpus luteum does decrease during luteolysis,
blood flow to the corpus luteum is still 5 to 20 times
greater than to the sun ounding ovarian tissue. Thus,
ischemia (reduced blood flo w) as a primary mode for
luteolysis seems unlikely. It is known that capillaries
in the corpus luteum undergo degeneration during
luteo lysis. It is possible that this cap illary degenera-
tion is more responsible for reducing blood flow than
vasoconstriction associated w ith PGF2u . Neverthe-
less, a degree of circulatory disruption is associated
Luteal Cell
T h e PGF2a rece pto r
comp lex also activates
protein kin ase-C (PK-C)
wh ich inh ib its proges-
terone synthesis.
with the luteo lytic process. However, it is unlikely that
disruption to the luteal vasculature can totally account
for luteolys is.
A second line of thinking is the theory that PGF2a
binds to specific receptors on large luteal cells a nd trig-
gers a cascade of events resulting in the death of these
cells and thus , cessation of steroidogenesis. These events
are presented in Figure 9-1 2 .
The Immune System May Be Involved
in Regression of the Corpus Luteum
It is well -known that immune cells are present
in the corpus luteum at the time of luteo lysis . These
cells are capable of performing phagocytosis of lutea l
cells. P hagocytic cells increase prior to the onset of lu-
teo lysis . Lymphocytes secrete cytoldnes. C ytokines are
non-antibody proteins secre ted by a variety of immune
cells that activate macrophages that then phagocytize
damaged dead luteal ce lls and cellular debris . Examples
of cytokines are interferons , interleukins and tu mor
necrosis factors (TNF). Cytokines have been shown to
cause luteal cell death in vitro. They a lso inhibit proges-
terone synthes is by luteal cells. While the mechanism
involving the role s of cytokines in luteolysis is far from
clear, it appears that normal morphologic and fu ncti onal
integrity of the corpus luteum can be reduced when
cytokines are present.
V
et
B
oo
ks
.ir
I
196 The Luteal Phase
In addition to a direct effect on the luteal cell,
cytokines may serve as triggering agents for a process
called apoptosis. Apoptosis (pronounced “a-pa-toe-
sis”) is a phenomenon known as ” programmed cell
death” . It is quite nonnal for cells throughout the body
to die on a daily basis. Cell death occurs by one of two
processes. The first, cell necrosis, is brought about
by pathologic damage. The second type of cell death,
apoptosis, is an ordered biochemical process. This
process involves distinct biochemical and morphologic
changes in the cell. The process of apoptosis is probably
the final step resulting in the death of the luteal cell.
Final destruction and ” clean-up” of the non-functional
luteal cells is probably perfom1ed by macrophages that
phagocytize damaged luteal cells. Over time the luteal
cells disappear completely, leaving only connective tis-
sue behind. Thus, the scar-like corpus albicans (white
body) is formed.
Luteolysis in Women is an Intra-Ovarian
Event. The Uterus is Not Required.
Uterectomy in the woman does not influence
ovarian cyclicity. In other words, the normal pattern
of folliculogenesis , luteal development and Juteolysis
occurs in a rhythmic fashion about every 28 days after
the removal of the uterus. A proposed mechanism for
luteolysis in primates is presented in Figure 9-13. Even
though traces of luteal oxytocin have been identified,
it is thought that oxytocin from the posterior pituitary
acts on ovarian oxytocin receptors to generate small
amounts of intraovarian PGF2u. Luteolysis is thought
to be a local effect and therefore only small amounts of
PGF2u are required to lyse the CL. As a result of oxytocin
receptors binding oxytocin, the synthetic pathway for
PGF2uis activated and this causes luteolysis. Luteolysis
therefore causes a marked reduction in progesterone
that is thought to cause endometrial synthesis ofPGF2a·
Endometrial PGF2u is important because it causes local
vasoconstriction ofthe endometrial arterioles and initi-
ates menstruation. This significant reduction in blood
flow brought about by vasoconstriction in the luminal
region of the endometrium causes necrosis and slough-
ing of the endometrial tissue. A more detailed descrip-
tion of the mechanism of menstruation is presented in
Chapter 16.
Administration of Progesterone Results in
Manipulation of the Estrous Cycle
Now that you understand the mechanisms that
control progesterone synthesis, secretion and luteolysis,
an understanding of how progesterone is used to control/
manipulate cyclicity will provide you with practical
knowledge that is based on the physiologic principles.
Figure 9-13. Proposed Mechanism
of Luteolysis in Primates
??
Hypothalamus
Endometrial synthesis of PGF2u
(See Ch apt er 16)
Vasoconstriction of e ndometrial arterioles
(See Chapt er 16)
Endometrial necrosis and sloughing (menses)
(See Chapte r 16)
A s you know, progesterone provides negative
feedback to the hypothalamus to suppress GnRH . This
fact has been appl ied to the developme nt of many ap-
plications designed to manipulate the repr oductive cycles
in domestic animals. The administration of progesterone
serves as an “artificial corpus luteum”.
Exogenous progesterone suppresses estrus and
ovulation. H owever, when the exogenous progesterone
is removed or w ithdrawn, the animal will enter proestrus
and estrus within two to t hree days after progesterone
removal. This approach enables estrus to be synchronized
in large groups offemales so that artific ial insemination
can be accomplished within a few days. This appl ica-
tion is intended to increase the conven ience of artificial
insemination programs and to f aci litate ferti lity (higher
pregnancy rates). In contrast, the use of exogenou s pro-
gesterone in women is intended to b lock ovulation and
minimize the likelihood of pregnancy. Mechanisms o f
this application are presented in Chapter 16.
Intravaginal Progesterone is Effective
at Synchronizing Estrus in Cattle
The EAZI-BREED™ CIDR® C attle Insert is
an intravaginal progesterone-releasing device used for
of estrus in beef cattle and dairy heifers.
CIDR is an acronym for ” Controlled Internal Drug Re-
lease” . T he product has also been approved for advanc-
ing first estrus in anestr us postp artum beef cows and in
prepubertal beef heifers. The CIDR® is inserted into the
vag ina of the cow/heifer and remains there for 7 days.
While in the vagina, progesterone diffuses out
of the CIDR® Insert, crosses the vagina l mucosa and
enters the vasculature of the vagina . The blood profiles
of progesterone in ovar iectom ized cows immediate ly
follow ing insertion, during a 7 day administration and
immediately after CID R® Insert removal are shown in
Figure 9- 14.
For synchronization of estrus the CIDR® Insert
is administered for 7 days w ith an inj ectio n of 5ml
Lutalyse® Sterile Solution (25mg prostag landin F 2 u) on
the sixth day. Progesterone from the CIDR® Insert sup-
presses GnRH release, gonadotropin r elease, fo llicular
development and ovulation in those cows and heifers that
have corpora lutea that regress spon taneously during the
7 day adminis tration period. L utalyse® is adm inistered
to initiate luteal regression in those cows and he ifers
that have a functional corpus luteum at the end of the
CIDR® Insert administration period. Upon removal of
the CIDR® Insert and injection of Lutalyse®, cows and
heifers w ill experience a rapid decline in the concen-
tration of progesterone followed by elevated GnRH ,
elevated gonadotropins and folli cular deve lopment and
will enter proestrus and estrus with in t\vo to three days
(a synchronized estrus).
The Luteal Phase 197
Figure 9-14. Blood
Progesterone Profiles After the
CIDR® Insertion and Remova l
I Plasma Progesterone Absorption
1 Profile Following CIDR® Insertion ,
, into Cows (n = 8) ‘
t
0.00 1.00 2.00 3.00
t Time after insertion (hr)
C ID R® inserted
Bars = Standard error of t he mean
I – —
: Plasma Progesterone Absorption
4.00
Profile for the Entire Admin istration
‘— __ in Cows (n=8)
CII C’
5
Re moval of 5.€
‘;; !!- ! -…….__ C ID R®
4 • I
… 3 -!-!- t – i t
1 –f
21 •
., u I I i
£s o+ I I I tt. T
0 2 4 6 8
t Time (days)
CID R® inserted
Bars = Standard error of the mean
‘ –
Plasma Progesterone Clearance
i Profile Following Removal of CIDR®
I_ from Cows (n=8)
CIIC’
3.0
2.5
2.0 .
1.5
c…… •
ns ‘l: 1.0 ‘\
0.5 ns c – ………_
ii: 8 0.0 • •
0 5 10 IS 20 25
t Time after removal (hr)
C ID R® removed
Bars = Stand ard error o f the mean
V
et
B
oo
ks
.ir
I
196 The Luteal Phase
In addition to a direct effect on the luteal cell,
cytokines may serve as triggering agents for a process
called apoptosis. Apoptosis (pronounced “a-pa-toe-
sis”) is a phenomenon known as ” programmed cell
death” . It is quite nonnal for cells throughout the body
to die on a daily basis. Cell death occurs by one of two
processes. The first, cell necrosis, is brought about
by pathologic damage. The second type of cell death,
apoptosis, is an ordered biochemical process. This
process involves distinct biochemical and morphologic
changes in the cell. The process of apoptosis is probably
the final step resulting in the death of the luteal cell.
Final destruction and ” clean-up” of the non-functional
luteal cells is probably perfom1ed by macrophages that
phagocytize damaged luteal cells. Over time the luteal
cells disappear completely, leaving only connective tis-
sue behind. Thus, the scar-like corpus albicans (white
body) is formed.
Luteolysis in Women is an Intra-Ovarian
Event. The Uterus is Not Required.
Uterectomy in the woman does not influence
ovarian cyclicity. In other words, the normal pattern
of folliculogenesis , luteal development and Juteolysis
occurs in a rhythmic fashion about every 28 days after
the removal of the uterus. A proposed mechanism for
luteolysis in primates is presented in Figure 9-13. Even
though traces of luteal oxytocin have been identified,
it is thought that oxytocin from the posterior pituitary
acts on ovarian oxytocin receptors to generate small
amounts of intraovarian PGF2u. Luteolysis is thought
to be a local effect and therefore only small amounts of
PGF2u are required to lyse the CL. As a result of oxytocin
receptors binding oxytocin, the synthetic pathway for
PGF2uis activated and this causes luteolysis. Luteolysis
therefore causes a marked reduction in progesterone
that is thought to cause endometrial synthesis ofPGF2a·
Endometrial PGF2u is important because it causes local
vasoconstriction ofthe endometrial arterioles and initi-
ates menstruation. This significant reduction in blood
flow brought about by vasoconstriction in the luminal
region of the endometrium causes necrosis and slough-
ing of the endometrial tissue. A more detailed descrip-
tion of the mechanism of menstruation is presented in
Chapter 16.
Administration of Progesterone Results in
Manipulation of the Estrous Cycle
Now that you understand the mechanisms that
control progesterone synthesis, secretion and luteolysis,
an understanding of how progesterone is used to control/
manipulate cyclicity will provide you with practical
knowledge that is based on the physiologic principles.
Figure 9-13. Proposed Mechanism
of Luteolysis in Primates
??
Hypothalamus
Endometrial synthesis of PGF2u
(See Ch apt er 16)
Vasoconstriction of e ndometrial arterioles
(See Chapt er 16)
Endometrial necrosis and sloughing (menses)
(See Chapte r 16)
A s you know, progesterone provides negative
feedback to the hypothalamus to suppress GnRH . This
fact has been appl ied to the developme nt of many ap-
plications designed to manipulate the repr oductive cycles
in domestic animals. The administration of progesterone
serves as an “artificial corpus luteum”.
Exogenous progesterone suppresses estrus and
ovulation. H owever, when the exogenous progesterone
is removed or w ithdrawn, the animal will enter proestrus
and estrus within two to t hree days after progesterone
removal. This approach enables estrus to be synchronized
in large groups offemales so that artific ial insemination
can be accomplished within a few days. This appl ica-
tion is intended to increase the conven ience of artificial
insemination programs and to f aci litate ferti lity (higher
pregnancy rates). In contrast, the use of exogenou s pro-
gesterone in women is intended to b lock ovulation and
minimize the likelihood of pregnancy. Mechanisms o f
this application are presented in Chapter 16.
Intravaginal Progesterone is Effective
at Synchronizing Estrus in Cattle
The EAZI-BREED™ CIDR® C attle Insert is
an intravaginal progesterone-releasing device used for
of estrus in beef cattle and dairy heifers.
CIDR is an acronym for ” Controlled Internal Drug Re-
lease” . T he product has also been approved for advanc-
ing first estrus in anestr us postp artum beef cows and in
prepubertal beef heifers. The CIDR® is inserted into the
vag ina of the cow/heifer and remains there for 7 days.
While in the vagina, progesterone diffuses out
of the CIDR® Insert, crosses the vagina l mucosa and
enters the vasculature of the vagina . The blood profiles
of progesterone in ovar iectom ized cows immediate ly
follow ing insertion, during a 7 day administration and
immediately after CID R® Insert removal are shown in
Figure 9- 14.
For synchronization of estrus the CIDR® Insert
is administered for 7 days w ith an inj ectio n of 5ml
Lutalyse® Sterile Solution (25mg prostag landin F 2 u) on
the sixth day. Progesterone from the CIDR® Insert sup-
presses GnRH release, gonadotropin r elease, fo llicular
development and ovulation in those cows and heifers that
have corpora lutea that regress spon taneously during the
7 day adminis tration period. L utalyse® is adm inistered
to initiate luteal regression in those cows and he ifers
that have a functional corpus luteum at the end of the
CIDR® Insert administration period. Upon removal of
the CIDR® Insert and injection of Lutalyse®, cows and
heifers w ill experience a rapid decline in the concen-
tration of progesterone followed by elevated GnRH ,
elevated gonadotropins and folli cular deve lopment and
will enter proestrus and estrus with in t\vo to three days
(a synchronized estrus).
The Luteal Phase 197
Figure 9-14. Blood
Progesterone Profiles After the
CIDR® Insertion and Remova l
I Plasma Progesterone Absorption
1 Profile Following CIDR® Insertion ,
, into Cows (n = 8) ‘
t
0.00 1.00 2.00 3.00
t Time after insertion (hr)
C ID R® inserted
Bars = Standard error of t he mean
I – —
: Plasma Progesterone Absorption
4.00
Profile for the Entire Admin istration
‘— __ in Cows (n=8)
CII C’
5
Re moval of 5.€
‘;; !!- ! -…….__ C ID R®
4 • I
… 3 -!-!- t – i t
1 –f
21 •
., u I I i
£s o+ I I I tt. T
0 2 4 6 8
t Time (days)
CID R® inserted
Bars = Standard error of the mean
‘ –
Plasma Progesterone Clearance
i Profile Following Removal of CIDR®
I_ from Cows (n=8)
CIIC’
3.0
2.5
2.0 .
1.5
c…… •
ns ‘l: 1.0 ‘\
0.5 ns c – ………_
ii: 8 0.0 • •
0 5 10 IS 20 25
t Time after removal (hr)
C ID R® removed
Bars = Stand ard error o f the mean
V
et
B
oo
ks
.ir
I
198 The Luteal Phase
Another use of an exogenous progesterone-
like compound is in mares. A material with the trade
name Regu-Mate® is used to control cyclicity. The ac-
tive ingredient in Regu-Mate® is a synthetic progestin
called altrenogest. The physiologic action of altrenogest
is the same as progesterone. It is used in mares for the
following reasons: I) to induce regular cyclicity in
mares making the transition from winter anestrus to
the breeding season, 2) to suppress undesired estrous
behavior and 3) allow for scheduled breeding during
the breeding season .
Altrenogest is administered by placing the
appropriate dose on the posterio-dorsal smface of the
mare’s tongue or is applied to the grain ration. It is
given daily for 15 consecutive days. During the time
that altrenogest is being administered Gn.RH is sup-
pressed, and behavioral estrus does not occur. After
cessation of the treatment, mares will display estrus
four to five days later.
Exogenous Prostaglandin F za is a Potent
Luteolysin and Can Synchronize Estrus
Following the discovery that PGF2o. was the
luteolysin, a major research emphasis was placed on us-
ing this hormone to shorten the estrous cycle and induce
estr us in cattle. Injections ofPGF2u between day seven
and day 18 will cause the cow to begin to show estrus
in about three days (60-80 hours after the injection).
Figure 9-15 illustrates the effect of prostaglandin for
inducing estrus. It must be emphasized that the corpus
luteum ofthe cow is not sensitive to PGFza between days
one and six of the cycle. In other words, injecting the
cow with PGF2a during this time will not have an effect
(See Figure 9-15).
Reproductive physiologists at the University of
Wisconsin and Michigan State University have devel-
oped an innovative use of GnRH and PGF2a that syn-
chronizes ovulation. This protocol is named Ovsynch
(See Figure 9- I 6, green section). When Gn.RH and
PGF2a are used together in the proper timed-sequence,
visual detection of estrus can be eliminated and timed
artificial insemination (TAI) can be performed. This
program is being used routinely as a reproductive man-
agement tool in the dairy industry. The Ovsynch innova-
tion incorporates the mechanisms of follicular dynamics
described in Chapter 8 and the mechanisms ofluteolysis
(described earlier in this chapter) into a practical appli-
cation of physiologic principles. A solid understanding
of these mechanisms will translate into understanding
of the Ovsynch protocol described later.
The basic strategy for the Ovsynch program
is presented in the steps that follow. Step 1- GnRH is
injected into cows that are eligible to be inseminated
(fully recovered from their last parturition). The GnRH
injection causes one of two events to take place. First,
if there is a dominant follicle on the ovary (a fol-
licle that is greater than I Omm and has an adequate
population of LH receptors) the cow will ovulate in
response to GnRH. A CL will then form. Second,
if the cow does not have a dominant follicle (an im-
mature follicle that has few LH receptors), GnRH will
promote continued follicular growth. In this case,
there is a CL present from the previous ovulation;
Step 2- An injection ofPGFza seven days after GnRH
causes luteolysis and the cow will enter the follicular
phase; Step 3- A second injection of GniUJ 48 hours
later causes the cow to ovulate. Step 4- The cow can
then be inseminated without detection of estrus I 6 hours
after the second Gn.RH injection.
This strategy, when properly applied in
commercial dairy herds has resulted in acceptable
conception rates without detection of estrus in lactat-
ing dairy cows. The Ovsynch strategy will enable
almost l 00% of the cows to be inseminated after the
designated postpartum waiting p eriod (typically 60
days and called the “voluntary wait period”). The first
GnRH injection in the Ovsynch program is given at
random (without knowledge of the specific day of
the cycle). This can result in several problems. If
cows are not cyclic, GnRH will not initiate cyclic-
ity in all of them. Those that do ovulate in response
to GnRH have reduced conception. Some GnRH-
treated cows will recruit follicles from the second or
third follicular wave and the follicle may not ovulate.
Therefore, the PGFza injection is not totally effec-
tive (because there is no CL present) in these cows.
In order to help minimize the above problems,
a strategy has been developed that is called Presynch
(See Figure 9-16, brown section). The Presynch pro-
gram begins 26 days prior to the first GnRH injection.
At random, all cows are given PGF20 . Fourteen days
later a second PGF2a injection is given. Remember, the
first PGF2o. will regress an existing corpus luteum if it
is between days 7 and 17. Obviously, all cows will not
fall into this range and the second PGF2o. regresses all
corpora lutea that are present because they are in the
” sensitive window” between days 7 and I 7. Twelve
days after the prostaglandin injection, GnRH is injected.
GnRH may cause a new follicle to ovulate, forming a
new CL as per the original Ovsynch protocol. More
detail about each method can be obtained from the Key
References section at the end of the chapter.
The Luteal Phase 199
Figure 9-15. Influence of Prostaglandin F2a Upon Cycle Length in the Cow
…….. 10
‘E
8 .._,
] 6
0 0 ma
t: 4
Qj
llO e 2
c.
0
Estrus
Normal Cycle – Estrus Every 21 Days
5 10 15
Day of cycle (cow)
21
Estrus
PGFza Injections – Day 0 to Day 6 – No Effect
…….. 10
‘E
8 …….
] 6
0 0
t: 4
Qj
llO e 2
c.
0
Estrus
…….. 10
‘E
8 …….
-g 6
0 0
iiit
t: 4
Qj
llO e 2
c.
0
Estrus
5 10 IS
Day of cycle (cow)
‘
5 7 10 IS
Day of cycle (cow)
\
‘ ‘ I
•
21
Estrus
21
Estrus
In the normal cyclic cow estrus and ovula-
tion occurs every 21 days. Luteolysis (in-
duced naturally by PGF2a from the uterus)
causes the animal to enter a new follicular
phase and subsequent estrus.
If a single injection of PGF2a is given be-
tween day zero and about day six, luteoly-
sis will not occur and the cycle will be of
normal length. This is because the corpus
luteum must reach a certain stage of devel-
opment before it is sensitive to PGF2a.
If PGF2a is injected on day 7-17, luteolysis
will occur. Progesterone will drop and the
animal will come into estrus in about three
days after the injection. Such a strategy is
used to synchronize estrus in large groups
of animals .
V
et
B
oo
ks
.ir
I
198 The Luteal Phase
Another use of an exogenous progesterone-
like compound is in mares. A material with the trade
name Regu-Mate® is used to control cyclicity. The ac-
tive ingredient in Regu-Mate® is a synthetic progestin
called altrenogest. The physiologic action of altrenogest
is the same as progesterone. It is used in mares for the
following reasons: I) to induce regular cyclicity in
mares making the transition from winter anestrus to
the breeding season, 2) to suppress undesired estrous
behavior and 3) allow for scheduled breeding during
the breeding season .
Altrenogest is administered by placing the
appropriate dose on the posterio-dorsal smface of the
mare’s tongue or is applied to the grain ration. It is
given daily for 15 consecutive days. During the time
that altrenogest is being administered Gn.RH is sup-
pressed, and behavioral estrus does not occur. After
cessation of the treatment, mares will display estrus
four to five days later.
Exogenous Prostaglandin F za is a Potent
Luteolysin and Can Synchronize Estrus
Following the discovery that PGF2o. was the
luteolysin, a major research emphasis was placed on us-
ing this hormone to shorten the estrous cycle and induce
estr us in cattle. Injections ofPGF2u between day seven
and day 18 will cause the cow to begin to show estrus
in about three days (60-80 hours after the injection).
Figure 9-15 illustrates the effect of prostaglandin for
inducing estrus. It must be emphasized that the corpus
luteum ofthe cow is not sensitive to PGFza between days
one and six of the cycle. In other words, injecting the
cow with PGF2a during this time will not have an effect
(See Figure 9-15).
Reproductive physiologists at the University of
Wisconsin and Michigan State University have devel-
oped an innovative use of GnRH and PGF2a that syn-
chronizes ovulation. This protocol is named Ovsynch
(See Figure 9- I 6, green section). When Gn.RH and
PGF2a are used together in the proper timed-sequence,
visual detection of estrus can be eliminated and timed
artificial insemination (TAI) can be performed. This
program is being used routinely as a reproductive man-
agement tool in the dairy industry. The Ovsynch innova-
tion incorporates the mechanisms of follicular dynamics
described in Chapter 8 and the mechanisms ofluteolysis
(described earlier in this chapter) into a practical appli-
cation of physiologic principles. A solid understanding
of these mechanisms will translate into understanding
of the Ovsynch protocol described later.
The basic strategy for the Ovsynch program
is presented in the steps that follow. Step 1- GnRH is
injected into cows that are eligible to be inseminated
(fully recovered from their last parturition). The GnRH
injection causes one of two events to take place. First,
if there is a dominant follicle on the ovary (a fol-
licle that is greater than I Omm and has an adequate
population of LH receptors) the cow will ovulate in
response to GnRH. A CL will then form. Second,
if the cow does not have a dominant follicle (an im-
mature follicle that has few LH receptors), GnRH will
promote continued follicular growth. In this case,
there is a CL present from the previous ovulation;
Step 2- An injection ofPGFza seven days after GnRH
causes luteolysis and the cow will enter the follicular
phase; Step 3- A second injection of GniUJ 48 hours
later causes the cow to ovulate. Step 4- The cow can
then be inseminated without detection of estrus I 6 hours
after the second Gn.RH injection.
This strategy, when properly applied in
commercial dairy herds has resulted in acceptable
conception rates without detection of estrus in lactat-
ing dairy cows. The Ovsynch strategy will enable
almost l 00% of the cows to be inseminated after the
designated postpartum waiting p eriod (typically 60
days and called the “voluntary wait period”). The first
GnRH injection in the Ovsynch program is given at
random (without knowledge of the specific day of
the cycle). This can result in several problems. If
cows are not cyclic, GnRH will not initiate cyclic-
ity in all of them. Those that do ovulate in response
to GnRH have reduced conception. Some GnRH-
treated cows will recruit follicles from the second or
third follicular wave and the follicle may not ovulate.
Therefore, the PGFza injection is not totally effec-
tive (because there is no CL present) in these cows.
In order to help minimize the above problems,
a strategy has been developed that is called Presynch
(See Figure 9-16, brown section). The Presynch pro-
gram begins 26 days prior to the first GnRH injection.
At random, all cows are given PGF20 . Fourteen days
later a second PGF2a injection is given. Remember, the
first PGF2o. will regress an existing corpus luteum if it
is between days 7 and 17. Obviously, all cows will not
fall into this range and the second PGF2o. regresses all
corpora lutea that are present because they are in the
” sensitive window” between days 7 and I 7. Twelve
days after the prostaglandin injection, GnRH is injected.
GnRH may cause a new follicle to ovulate, forming a
new CL as per the original Ovsynch protocol. More
detail about each method can be obtained from the Key
References section at the end of the chapter.
The Luteal Phase 199
Figure 9-15. Influence of Prostaglandin F2a Upon Cycle Length in the Cow
…….. 10
‘E
8 .._,
] 6
0 0 ma
t: 4
Qj
llO e 2
c.
0
Estrus
Normal Cycle – Estrus Every 21 Days
5 10 15
Day of cycle (cow)
21
Estrus
PGFza Injections – Day 0 to Day 6 – No Effect
…….. 10
‘E
8 …….
] 6
0 0
t: 4
Qj
llO e 2
c.
0
Estrus
…….. 10
‘E
8 …….
-g 6
0 0
iiit
t: 4
Qj
llO e 2
c.
0
Estrus
5 10 IS
Day of cycle (cow)
‘
5 7 10 IS
Day of cycle (cow)
\
‘ ‘ I
•
21
Estrus
21
Estrus
In the normal cyclic cow estrus and ovula-
tion occurs every 21 days. Luteolysis (in-
duced naturally by PGF2a from the uterus)
causes the animal to enter a new follicular
phase and subsequent estrus.
If a single injection of PGF2a is given be-
tween day zero and about day six, luteoly-
sis will not occur and the cycle will be of
normal length. This is because the corpus
luteum must reach a certain stage of devel-
opment before it is sensitive to PGF2a.
If PGF2a is injected on day 7-17, luteolysis
will occur. Progesterone will drop and the
animal will come into estrus in about three
days after the injection. Such a strategy is
used to synchronize estrus in large groups
of animals .
V
et
B
oo
ks
.ir
200 The Luteal Phase
I STEP •I
PGF2a
t
STEP
I
2
Figure 9-16. Presynch and Ovsynch as Methods to
Synchronize Ovulation in Cows
14d
ACTION
PGF2a
PGF2a
I STEP 21 I STEP •I
Gn RH PGF2a
t 12d t
Presynch
cow
WHEN REASON RESPONSE
Regress existing CL Cow ovula tes
Anytime and induce new and produces
ovulation “new” CL
14 days Regress “ne w” CL. New follicular
after I st In cows not phase ( + P., )
PGF2a responding to
Jst PGF2a
injection – regress
“old” CL.
STEP ACTION WHEN REASON
Situation A
To cause ovulat ion
in existi ng d ominant
follicle (greater t h an
IOmm a nd t hat have
LH receptors)
12 days
Situation B I Gn RH after last
PGF2a To cause continued growth o f exis ting
immature follicles
(few LH rece ptors).
A CL from the
current cycle is
present at the same
time (“old” C L).
Situation A
To regress “new· CL
(resul t! ng fro m
previo us Gn RH
7 days injection
2 PGF2a after last Situation B
GnRH To regress either “old ‘
CL or both the “ol d”
CL and t h e “new” CL
(from Gn RH Injec t ion)
Situation A
2 days To cause ovul ati on of
3 GnRH after last
dominant follicle
PGF2a Situation B
To cause ovulati on
o f dom inant follicle
lnsemi- 16 hours
Sperm in tract
4 nation after before ov ulatio n GnRH fertilization
I STEP 21 I STEP 31
7d
PGF20. l Gn RH
t
All STEP41
cow
RESPONSE
Situation A
Th e LH surge
cau ses ovulation
of the existing
dominant fo llicle,
a ne w C L Is
formed and a
new foll icular
wave starts.
Situation B
Co ntinued fo ll icula r
growth towards
fo lli cular d omina nce
(CL o f the cycle
still present)
Situation A
Co w enters
fo ll icular phase
an d new dominan t
fo llicle devel ops
Situation B
Follicl e from first
GnRH injection
(Step 3) co ntinues
to grow a nd
beco mes d o minant .
The”old” CL
reg resses.
Situat ion A
Ovulation In
24-32 hou rs
Situation B
Ovulation in
24-32 hou rs
30-40%
co nce ption
Further
PHENOMENA
for Fertility
Female elephants have a uniquely long es-
trous cycle (16 weeks) am/ a gestation of 22
months. What does this say about elephant
CL?
The regression of the cmpus luteum in 1m-
mans and other primates is not controlled by
the utems. However, PGF2a will induce lu-
teolysis in primates. It is believed that PGF2a
of ovarian origin is responsible for causing
luteal regression.
The co1pus lutermz of mos t rode1zts (rats,
mice, hamsters and g erbils) does not develop
unless copulation occurs. P enile stimulation
of the cervix causes prolactin release from the
female. Prolactin is luteotropic and causes
the formation of cotpora lutea.
Some spiders have no p enis. They ej ect sperm
from their abdomen onto their web. The male
spider picks up the ejaculate with a special
set of antennae and searches for a 1·eceptive
female who produces a pheromone. Th e male
has to be vety careful and deposit the sem en
by surprise because th e female will eat hi m if
she catches him.
The luteal phase of the estrous cy cle of the
kangaroo is longer than preg nancy.
Researchers at N .C. State University obser ved
a sow that had 128 corpora Iuten on both of
h er ovaries. This is ten times the normal
number of corpora lutea. The caus e of such
a high number of ovulations is unknown.
The Luteal Phase 201
Key References
Leymarie, P. and Marta! , J. 1993 . ” T he corpus
luteum from cycle to gestation” in Re production in
Mammals and Man . p 413 -434. C. T hibaul t, M.C.
Levasseur and R .H.F. Hunter, eds. , E llipses, Paris.
ISBN 2- 7298- 9354-7.
McCracken, J .A. 1998. “Luteolysis” in Encvclopedia
o(Reproduction . Vol. 2 . p1083 – 1094. Knobil, E. and
J. D. Nei ll, eds. Academic Press, San Diego ISBN
0- 12-227022-3.
Niswender, G.D. and T. M. Nett. 1994. “Corpus
luteum and its contro l in infr aprimate species” in
Th e Phvsiolof?Y of Reproduction, 2nd Editi on. Vol.
I p78 1-816. E. Knob il and J .D. Neill, eds. , R aven
Press, Ltd., New York. ISBN 0- 78 17-0086-8.
Pate, J. L. and D .H. Townson. 1994. “Novel local
regulators in luteal regression.” XXI Biennia l Sym-
pos ium on Anima l Reproduction. J. Anim.
Sci. 72 (Suppl. 3):3 1-42.
Pursley, J.R., M.R. Kosorok and M.C. Wi ltbank,
1997. ” Reproductive management of lactating dairy
cows using synchronization of ovulation” in J. Daily
Sci. 80:30 1-306.
Salamonsen, L.A. 2003. ” Tissue inj ury and repa ir
in the fe male human reproductive tract.” Reprod.
125(3):30 1-3 11 .
9
V
et
B
oo
ks
.ir
200 The Luteal Phase
I STEP •I
PGF2a
t
STEP
I
2
Figure 9-16. Presynch and Ovsynch as Methods to
Synchronize Ovulation in Cows
14d
ACTION
PGF2a
PGF2a
I STEP 21 I STEP •I
Gn RH PGF2a
t 12d t
Presynch
cow
WHEN REASON RESPONSE
Regress existing CL Cow ovula tes
Anytime and induce new and produces
ovulation “new” CL
14 days Regress “ne w” CL. New follicular
after I st In cows not phase ( + P., )
PGF2a responding to
Jst PGF2a
injection – regress
“old” CL.
STEP ACTION WHEN REASON
Situation A
To cause ovulat ion
in existi ng d ominant
follicle (greater t h an
IOmm a nd t hat have
LH receptors)
12 days
Situation B I Gn RH after last
PGF2a To cause continued growth o f exis ting
immature follicles
(few LH rece ptors).
A CL from the
current cycle is
present at the same
time (“old” C L).
Situation A
To regress “new· CL
(resul t! ng fro m
previo us Gn RH
7 days injection
2 PGF2a after last Situation B
GnRH To regress either “old ‘
CL or both the “ol d”
CL and t h e “new” CL
(from Gn RH Injec t ion)
Situation A
2 days To cause ovul ati on of
3 GnRH after last
dominant follicle
PGF2a Situation B
To cause ovulati on
o f dom inant follicle
lnsemi- 16 hours
Sperm in tract
4 nation after before ov ulatio n GnRH fertilization
I STEP 21 I STEP 31
7d
PGF20. l Gn RH
t
All STEP41
cow
RESPONSE
Situation A
Th e LH surge
cau ses ovulation
of the existing
dominant fo llicle,
a ne w C L Is
formed and a
new foll icular
wave starts.
Situation B
Co ntinued fo ll icula r
growth towards
fo lli cular d omina nce
(CL o f the cycle
still present)
Situation A
Co w enters
fo ll icular phase
an d new dominan t
fo llicle devel ops
Situation B
Follicl e from first
GnRH injection
(Step 3) co ntinues
to grow a nd
beco mes d o minant .
The”old” CL
reg resses.
Situat ion A
Ovulation In
24-32 hou rs
Situation B
Ovulation in
24-32 hou rs
30-40%
co nce ption
Further
PHENOMENA
for Fertility
Female elephants have a uniquely long es-
trous cycle (16 weeks) am/ a gestation of 22
months. What does this say about elephant
CL?
The regression of the cmpus luteum in 1m-
mans and other primates is not controlled by
the utems. However, PGF2a will induce lu-
teolysis in primates. It is believed that PGF2a
of ovarian origin is responsible for causing
luteal regression.
The co1pus lutermz of mos t rode1zts (rats,
mice, hamsters and g erbils) does not develop
unless copulation occurs. P enile stimulation
of the cervix causes prolactin release from the
female. Prolactin is luteotropic and causes
the formation of cotpora lutea.
Some spiders have no p enis. They ej ect sperm
from their abdomen onto their web. The male
spider picks up the ejaculate with a special
set of antennae and searches for a 1·eceptive
female who produces a pheromone. Th e male
has to be vety careful and deposit the sem en
by surprise because th e female will eat hi m if
she catches him.
The luteal phase of the estrous cy cle of the
kangaroo is longer than preg nancy.
Researchers at N .C. State University obser ved
a sow that had 128 corpora Iuten on both of
h er ovaries. This is ten times the normal
number of corpora lutea. The caus e of such
a high number of ovulations is unknown.
The Luteal Phase 201
Key References
Leymarie, P. and Marta! , J. 1993 . ” T he corpus
luteum from cycle to gestation” in Re production in
Mammals and Man . p 413 -434. C. T hibaul t, M.C.
Levasseur and R .H.F. Hunter, eds. , E llipses, Paris.
ISBN 2- 7298- 9354-7.
McCracken, J .A. 1998. “Luteolysis” in Encvclopedia
o(Reproduction . Vol. 2 . p1083 – 1094. Knobil, E. and
J. D. Nei ll, eds. Academic Press, San Diego ISBN
0- 12-227022-3.
Niswender, G.D. and T. M. Nett. 1994. “Corpus
luteum and its contro l in infr aprimate species” in
Th e Phvsiolof?Y of Reproduction, 2nd Editi on. Vol.
I p78 1-816. E. Knob il and J .D. Neill, eds. , R aven
Press, Ltd., New York. ISBN 0- 78 17-0086-8.
Pate, J. L. and D .H. Townson. 1994. “Novel local
regulators in luteal regression.” XXI Biennia l Sym-
pos ium on Anima l Reproduction. J. Anim.
Sci. 72 (Suppl. 3):3 1-42.
Pursley, J.R., M.R. Kosorok and M.C. Wi ltbank,
1997. ” Reproductive management of lactating dairy
cows using synchronization of ovulation” in J. Daily
Sci. 80:30 1-306.
Salamonsen, L.A. 2003. ” Tissue inj ury and repa ir
in the fe male human reproductive tract.” Reprod.
125(3):30 1-3 11 .
9
V
et
B
oo
ks
.ir
The Puerperium & Lactation
Parturition
Fetal Attachment & Gestation
Early Embryogenesis &
Maternal Recognition of Pregnancy
Ovulation & Fertilization
Cyclicity
Regulation of
Reproduction
Tract Function
Puberty
Prenatal
Development
… , … ,
\ I
Spermatogenesis
Regulation of
Reproduction
Tract Function
Puberty
Prenatal
Development
Take Home Message
Reproductive behavior is an obligatmy component oftlze reproductive process. It consists
ofprecopulatory, copulatory and postcopulatory stages. In the female, sexual receptivity
occurs only during estrus and is characterized by distinct behavior and mating posture (lm·-
dosis). In the male, reproductive behavior can occur potentially any time. Sexual arousal
in the male involves a cascade of endocrine and neural events that result in erection oftlte
penis, mounting oftlze sexually receptive female, intromission and ejaculation. Erection of
the penis involves specific neural and biochemical events that culminate in penile vasodila-
tion. Ejaculation is a reflex that is initiated by stimulation of the glans penis and concludes
with expulsion ofsemen
Reproductive behavior has evolved as one of
the s trongest drives in the animal kingdom and usually
takes precedence over all other forms of activity such as
eating, resting and s leeping. The purpose of reproduc-
tive behavior is to promote the opporhmi ty fo r copula-
tion and thus increase the probability that the spem1 and
the egg will meet. The ultimate goals of copulation are
pregnancy, successful embryogenesis and parturition.
Reproductive behavior in the male
consists of three distinct stages:
• the precopulatory stage
• the copulatory stage
• the postcopulatory stage
Reproductive behavior in the male can be di-
vided into thr ee distinct stages. These stages are : the
precopulatory stage; the copulatory stage; and the
postcopulatory stage. T he specific events that occur
during each of these stages are presented in F igure 11-1 .
Reproductive behavior in the female
can be considered to serve the following
functions:
• attractivity
• proceptivity
• receptivity
Figure 11-1. Stages of Male
Reproductive Behavior and
Specific Events in Each Stage
Precopulatory Behavior
Search for
sexual partner
•
Courtship –
Sexual arousal –
Erection –
Penile protr usion
• ..
Copulatory Behavior
Mounting
Intromission
Ejaculation
Postcopulatory Behavior
I Dismount I –
I T I Refractory period –
I Memory I
V
et
B
oo
ks
.ir
The Puerperium & Lactation
Parturition
Fetal Attachment & Gestation
Early Embryogenesis &
Maternal Recognition of Pregnancy
Ovulation & Fertilization
Cyclicity
Regulation of
Reproduction
Tract Function
Puberty
Prenatal
Development
… , … ,
\ I
Spermatogenesis
Regulation of
Reproduction
Tract Function
Puberty
Prenatal
Development
Take Home Message
Reproductive behavior is an obligatmy component oftlze reproductive process. It consists
ofprecopulatory, copulatory and postcopulatory stages. In the female, sexual receptivity
occurs only during estrus and is characterized by distinct behavior and mating posture (lm·-
dosis). In the male, reproductive behavior can occur potentially any time. Sexual arousal
in the male involves a cascade of endocrine and neural events that result in erection oftlte
penis, mounting oftlze sexually receptive female, intromission and ejaculation. Erection of
the penis involves specific neural and biochemical events that culminate in penile vasodila-
tion. Ejaculation is a reflex that is initiated by stimulation of the glans penis and concludes
with expulsion ofsemen
Reproductive behavior has evolved as one of
the s trongest drives in the animal kingdom and usually
takes precedence over all other forms of activity such as
eating, resting and s leeping. The purpose of reproduc-
tive behavior is to promote the opporhmi ty fo r copula-
tion and thus increase the probability that the spem1 and
the egg will meet. The ultimate goals of copulation are
pregnancy, successful embryogenesis and parturition.
Reproductive behavior in the male
consists of three distinct stages:
• the precopulatory stage
• the copulatory stage
• the postcopulatory stage
Reproductive behavior in the male can be di-
vided into thr ee distinct stages. These stages are : the
precopulatory stage; the copulatory stage; and the
postcopulatory stage. T he specific events that occur
during each of these stages are presented in F igure 11-1 .
Reproductive behavior in the female
can be considered to serve the following
functions:
• attractivity
• proceptivity
• receptivity
Figure 11-1. Stages of Male
Reproductive Behavior and
Specific Events in Each Stage
Precopulatory Behavior
Search for
sexual partner
•
Courtship –
Sexual arousal –
Erection –
Penile protr usion
• ..
Copulatory Behavior
Mounting
Intromission
Ejaculation
Postcopulatory Behavior
I Dismount I –
I T I Refractory period –
I Memory I
V
et
B
oo
ks
.ir
I
230 Reproductive Behavior
Precopulatory, copulatory and postcopulatory
behaviors in the female can be considered as serving
the functions of: attractivity, proceptivity and receptiv-
ity. Attractivity refers to behaviors and other signals
that serve to attract males. This can include postures,
vocalizations, behaviors and chemical cues such as
pheromones that attract the male to approach and en-
gage in precopulatory behavior. Proceptivity refers to
the behaviors exhibited by females toward males that
stimulate the male to copulate or that reinitiate sexual
behavior after copulation. For example, head butting
of the male and mounting the male are two of the most
common preceptive behaviors exhibited by females.
Proceptivity may also include behaviors among fe-
males, such as female-female mounting that sexually
stimulate males. Finally, r·eceptivity is the copulatory
behavior of females that ensures insemination. This
may include the immobility or standing response (lor-
dosis) as well as tail deviation or backing-up toward
the male.
As you have already learned, sexual activity
of the postpubertal female is confined to estrus (heat).
This short period of sexual receptivity limits the time
during which precopulatory behavior occurs in most
females. In contrast, the male is potentially capable
of initiating reproductive behavior at any time after
puberty. The initiation of courtship-specific behavior
is generally under the influence of the female. She
will send subtle, or sometimes overt signals to the
male (attractivity) to initiate courtship behavior. Fac-
tors such as sexual signaling pheromones, vocaliza-
tion, increased physical activity and subtle postural
changes are signals provided by the female that will
initiate more intense courtship behavior on the part of
the male. In addition, it has been hypothesized that
female-female (proceptivity) interactions such as ho-
mosexual mounting activity among cattle may serve
as signals to initiate male-female courtship behavior.
In general, the postpubertal male is almost constantly
searching for signals sent by the female to indicate that
she is sexually receptive.
Identification of a sexual partner probably
requires mostofthe senses (olfactory, visual, auditory
and tactile). The relative importance of these sensory
stimuli has not been described critically in most spe-
CleS.
Females of almost all species appear to show
a marked increase in general physical activity as
they come into estrus (See Figure 11-2). Elevated
physical activity is generally manifested by increased
locomotion. In addition, milling around, exploration,
increased vocalization and agonistic behavior towards
other females can be observed. In almost all species
studied, including humans, there is a marked increase
1/)
a..
w
I-
I/)
1/)
a..
w
I-
I/)
1/)
a..
w
I-
I/)
1/)
a..
w
I-
I/)
Figure 11-2. Relationship
Between Physical Activity and
Reproductive Cycles in
Various Female Mammals
!cows I
Estrus Estrus
sows
Estrus Estrus
RATS
Estrus Estrus
!woMEN!
Menses Menses
Physical activity increases significantly around
the time of estrus and /or ovulation.
in physical activity that accompanies the time of ovula-
tion. Presumably, this physical activity is associated
with searching for a mate . This increased physical
activity can be measured by equipping females with
pedometers. Pedometers are devices that monitor and
quantitate steps taken by the animal and are currently
used in commercial dairy enterprises for detection of
estrus.
Courtship-specific behavior is initiated after
a sexual partner has been identified.
Once a sexual partner has been identified, a
series of highly specific courtship behav iors begin.
Courtship-specific behaviors include sniffing of the
vulva by the male, urination by the female in the pres-
ence of the male, exhibiting flehmen behavior (See
Figure 11 -5), chin resting, circling and increased pho-
nation. In many species the sense of vision appear s
to be the most important w ith regard to sexual arousal
in the male. This should not be interpreted to mean
that other stimuli, such as auditory or o lfac tmy are not
important.
Copulatmy behavior varies significantly
among species with regard to duration.
Lordosis (mating posture) by the fema le (re-
ceptivity) triggers significant sexual arousal behavior
on the part of the male. Once the male discovers that
the fema le will display lordosis, he becomes sexually
stimulated. It should be emphasized that lordos is is a
highly specific female motor response associated with
the “willingness” to mate.
Sexual arousal is followed by erection
and penile protrusion.
Following expo sure to the appropriate stimuli ,
erection and protmsion of the penis occur. T hese highly
specific m otor events are controlled by the central
nervous system. The mechanisms of peni le protru sion
and erection will be presented later. Typical behavior
dming search, courtship and sexual arousal for domestic
animals is presented in Tabl e 11 -1 .
Reproductive Behavior 231
After significant sexual s timulation, mount-
ing, intromission and ejaculation follow. In genera l,
mammals can be c la ssified as sustained copulators
or short copulators. The bull, ram, buck and tom are
short copulators w hile the boar, dog and camel ids are
sustained copulators. The stallion is intem1edi ate with
regard to duration of copulation.
Mounting behavior generally requires immobi-
lization of the female and elevation ofthe front legs o f
the male to straddle the caudal region of the female ( See
Figure 11-1 0). Intromission is ent rance of the penis
into the vagi na. Ejaculation is expulsion of semen
from the penis into the female reproductive tract.
Copulatory behavior on the par t of the male
is learned. Past sexual experiences are important in
order for the male to develop appropriate reproductive
behavior. For example, negative experiences during
the precopulatory and copulatory stages will generally
result in less enthusiasm on the part of the male. From
a practical standpoint, m anagement of the breeding
male should always be directed towards providing the
ma le w ith totally p ositive sti m ul i. U ti lizing non-estrus
femal es to collect semen fi·om stallions, boars, rams and
bulls should be avo ided because these fe males do not
w illingly stand to be mounted. Inj my to both the female
and the male can occur under these circumstances.
Postcopulatmy behavior is a
period of refractivity.
Postcopulatory behavior involves dism ounting
and a per iod during which either the male, the fem ale
or both will not engage in copulatory behavior. T his
refractory period is a peri od of time during w hich
a sec ond copulation w ill not take pl ace. Memory is
important in both a positive and negative way. Positive
mating experiences promote reproductive behavior and
negative inhibit reproductive behavior. When semen is
collected for artificial insemination, it is important to re-
duce the duration of the refi·actmy period when multiple
ejaculations need to be collected in the shortest possible
time . Techniques to reduce the refractory period will be
presented later in the chapter. Both ma les and females
often display specific postcopulatory behavior such as
vocal emissions, genital groomi ng, changing postural
relationships and various tacti le behaviors , such as
licking and nuzzl ing.
V
et
B
oo
ks
.ir
I
230 Reproductive Behavior
Precopulatory, copulatory and postcopulatory
behaviors in the female can be considered as serving
the functions of: attractivity, proceptivity and receptiv-
ity. Attractivity refers to behaviors and other signals
that serve to attract males. This can include postures,
vocalizations, behaviors and chemical cues such as
pheromones that attract the male to approach and en-
gage in precopulatory behavior. Proceptivity refers to
the behaviors exhibited by females toward males that
stimulate the male to copulate or that reinitiate sexual
behavior after copulation. For example, head butting
of the male and mounting the male are two of the most
common preceptive behaviors exhibited by females.
Proceptivity may also include behaviors among fe-
males, such as female-female mounting that sexually
stimulate males. Finally, r·eceptivity is the copulatory
behavior of females that ensures insemination. This
may include the immobility or standing response (lor-
dosis) as well as tail deviation or backing-up toward
the male.
As you have already learned, sexual activity
of the postpubertal female is confined to estrus (heat).
This short period of sexual receptivity limits the time
during which precopulatory behavior occurs in most
females. In contrast, the male is potentially capable
of initiating reproductive behavior at any time after
puberty. The initiation of courtship-specific behavior
is generally under the influence of the female. She
will send subtle, or sometimes overt signals to the
male (attractivity) to initiate courtship behavior. Fac-
tors such as sexual signaling pheromones, vocaliza-
tion, increased physical activity and subtle postural
changes are signals provided by the female that will
initiate more intense courtship behavior on the part of
the male. In addition, it has been hypothesized that
female-female (proceptivity) interactions such as ho-
mosexual mounting activity among cattle may serve
as signals to initiate male-female courtship behavior.
In general, the postpubertal male is almost constantly
searching for signals sent by the female to indicate that
she is sexually receptive.
Identification of a sexual partner probably
requires mostofthe senses (olfactory, visual, auditory
and tactile). The relative importance of these sensory
stimuli has not been described critically in most spe-
CleS.
Females of almost all species appear to show
a marked increase in general physical activity as
they come into estrus (See Figure 11-2). Elevated
physical activity is generally manifested by increased
locomotion. In addition, milling around, exploration,
increased vocalization and agonistic behavior towards
other females can be observed. In almost all species
studied, including humans, there is a marked increase
1/)
a..
w
I-
I/)
1/)
a..
w
I-
I/)
1/)
a..
w
I-
I/)
1/)
a..
w
I-
I/)
Figure 11-2. Relationship
Between Physical Activity and
Reproductive Cycles in
Various Female Mammals
!cows I
Estrus Estrus
sows
Estrus Estrus
RATS
Estrus Estrus
!woMEN!
Menses Menses
Physical activity increases significantly around
the time of estrus and /or ovulation.
in physical activity that accompanies the time of ovula-
tion. Presumably, this physical activity is associated
with searching for a mate . This increased physical
activity can be measured by equipping females with
pedometers. Pedometers are devices that monitor and
quantitate steps taken by the animal and are currently
used in commercial dairy enterprises for detection of
estrus.
Courtship-specific behavior is initiated after
a sexual partner has been identified.
Once a sexual partner has been identified, a
series of highly specific courtship behav iors begin.
Courtship-specific behaviors include sniffing of the
vulva by the male, urination by the female in the pres-
ence of the male, exhibiting flehmen behavior (See
Figure 11 -5), chin resting, circling and increased pho-
nation. In many species the sense of vision appear s
to be the most important w ith regard to sexual arousal
in the male. This should not be interpreted to mean
that other stimuli, such as auditory or o lfac tmy are not
important.
Copulatmy behavior varies significantly
among species with regard to duration.
Lordosis (mating posture) by the fema le (re-
ceptivity) triggers significant sexual arousal behavior
on the part of the male. Once the male discovers that
the fema le will display lordosis, he becomes sexually
stimulated. It should be emphasized that lordos is is a
highly specific female motor response associated with
the “willingness” to mate.
Sexual arousal is followed by erection
and penile protrusion.
Following expo sure to the appropriate stimuli ,
erection and protmsion of the penis occur. T hese highly
specific m otor events are controlled by the central
nervous system. The mechanisms of peni le protru sion
and erection will be presented later. Typical behavior
dming search, courtship and sexual arousal for domestic
animals is presented in Tabl e 11 -1 .
Reproductive Behavior 231
After significant sexual s timulation, mount-
ing, intromission and ejaculation follow. In genera l,
mammals can be c la ssified as sustained copulators
or short copulators. The bull, ram, buck and tom are
short copulators w hile the boar, dog and camel ids are
sustained copulators. The stallion is intem1edi ate with
regard to duration of copulation.
Mounting behavior generally requires immobi-
lization of the female and elevation ofthe front legs o f
the male to straddle the caudal region of the female ( See
Figure 11-1 0). Intromission is ent rance of the penis
into the vagi na. Ejaculation is expulsion of semen
from the penis into the female reproductive tract.
Copulatory behavior on the par t of the male
is learned. Past sexual experiences are important in
order for the male to develop appropriate reproductive
behavior. For example, negative experiences during
the precopulatory and copulatory stages will generally
result in less enthusiasm on the part of the male. From
a practical standpoint, m anagement of the breeding
male should always be directed towards providing the
ma le w ith totally p ositive sti m ul i. U ti lizing non-estrus
femal es to collect semen fi·om stallions, boars, rams and
bulls should be avo ided because these fe males do not
w illingly stand to be mounted. Inj my to both the female
and the male can occur under these circumstances.
Postcopulatmy behavior is a
period of refractivity.
Postcopulatory behavior involves dism ounting
and a per iod during which either the male, the fem ale
or both will not engage in copulatory behavior. T his
refractory period is a peri od of time during w hich
a sec ond copulation w ill not take pl ace. Memory is
important in both a positive and negative way. Positive
mating experiences promote reproductive behavior and
negative inhibit reproductive behavior. When semen is
collected for artificial insemination, it is important to re-
duce the duration of the refi·actmy period when multiple
ejaculations need to be collected in the shortest possible
time . Techniques to reduce the refractory period will be
presented later in the chapter. Both ma les and females
often display specific postcopulatory behavior such as
vocal emissions, genital groomi ng, changing postural
relationships and various tacti le behaviors , such as
licking and nuzzl ing.
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232 Reproductive Behavior
Table 11-1 . Typical Behavior During Search, Courtship and Consummation by Female and Male
Domestic Animals
Species Search
Cow Increased locomotion,
increased vocalization,
twitching & elevation
of the tail
Mare Increased locomotion,
tail erected (“flagging”)
Ewe Short period of
restlessness
ram “seeking”
Sow Mild restlessness
Bitch Roaming
Queen Vocalization
(calling)
Species Search
Bull Approach sexually
active group of females
testing for
lordosis, flelm1en
Stallion Visual search, flehmen
Sniffing and licking
of ana-genital region,
nudging ewe, flehmen
Moving among females
Roaming around territory
Prowling
FEMALE
Courtship
Increased grooming,
mounting attempts
with other females
Urination stance,
urination in presence
of stallion
Urination in
presence of ram
Immobile stance
limnobile stance
Crouching,
affectionate
head rubbing, rolling
MALE
Courtship
Nuzzling and licking
of perineal region:
chin resting, testing for
lordosis
High degree of
excitement
Neck outstretched
and head held
horizontally
Nuzzling, grinding of
teeth, foams at mouth
Sniffing, licking of the
vulva
Biting queen on dorsal
neck
Consummation
Homosexual mounting & immobile stance
(standing to be mounted)
Presents hindquarters to male,
clitoral exposure by labial eversion,
pulsati le contractions of labia
Immobile stance
Immobile stance
Tail defl ected to one side
Urination in presence of ma le
affectionate head rubbing
Elevation of rear quarters and hyper-
extension ofback (lordosis),
presentation of vulva, tai l deviation
Consummation
Penile protrusion
w ith dribbling
of seminal fluid with few sperm-
atozoa, erection and attempted mounts
Penile protrusion with no
preejaculatmy expulsion of
seminal fluid
Repeated dorsal retraction of scrotum,
penile protrusion with no dribbling of
seminal flu id
Penile protrusion, shallow pelvic
thrusts, attempted mounting
Erection, protrusion of penis, mounting
Mounting
Reproductive Behavior is Programmed
During Prenatal Development
During embryogenesis, sexual differentiation
occurs, during which the brain is programmed to be
either male or female. Recent findings suggest that the
very early embryo is neutral with regard to sex (gender).
Under the influence of extremely small quantities of
estradiol the brain becomes fem inized. Feminiza-
tion is the development of female-like behavior. As
you learned in Chapter 6, during feta l development,
a.-fetoprotein is produced that prevents most fetal and
maternal estradiol from crossing the blood-brain barrier
and entering the brain. When a.-fetoprote in prevents
estradiol from entering the brain, the embryo becomes
“fully feminized,” because it has not been exposed to
estrogen (See Chapter 6). Alpha-fetoprotein does not
bind to testosterone, which can then enter the brain and
be converted to estradiol. In developing males this high
concentration of estradiol in the brain causes defemini-
zation and masculinization of the brain. Defeminiza-
tion reduces the likelihood that the animal will express
female-like behavior postpubertally. Masculinization
results in the potential of the animal to develop male-
like behavior after puberty.
Sex differences in specific brain structures
for the control of reproductive behavior have been
observed. For example, in the male, the preoptic area
Reproductive Behavior 233
of the hypothalamus is larger than in female s. I n the
male, the size of neurons, the neuron nuclei and the
dendritic arborizations are greater. In the fema le, the
ventromedial hypothalamus is more important with
regard to reproductive behavior.
In most mammals, reproductive behaviors
are sexually differentiated. For example, mounting,
erection and ejaculation are typically male behavi ors,
while standing to be mounted (lordosis), crouching and
increased locomotion are typically female behaviors.
These behaviors are endocrine controlled. For example,
sequential treatment with progesterone and estradiol
induces sexual receptivity in ovariectomized fem ales
and testosterone will restore reproductive behavior in
castrated males. In some species, inj ections of testos-
terone into castrated females will even induce male-like
reproductive behavior. Female fetuses exposed to
androgens prenatally will display significantly reduced
female behavior (defeminized) and acquire male-like
behavior pos tna tally (mascu lini zed). In contrast,
males exposed to estrogen or progesterone prena-
tally are unaffected. A class ic example illustrating the
behavioral manifestations of prenatal exposure to andro-
gens is the freemartin heifer. As previous ly discussed
(See Chapter 4), thi s animal has abnormal development
of the reproductive tract for two reasons . First, from a
genetic perspective, freemartins are chimeras that are
XX/XY and therefore they have an ovotestis. Second,
Figure 11-3. Influence of Various Steroid Treatments Upon
Reproductive Behavior
PRE NATAL
Fetus + E2 ——• f Estrous behavior + male-like behavior
Fetus + Testosterone f Estrous behavior + male-like behavior
‘b Fetus + E2 or P4 No effect (normal ‘b behavior)
‘b Fetus+ Testosterone – No effect (normal ‘b behavior)
POSTNATAL
No estrous behavior
Estrous behavior
——+- Maximum estrous behavior
+ E2 ——-
+ P4 and E2
+ Testosterone Male-like behavior
———-.- f Sexual behavior
+ Testosterone —-•
Ovaries remo ved (ovariect omy)
Sexual behavior restored
Sexual behavior restored
removed (orchidectomy)
V
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232 Reproductive Behavior
Table 11-1 . Typical Behavior During Search, Courtship and Consummation by Female and Male
Domestic Animals
Species Search
Cow Increased locomotion,
increased vocalization,
twitching & elevation
of the tail
Mare Increased locomotion,
tail erected (“flagging”)
Ewe Short period of
restlessness
ram “seeking”
Sow Mild restlessness
Bitch Roaming
Queen Vocalization
(calling)
Species Search
Bull Approach sexually
active group of females
testing for
lordosis, flelm1en
Stallion Visual search, flehmen
Sniffing and licking
of ana-genital region,
nudging ewe, flehmen
Moving among females
Roaming around territory
Prowling
FEMALE
Courtship
Increased grooming,
mounting attempts
with other females
Urination stance,
urination in presence
of stallion
Urination in
presence of ram
Immobile stance
limnobile stance
Crouching,
affectionate
head rubbing, rolling
MALE
Courtship
Nuzzling and licking
of perineal region:
chin resting, testing for
lordosis
High degree of
excitement
Neck outstretched
and head held
horizontally
Nuzzling, grinding of
teeth, foams at mouth
Sniffing, licking of the
vulva
Biting queen on dorsal
neck
Consummation
Homosexual mounting & immobile stance
(standing to be mounted)
Presents hindquarters to male,
clitoral exposure by labial eversion,
pulsati le contractions of labia
Immobile stance
Immobile stance
Tail defl ected to one side
Urination in presence of ma le
affectionate head rubbing
Elevation of rear quarters and hyper-
extension ofback (lordosis),
presentation of vulva, tai l deviation
Consummation
Penile protrusion
w ith dribbling
of seminal fluid with few sperm-
atozoa, erection and attempted mounts
Penile protrusion with no
preejaculatmy expulsion of
seminal fluid
Repeated dorsal retraction of scrotum,
penile protrusion with no dribbling of
seminal flu id
Penile protrusion, shallow pelvic
thrusts, attempted mounting
Erection, protrusion of penis, mounting
Mounting
Reproductive Behavior is Programmed
During Prenatal Development
During embryogenesis, sexual differentiation
occurs, during which the brain is programmed to be
either male or female. Recent findings suggest that the
very early embryo is neutral with regard to sex (gender).
Under the influence of extremely small quantities of
estradiol the brain becomes fem inized. Feminiza-
tion is the development of female-like behavior. As
you learned in Chapter 6, during feta l development,
a.-fetoprotein is produced that prevents most fetal and
maternal estradiol from crossing the blood-brain barrier
and entering the brain. When a.-fetoprote in prevents
estradiol from entering the brain, the embryo becomes
“fully feminized,” because it has not been exposed to
estrogen (See Chapter 6). Alpha-fetoprotein does not
bind to testosterone, which can then enter the brain and
be converted to estradiol. In developing males this high
concentration of estradiol in the brain causes defemini-
zation and masculinization of the brain. Defeminiza-
tion reduces the likelihood that the animal will express
female-like behavior postpubertally. Masculinization
results in the potential of the animal to develop male-
like behavior after puberty.
Sex differences in specific brain structures
for the control of reproductive behavior have been
observed. For example, in the male, the preoptic area
Reproductive Behavior 233
of the hypothalamus is larger than in female s. I n the
male, the size of neurons, the neuron nuclei and the
dendritic arborizations are greater. In the fema le, the
ventromedial hypothalamus is more important with
regard to reproductive behavior.
In most mammals, reproductive behaviors
are sexually differentiated. For example, mounting,
erection and ejaculation are typically male behavi ors,
while standing to be mounted (lordosis), crouching and
increased locomotion are typically female behaviors.
These behaviors are endocrine controlled. For example,
sequential treatment with progesterone and estradiol
induces sexual receptivity in ovariectomized fem ales
and testosterone will restore reproductive behavior in
castrated males. In some species, inj ections of testos-
terone into castrated females will even induce male-like
reproductive behavior. Female fetuses exposed to
androgens prenatally will display significantly reduced
female behavior (defeminized) and acquire male-like
behavior pos tna tally (mascu lini zed). In contrast,
males exposed to estrogen or progesterone prena-
tally are unaffected. A class ic example illustrating the
behavioral manifestations of prenatal exposure to andro-
gens is the freemartin heifer. As previous ly discussed
(See Chapter 4), thi s animal has abnormal development
of the reproductive tract for two reasons . First, from a
genetic perspective, freemartins are chimeras that are
XX/XY and therefore they have an ovotestis. Second,
Figure 11-3. Influence of Various Steroid Treatments Upon
Reproductive Behavior
PRE NATAL
Fetus + E2 ——• f Estrous behavior + male-like behavior
Fetus + Testosterone f Estrous behavior + male-like behavior
‘b Fetus + E2 or P4 No effect (normal ‘b behavior)
‘b Fetus+ Testosterone – No effect (normal ‘b behavior)
POSTNATAL
No estrous behavior
Estrous behavior
——+- Maximum estrous behavior
+ E2 ——-
+ P4 and E2
+ Testosterone Male-like behavior
———-.- f Sexual behavior
+ Testosterone —-•
Ovaries remo ved (ovariect omy)
Sexual behavior restored
Sexual behavior restored
removed (orchidectomy)
V
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234 Reproductive Behavior
androgen exposure per se causes abnonnal develop-
ment of the female tract. In addition, the freemartin
displays more male-like behavior than do her normal
heifer counterparts. Figure 11-3 summarizes the influ-
ence of reproductive steroids on behavior in the male
and the female.
The presence of gonadal steroids (estradiol
and testosterone) is obligatory for normal reproduc-
tive behavior in both the male and the female. For
example, ovariectomized females display no estrous
behavior (See Figure 11-3 ). Likewise, castrated
males have significantly reduced reproductive behav-
ior. However, the abolition of reproductive behavior
depends on the duration of time between castration
and the opportunity to copulate. For example, males
that have reached puberty and established a sustained
pattern of reproductive behavior require a longer
period of time between abolition of sexual behavior
after castration than do males that have not estab-
lished a sustained pattern of reproductive behavior.
Females will display male reproductive be-
havior following injections of testosterone.
When ovariectomized fema les receive injections
of estradiol, estrous behavior is reestablished, but at
a less than maximum leve l. Among farm animals,
ovariectomized fe males that are tTeated fi rst with pro-
gesterone (to mimic the luteal phase of the cycle) and
then treate d with estradio l displ ay maximum estrous
behavior. In other species, estradiol must precede
progesterone to produce maximal behavior. It is not
clear why progesterone “priming” of the central ner-
vous system fo r maximal stimulation is necessary. It
would be logical to propose that progesterone promotes
upregulation of estradiol receptors in the brain. Ova-
riectomized females that are treated with testosterone
develop male-like behavior. They w ill even develop
secondary sex characteristics (reduced pitch of voice,
hump on the back of the neck and atrophy of the fe male
reproductive tract).
Figure 11-4. Hypothetical NeNous Pathway Eliciting
Reproductive-Specific Motor Behavior
• Visual
• Olfactory
• Auditory
• Tactile
• Estrogen receptors
• t E2 -+ t increased
nerve excitability
• Neurons produce
behavior specific
peptides
OC = Optic Chiasm
AL = Anterior Lobe
of Pituitary
PL = Posterior Lobe
of Pituitary
• “Receiving zone”
for hypothalam ic
peptides
• Speeds up impulses
and mounting
Spinal cord
• Generates
signals to
specific muscles
fo r lordosis
and moun ting
Specific muscles
responsible
for lordos is and
Reproductive Behavior is Controlled by the
Central Nervo us System
The neural pathways and key anatomical com-
ponents for the control of reproductive behavior are pre-
sented in Figure 11 -4 . Reproductive behavior can take
place only if the nemons in the hypothalamus have been
sensitized to respond to sensory signals. Testosterone
in the male is aromatized to esh·adio l in the brain and
estradiol promotes reproduc tive behav ior. Recall that
tes tosterone is produced in small episodes every 4 to 6
hours. Therefore, there is a relatively constant supply of
testosterone and thus estradiol, to the hypothalamus in
the male. This allows the male to initiate reproductive
behavior at any time. In contrast, the female experi-
ences high esh·adiol during the follicu lar phase only and
will display sexual receptivity during estrus only.
Figure 11-4 outlines a generic neural pathway
for sexual behavior. Under the influence of estrogen,
sensory inputs such as olfaction, audition, v ision and
tactility send neural messages to the hypothalamus.
These neurons synapse directly on neurons in the ven-
h·omedial hypo thal amus as well as the preoptic and
anterior hypothalami c regions. These sensory inputs
cause neurons in the hypothalamus to release behav ior
spec ific peptides that serve as ne urotransmitters. These
neurotransmitters act on neurons in the midbrain. The
neurons in the midbrain serve as receiving zones for
the peptides produced by the hypothalamic neurons.
The midbrain h·anslates neuropeptide signals released
by hypothalamic neurons into a fast response . Neu-
rons in the midbrain synapse with neurons in the brain
stem (medulla). These nervous s ignals are integrated
in the medulla. From the medulla, nerve tracts extend
to the spinal cord where the nerves synapse with mo-
tor neurons that innervate muscles that cause lordosis
and mounting. It should be emphasized that the model
presented in Figure 11-4 does not account for all of the
nerve pathways involved in reproductive behavior.
Reproductive behavior is initiated by:
• olfaction
• vision
• audition
• tactility
The primary sensory inputs for reproductive
behavior are olfaction, audition, vision and tactility. The
degree to which these sensory inputs influence repro-
ductive behavior, particularly precopulatmy behavior,
varies significantly among species.
Reproductive Behavior 235
Figure 11-5. Flehmen
Response in the Stallion and
Bull and the Vomeronasal
Pathway
0;2/
Flu ids
Nasopalatine
Flu ids duct
The flehmen res ponse involves curling of the
upper lip so that airflow through the nasal pas-
sages is restricted. A subatmospheric pres-
sure is thus created in the nasopalatine duct.
Therefore , flu ids can be aspirated through
the duct and into the sensory surfaces of the
vomeronasal organ. Arrows in the bull indicate
the approximate openings of the nasopalatine
ducts. (Photo of stallion courtesy of Dr. A T1bary, Washington
State University, College of Veterinary Medicine; Photo of bull
courtesy of Select S ires, Inc. www.selectsires.com)
V
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234 Reproductive Behavior
androgen exposure per se causes abnonnal develop-
ment of the female tract. In addition, the freemartin
displays more male-like behavior than do her normal
heifer counterparts. Figure 11-3 summarizes the influ-
ence of reproductive steroids on behavior in the male
and the female.
The presence of gonadal steroids (estradiol
and testosterone) is obligatory for normal reproduc-
tive behavior in both the male and the female. For
example, ovariectomized females display no estrous
behavior (See Figure 11-3 ). Likewise, castrated
males have significantly reduced reproductive behav-
ior. However, the abolition of reproductive behavior
depends on the duration of time between castration
and the opportunity to copulate. For example, males
that have reached puberty and established a sustained
pattern of reproductive behavior require a longer
period of time between abolition of sexual behavior
after castration than do males that have not estab-
lished a sustained pattern of reproductive behavior.
Females will display male reproductive be-
havior following injections of testosterone.
When ovariectomized fema les receive injections
of estradiol, estrous behavior is reestablished, but at
a less than maximum leve l. Among farm animals,
ovariectomized fe males that are tTeated fi rst with pro-
gesterone (to mimic the luteal phase of the cycle) and
then treate d with estradio l displ ay maximum estrous
behavior. In other species, estradiol must precede
progesterone to produce maximal behavior. It is not
clear why progesterone “priming” of the central ner-
vous system fo r maximal stimulation is necessary. It
would be logical to propose that progesterone promotes
upregulation of estradiol receptors in the brain. Ova-
riectomized females that are treated with testosterone
develop male-like behavior. They w ill even develop
secondary sex characteristics (reduced pitch of voice,
hump on the back of the neck and atrophy of the fe male
reproductive tract).
Figure 11-4. Hypothetical NeNous Pathway Eliciting
Reproductive-Specific Motor Behavior
• Visual
• Olfactory
• Auditory
• Tactile
• Estrogen receptors
• t E2 -+ t increased
nerve excitability
• Neurons produce
behavior specific
peptides
OC = Optic Chiasm
AL = Anterior Lobe
of Pituitary
PL = Posterior Lobe
of Pituitary
• “Receiving zone”
for hypothalam ic
peptides
• Speeds up impulses
and mounting
Spinal cord
• Generates
signals to
specific muscles
fo r lordosis
and moun ting
Specific muscles
responsible
for lordos is and
Reproductive Behavior is Controlled by the
Central Nervo us System
The neural pathways and key anatomical com-
ponents for the control of reproductive behavior are pre-
sented in Figure 11 -4 . Reproductive behavior can take
place only if the nemons in the hypothalamus have been
sensitized to respond to sensory signals. Testosterone
in the male is aromatized to esh·adio l in the brain and
estradiol promotes reproduc tive behav ior. Recall that
tes tosterone is produced in small episodes every 4 to 6
hours. Therefore, there is a relatively constant supply of
testosterone and thus estradiol, to the hypothalamus in
the male. This allows the male to initiate reproductive
behavior at any time. In contrast, the female experi-
ences high esh·adiol during the follicu lar phase only and
will display sexual receptivity during estrus only.
Figure 11-4 outlines a generic neural pathway
for sexual behavior. Under the influence of estrogen,
sensory inputs such as olfaction, audition, v ision and
tactility send neural messages to the hypothalamus.
These neurons synapse directly on neurons in the ven-
h·omedial hypo thal amus as well as the preoptic and
anterior hypothalami c regions. These sensory inputs
cause neurons in the hypothalamus to release behav ior
spec ific peptides that serve as ne urotransmitters. These
neurotransmitters act on neurons in the midbrain. The
neurons in the midbrain serve as receiving zones for
the peptides produced by the hypothalamic neurons.
The midbrain h·anslates neuropeptide signals released
by hypothalamic neurons into a fast response . Neu-
rons in the midbrain synapse with neurons in the brain
stem (medulla). These nervous s ignals are integrated
in the medulla. From the medulla, nerve tracts extend
to the spinal cord where the nerves synapse with mo-
tor neurons that innervate muscles that cause lordosis
and mounting. It should be emphasized that the model
presented in Figure 11-4 does not account for all of the
nerve pathways involved in reproductive behavior.
Reproductive behavior is initiated by:
• olfaction
• vision
• audition
• tactility
The primary sensory inputs for reproductive
behavior are olfaction, audition, vision and tactility. The
degree to which these sensory inputs influence repro-
ductive behavior, particularly precopulatmy behavior,
varies significantly among species.
Reproductive Behavior 235
Figure 11-5. Flehmen
Response in the Stallion and
Bull and the Vomeronasal
Pathway
0;2/
Flu ids
Nasopalatine
Flu ids duct
The flehmen res ponse involves curling of the
upper lip so that airflow through the nasal pas-
sages is restricted. A subatmospheric pres-
sure is thus created in the nasopalatine duct.
Therefore , flu ids can be aspirated through
the duct and into the sensory surfaces of the
vomeronasal organ. Arrows in the bull indicate
the approximate openings of the nasopalatine
ducts. (Photo of stallion courtesy of Dr. A T1bary, Washington
State University, College of Veterinary Medicine; Photo of bull
courtesy of Select S ires, Inc. www.selectsires.com)
V
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236 Reproductive Behavior
The Olfactory and Vomeronasal Systems
Respond to Pheromones that Trigger
Reproductive Behavior
Secretions from the female reproductive tract
serve to sexually stimulate and attract the male to the
female. Vaginal and urinary secretions from females in
estrus smell different to the male than secretions from
females not in estrus. There is good scientific evidence
that females produce pheromonal substances that are
identifiable both within species and among species.
However, their action is species specific. Recall that a
phe1·omone is a volatile substance secreted or released
to the outside of the body and perceived by the olfac-
tory system and/or activated by the vomeronasal organ.
Releasing pheromones can cause specific behavior in
the recipient. Pheromones can also be priming phero-
mones that have physiologic rather than behavioral
effects on the recipient.
Males also produce sex pheromones that attract
and stimulate females. Among food producing animals,
the best documentation for a male sex pheromone is in
swine. Boars produce specific substances that cause
sows and gilts to become sexually aroused when they are
in estms. Two sexual attractants are produced by boars.
One of these attractants is a preputial pouch secretion.
The second pheromonal-like substance is present in
saliva secreted by the submaxillary salivary glands.
During sexual excitement and precopulatory interac-
tions, the boar produces copious quantities of foamy
saliva. The active components in saliva are the androgen
metabolites 3a.-androstenol and 5a.-androstenone. Both
compounds have a musk-like odor.
It has been demonstrated that dogs have the
ability to identifY cows in estrus by olfactory discrimi-
nation. In addition, rats can be trained to press a lever
in response to air bubbled through urine from cows in
estms. Rats did not press the lever when air was bubbled
through urine fi·om nonestrous cows. Clearly, urine from
cows in estrus contains a material that can be identified
by olfaction by other species (dogs and rats).
Figure 11-6. “Warm-Up” Stalls Used for Stimulating Sexual Behavior in
Bulls Providing Semen for Artificial Insemination
Bulls waiting to be ejaculated (arrows) watch mounting and ejaculatory behavior of another bull. Such a
practice “prestimulates” bulls and reduces stimulation time when they enter the collection arena. It also in-
creases sperm harvest. A false-mount is being performed by the bull mounting the stimulus animal (SA).
(Photo courtesy of Select Sires, Inc., www.selectsires.com)
Flehmen Behavior is a Close-Range
Investigative Behavior
Some pheromones appear to be less volati le
and need to be detected by the vomeronasal organ
in the bull, ram, stallion and to some extent, the boar.
The male needs to closely approach the source of
pheromones and he wi ll nuzzle the genital region of the
female. The vomeronasal organ (See Figure 11-5) is an
accessory ol factory organ. It is connected to two small
openings in the anterior roof of the mouth j ust behind
the upper lip. Fluid-borne, less vo latile chemicals can
enter the vomeronasal organ through the oral cavity
by means of the nasopalatine ( incis ive) ducts. Many
species, such as bulls, rams and stallions, perfom1 a
special investigative maneuver when in close proxim-
ity to a female. Vaginal secretions and urine evoke an
investigative behavior known as the flehmen response.
Flehmen behavior allows less vo latile materials to be
“examined” by sensory neurons in the vomeronasal
organ. Flehmen behavior is characterized by head el-
evation and curling of the upper lip (See Figure 11 -5).
Curling of the upper lip closes the nostrils and allows
a negative pressure to forn1 in the nasopalatine duct.
Thus, less vo latile materials (like mucous and urine)
can be aspirated through the duct into the vomeronasal
organ where they can be “evaluated” by sensory neurons
in the organ. Olfactory bulbec tomy in goats inhibits
the flehmen response. Flehmen behavior in males is
likely to be performed whether the material is from an
estrus or nonestrus female. It is believed that the fleh-
men behavior is used to help a male identifY mating
opportunities. Flehmen is occasionally performed by
females during sexual encounters with males. Cows
will frequently perform the maneuver when sniffing
other cows that are in estrus or proestrus. As in the
male, females will display flelunen to novel compmmds,
including fluids associated with the placenta, newborn
animals and other volatile materia ls. Flehmen is fre –
quently displayed by post-parturient fe males as they
make identity discriminations between their own versus
other’s neonates.
Auditory stimulation can serve as a
long-range signal.
In many species, sexual readiness is accompa-
nied by some fom1 of unique vocalization or “mating
calls”. For example, cows are known to increase their
bellowing during the time of estrus. Sows display a
characteristic grunting sound associated with estrus.
Queens often “yeow” repeatedly to call the tom. By
Reproductive Behavior 237
comparison, mares and ewes are relatively silent. El-
evated vocalization serves to alert or send a signa l to
males that sexual readiness is imminent. The auditory
stimulus is more useful in long-range discrimination,
rather than close discrimination. The classic example
of reproductive driven vocalization is bugling of the
bull e lk during rut (the breeding season).
Visual signals are valuable for
close encounters.
All females display a fonn of sexual postur-
ing that can be perceived by males. While posturing
can be quite subtle, especially to human observers, the
identifica tion of postures probably takes place easily
among members of the same species.
Tactile stimulation is generally the final
stimulus before copulation.
Almost all males experience a degree of
sexual stimulation when they observe mating behavior
among other individuals of the same species. It is well
documented that in bulls, visual observation of mating
behavior enhances sexual stimulation. This observa-
tion has led to the common practice of placing bulls
used for artificial insemination in “warm-up” stalls
(See Figure 11-6). Bulls are brought to the “warm-up”
stalls and are allowed to observe the mounting behav-
ior and collection of semen from other bulls prior to
entering the collection area themselves. This causes an
elevated level of sexual excitement and reduces the time
required for final sexual stimulation and collection of
semen. This is important because labor requirements
for semen collection are significant. This procedure
is also important because it tends to increase spem1
concentration in the ejaculate.
Tactile stimuli fro m male s appears to be im-
portant in evoking sexual postures or standing postures
by females. For example, biti ng on the neck and the
withers of mares by stallions appears to be important
for sexual stimulation. Biting of the neck of the queen
by the tom is also a characteristic reproductive behav-
ior among cats. Rubbing of the flanks and genitalia
of mares, whether done by the stallion or by a human
handler, evokes behavior signals of estrus from the mare
that othe1wise would not be displayed. Chin resting by
a bull on the back of a cow just prior to mounting may
have some stimulatory effect on the cow.
V
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236 Reproductive Behavior
The Olfactory and Vomeronasal Systems
Respond to Pheromones that Trigger
Reproductive Behavior
Secretions from the female reproductive tract
serve to sexually stimulate and attract the male to the
female. Vaginal and urinary secretions from females in
estrus smell different to the male than secretions from
females not in estrus. There is good scientific evidence
that females produce pheromonal substances that are
identifiable both within species and among species.
However, their action is species specific. Recall that a
phe1·omone is a volatile substance secreted or released
to the outside of the body and perceived by the olfac-
tory system and/or activated by the vomeronasal organ.
Releasing pheromones can cause specific behavior in
the recipient. Pheromones can also be priming phero-
mones that have physiologic rather than behavioral
effects on the recipient.
Males also produce sex pheromones that attract
and stimulate females. Among food producing animals,
the best documentation for a male sex pheromone is in
swine. Boars produce specific substances that cause
sows and gilts to become sexually aroused when they are
in estms. Two sexual attractants are produced by boars.
One of these attractants is a preputial pouch secretion.
The second pheromonal-like substance is present in
saliva secreted by the s ubmaxillary salivary glands.
During sexual excitement and precopulatory interac-
tions, the boar produces copious quantities of foamy
saliva. The active components in saliva are the androgen
metabolites 3a.-androstenol and 5a.-androstenone. Both
compounds have a musk-like odor.
It has been demonstrated that dogs have the
ability to identifY cows in estrus by olfactory discrimi-
nation. In addition, rats can be trained to press a lever
in response to air bubbled through urine from cows in
estms. Rats did not press the lever when air was bubbled
through urine fi·om nonestrous cows. Clearly, urine from
cows in estrus contains a material that can be identified
by olfaction by other species (dogs and rats).
Figure 11-6. “Warm-Up” Stalls Used for Stimulating Sexual Behavior in
Bulls Providing Semen for Artificial Insemination
Bulls waiting to be ejaculated (arrows) watch mounting and ejaculatory behavior of another bull. Such a
practice “prestimulates” bulls and reduces stimulation time when they enter the collection arena. It also in-
creases sperm harvest. A false-mount is being performed by the bull mounting the stimulus animal (SA).
(Photo courtesy of Select Sires, Inc., www.selectsires.com)
Flehmen Behavior is a Close-Range
Investigative Behavior
Some pheromones appear to be less volati le
and need to be detected by the vomeronasal organ
in the bull, ram, stallion and to some extent, the boar.
The male needs to closely approach the source of
pheromones and he wi ll nuzzle the genital region of the
female. The vomeronasal organ (See Figure 11-5) is an
accessory ol factory organ. It is connected to two small
openings in the anterior roof of the mouth j ust behind
the upper lip. Fluid-borne, less vo latile chemicals can
enter the vomeronasal organ through the oral cavity
by means of the nasopalatine ( incis ive) ducts. Many
species, s uch as bulls, rams and stallions, perfom1 a
special investigative maneuver when in close proxim-
ity to a female. Vaginal secretions and urine evoke an
investigative behavior known as the flehmen response.
Flehmen behavior allows less vo latile materials to be
“examined” by sensory neurons in the vomeronasal
organ. Flehmen behavior is characterized by head el-
evation and curling of the upper lip (See Figure 11 -5).
Curling of the upper lip closes the nostrils and allows
a negative pressure to forn1 in the nasopalatine duct.
Thus, less vo latile materials (like mucous and urine)
can be aspirated through the duct into the vomeronasal
organ where they can be “evaluated” by sensory neurons
in the organ. Olfactory bulbec tomy in goats inhibits
the flehmen response. Flehmen behavior in males is
likely to be performed whether the material is from an
estrus or nonestrus female. It is believed that the fleh-
men behavior is used to help a male identifY mating
opportunities. Flehmen is occasionally performed by
females during sexual encounters with males. Cows
will frequently perform the maneuver when sniffing
other cows that are in estrus or proestrus. As in the
male, females will display flelunen to novel compmmds,
including fluids associated with the placenta, newborn
animals and other volatile materia ls. Flehmen is fre –
quently displayed by post-parturient fe males as they
make identity discriminations between their own versus
other’s neonates.
Auditory stimulation can serve as a
long-range signal.
In many species, sexual readiness is accompa-
nied by some fom1 of unique vocalization or “mating
calls”. For example, cows are known to increase their
bellowing during the time of estrus. Sows display a
characteristic grunting sound associated with estrus.
Queens often “yeow” repeatedly to call the tom. By
Reproductive Behavior 237
comparison, mares and ewes are relatively silent. El-
evated vocalization serves to alert or send a signa l to
males that sexual readiness is imminent. The auditory
stimulus is more useful in long-range discrimination,
rather than close discrimination. The classic example
of reproductive driven vocalization is bugling of the
bull e lk during rut (the breeding season).
Visual signals are valuable for
close encounters.
All females display a fonn of sexual postur-
ing that can be perceived by males. While posturing
can be quite subtle, especially to human observers, the
identifica tion of postures probably takes place easily
among members of the same species.
Tactile stimulation is generally the final
stimulus before copulation.
Almost all males experience a degree of
sexual stimulation when they observe mating behavior
among other individuals of the same species. It is well
documented that in bulls, visual observation of mating
behavior enhances sexual stimulation. This observa-
tion has led to the common practice of placing bulls
used for artificial insemination in “warm-up” stalls
(See Figure 11-6). Bulls are brought to the “warm-up”
stalls and are allowed to observe the mounting behav-
ior and collection of semen from other bulls prior to
entering the collection area themselves. This causes an
elevated level of sexual excitement and reduces the time
required for final sexual stimulation and collection of
semen. This is important because labor requirements
for semen collection are significant. This procedure
is also important because it tends to increase spem1
concentration in the ejaculate.
Tactile stimuli fro m male s appears to be im-
portant in evoking sexual postures or standing postures
by females. For example, biti ng on the neck and the
withers of mares by stallions appears to be important
for sexual stimulation. Biting of the neck of the queen
by the tom is also a characteristic reproductive behav-
ior among cats. Rubbing of the flanks and genitalia
of mares, whether done by the stallion or by a human
handler, evokes behavior signals of estrus from the mare
that othe1wise would not be displayed. Chin resting by
a bull on the back of a cow just prior to mounting may
have some stimulatory effect on the cow.
V
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238 Reproductive Behavior
Penile Erection and Protrusion
Completes the Pr·ecopulatory P hase
of Reproductive Behavior
When sexua l receptiv ity of a f emale is es –
tablished and sufficie nt a rousal is acc omplished in
the ma le, erection and protrusion of the penis ensue .
Successful penil e erection requires a complex series
of neura l and vas omotor (blood v essel) reactions .
Erection of the penis is necessary for copulation and
depos ition of semen in the female repro ductive h·act.
Erection is character ized by a marked increase in the
rigidity of the peni s. T he increased rigidity is the result
of a marked incr ease in ar teri al inflow of blood w hen
compared to the ven ous outflow of blood. Erecti on
requires that blood b e trapped within the cavernous
sinuses of the penis . Increased blood flow to the pe-
nis is brought ab out by vasodilation of the arterioles
supplying it. In the bull, ram and boar erection not
only involves increased bl oo d flow and a subsequent
Figure 11-7. Steps in Penile Erection as They Relate to Cavernous
Blood Pressure and Contraction of the Bulbospongiosus and
Ischiocavernosus Muscles
“;)
:I
E
E -f
:s
Ul
Ul
f
D.
“C
0
0
a5
Ill :s
0 c
Q)
u
Sexual arousal
(visual, tactile,
olfactory)
(Modified from Beckett, et al. 1972. Bioi. of Rep rod. 7:359)
. .•. ,ljJJ..:::: of
bulbospongiosus
— – – —— –41’1-___,- , . A.\1 Contractions of ischiocavernosus
t Blood flow
to cavernous tissue +
+ venous outflow
Vasodilation
of helicine arteries
(tblood flow)
7
Time (seconds)
Cavernous
pressure
increase in pressur e, but a simultaneous relaxation ofthe
reh·actor peni s muscles. Thus, erection and protrusion
a lso involve s traighten ing of the penis to eliminate the
s igmo id flexure. The penis of the bull, boar and ram
is fi broe lastic in nature and therefore does not increase
significantly in diameter during erection and protrusion.
In contrast, the penis of the stallion increases signifi-
cantly in diameter during erection. The stallion has a
retractor penis muscle that, as in other spec ies, relaxes
during erection. H owever, the sta llion does not have
a sigmoid flex ure. Engorgement w ith blood plays a
much mor e sig nifi cant role in the highly vascular penis
of the stallion, dog and man than in the bu11, ram, boar
and camelids.
Erection of the p enis requires:
• elevated arterial blood inflow
• dilation of corporal sinusoids
• restricted venous outflow
• elevated intrapenile pressure
• relax ation of the retractor .
penis m uscle
Contractions of the ischiocavernosus muscles
cause compression of the penile veins. T his compres-
sion causes blockage of venous retum thus enabling
the cavem ous tissue to retain blood for maintenance of
an erection. As you w ill recall, the isch iocavernosus
muscles surround the two crura. Intem1ittent contrac-
tions of the muscles creates a pump-like action at the
base of the penis. T hese contractions result in a buildup
of blood within the corpus cavemosum of the penis
and exceptionally high pressures resu lt. For example,
during the fina l stages of erection, the pressures w ithin
the cavernous tissue of the goat penis can reach 7,000
nun Hg (S ee Figme 11-7). When the penis is flacc id,
pressures w ithin the corpus cavernosum are only 19
mm Hg. Pressures in the bull penis are around 1,700
mm Hg during peak erection and a bou t 30 mm Hg
when the cavernous spaces are collapsed. Figure 11-7
summarizes the steps of penile erection and intrapenile
pressures as they relate to contraction of the ischiocav-
emosus and bulbospongiosus muscles.
One of the most publ icized phannaceuticals
ever introduced is a material called Si ldenafil C itrate
(Viagra®). This pharmaceutical provides a therapy for
erectile dysfunction in men. Erectile dysfunction is
defined as the inability to achieve and maintain a penile
erection (tumescence) . Reports indicate that 10% of
men between the ages 40 and 70 years old are affl icted
Reproductive Behavior 239
Figure 11-8. Basic Steps
in the Erectile Process
STEP I
Erotogenic stimuli cause sensory nerves to fire
r
STEP 2
Sensory nerves activate
“Reproductive Behavior Center”
in hypothalamus- (See Figure 11- 4)
I …
STEP 3
St imu lat ion of parasympat heti c nerves that
innervate peni le arterio les
STEP4
Parasympathetic ne rve te rminals rele ase
nitric oxide (NO) – (See Figure 11-9)
STEP 5
Nitric oxide init iates biochemical cascade
that causes e rection – (See Figure 11 -9)
by complete erectile failure. Other reports have esti-
mated that up to 30 million men in the United States
may have some fonn of erectile dysfunction. E rectile
dysfunction is rare among dom estic anima ls because
such males ar e rapidly eliminated from the gene poo l
by artificia l selection (culling) or by natural selection
(no erection-no copulation-no offspring).
Erection of the Penis Requires Sensory Inp ut
and a Local Vascular Response
As mentioned earli er in the chapter, penile erec-
tion is a complex series of neural and vasomotor events.
These events can be broadly su bd ivided into a nervous
component (cerebral and sp inal) and a local vascular
component within the penis. The nervous component
is arousal-driv en. For example, there must be ap-
propriate sensory stim uli (tactile, visual, auditory and
olfactory) in order for the central nervous system to be
appropriately stimulated so that efferent neural events
V
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oo
ks
.ir
238 Reproductive Behavior
Penile Erection and Protrusion
Completes the Pr·ecopulatory P hase
of Reproductive Behavior
When sexua l receptiv ity of a f emale is es –
tablished and sufficie nt a rousal is acc omplished in
the ma le, erection and protrusion of the penis ensue .
Successful penil e erection requires a complex series
of neura l and vas omotor (blood v essel) reactions .
Erection of the penis is necessary for copulation and
depos ition of semen in the female repro ductive h·act.
Erection is character ized by a marked increase in the
rigidity of the peni s. T he increased rigidity is the result
of a marked incr ease in ar teri al inflow of blood w hen
compared to the ven ous outflow of blood. Erecti on
requires that blood b e trapped within the cavernous
sinuses of the penis . Increased blood flow to the pe-
nis is brought ab out by vasodilation of the arterioles
supplying it. In the bull, ram and boar erection not
only involves increased bl oo d flow and a subsequent
Figure 11-7. Steps in Penile Erection as They Relate to Cavernous
Blood Pressure and Contraction of the Bulbospongiosus and
Ischiocavernosus Muscles
“;)
:I
E
E -f
:s
Ul
Ul
f
D.
“C
0
0
a5
Ill :s
0 c
Q)
u
Sexual arousal
(visual, tactile,
olfactory)
(Modified from Beckett, et al. 1972. Bioi. of Rep rod. 7:359)
. .•. ,ljJJ..:::: of
bulbospongiosus
— – – —— –41’1-___,- , . A.\1 Contractions of ischiocavernosus
t Blood flow
to cavernous tissue +
+ venous outflow
Vasodilation
of helicine arteries
(tblood flow)
7
Time (seconds)
Cavernous
pressure
increase in pressur e, but a simultaneous relaxation ofthe
reh·actor peni s muscles. Thus, erection and protrusion
a lso involve s traighten ing of the penis to eliminate the
s igmo id flexure. The penis of the bull, boar and ram
is fi broe lastic in nature and therefore does not increase
significantly in diameter during erection and protrusion.
In contrast, the penis of the stallion increases signifi-
cantly in diameter during erection. The stallion has a
retractor penis muscle that, as in other spec ies, relaxes
during erection. H owever, the sta llion does not have
a sigmoid flex ure. Engorgement w ith blood plays a
much mor e sig nifi cant role in the highly vascular penis
of the stallion, dog and man than in the bu11, ram, boar
and camelids.
Erection of the p enis requires:
• elevated arterial blood inflow
• dilation of corporal sinusoids
• restricted venous outflow
• elevated intrapenile pressure
• relax ation of the retractor .
penis m uscle
Contractions of the ischiocavernosus muscles
cause compression of the penile veins. T his compres-
sion causes blockage of venous retum thus enabling
the cavem ous tissue to retain blood for maintenance of
an erection. As you w ill recall, the isch iocavernosus
muscles surround the two crura. Intem1ittent contrac-
tions of the muscles creates a pump-like action at the
base of the penis. T hese contractions result in a buildup
of blood within the corpus cavemosum of the penis
and exceptionally high pressures resu lt. For example,
during the fina l stages of erection, the pressures w ithin
the cavernous tissue of the goat penis can reach 7,000
nun Hg (S ee Figme 11-7). When the penis is flacc id,
pressures w ithin the corpus cavernosum are only 19
mm Hg. Pressures in the bull penis are around 1,700
mm Hg during peak erection and a bou t 30 mm Hg
when the cavernous spaces are collapsed. Figure 11-7
summarizes the steps of penile erection and intrapenile
pressures as they relate to contraction of the ischiocav-
emosus and bulbospongiosus muscles.
One of the most publ icized phannaceuticals
ever introduced is a material called Si ldenafil C itrate
(Viagra®). This pharmaceutical provides a therapy for
erectile dysfunction in men. Erectile dysfunction is
defined as the inability to achieve and maintain a penile
erection (tumescence) . Reports indicate that 10% of
men between the ages 40 and 70 years old are affl icted
Reproductive Behavior 239
Figure 11-8. Basic Steps
in the Erectile Process
STEP I
Erotogenic stimuli cause sensory nerves to fire
r
STEP 2
Sensory nerves activate
“Reproductive Behavior Center”
in hypothalamus- (See Figure 11- 4)
I …
STEP 3
St imu lat ion of parasympat heti c nerves that
innervate peni le arterio les
STEP4
Parasympathetic ne rve te rminals rele ase
nitric oxide (NO) – (See Figure 11-9)
STEP 5
Nitric oxide init iates biochemical cascade
that causes e rection – (See Figure 11 -9)
by complete erectile failure. Other reports have esti-
mated that up to 30 million men in the United States
may have some fonn of erectile dysfunction. E rectile
dysfunction is rare among dom estic anima ls because
such males ar e rapidly eliminated from the gene poo l
by artificia l selection (culling) or by natural selection
(no erection-no copulation-no offspring).
Erection of the Penis Requires Sensory Inp ut
and a Local Vascular Response
As mentioned earli er in the chapter, penile erec-
tion is a complex series of neural and vasomotor events.
These events can be broadly su bd ivided into a nervous
component (cerebral and sp inal) and a local vascular
component within the penis. The nervous component
is arousal-driv en. For example, there must be ap-
propriate sensory stim uli (tactile, visual, auditory and
olfactory) in order for the central nervous system to be
appropriately stimulated so that efferent neural events
V
et
B
oo
ks
.ir
I
240 Reproductive Behavior
Figure 11-9. Vascular and Biochemical Contra! of an Erection
(Modified from Korenman. 1998. Am. J. Med. 105.135.)
Su perficial
dorsal vei n
vein
· Erect Penis
Arte rl ol
inflo w
Internal
pudendal
— .
Circ umfl ex
vein
Emissory –+
Cavernosal
artery
Flaccid Penis
ve in
PDEs
+ Inhibi tion
Erect Penis
Sinusoid smooth
muscle relaxes
I ERElJoNI
Anatomy
The shaft of the penis co nsists of
tw o dorso-lateral co rpora cave r-
nosa and the corp us spongiosum.
Arteria l blood is supplied by the in-
ternal pudendal artery that supplies
the dorsal and deep cave rnosal ar-
teries. Corpo ral sinusoids are sup-
plied by helici ne arteries. The deep
dorsal vein and superficial dorsal
vein drain the erectil e tissues.
Flaccid penis
The sinuso ids are flattened be-
cause adrenergic nerves secrete
norepinephe rine that causes vaso-
constriction. Blood flow to the cav-
ernous tissue therefore is quite low
for the majority of the time. Since
no erotogenic sti muli a re p res-
ent, nonadrenergic noncholine rgic
(NANC) parasympathetic neurons
do not fire and thus do not release
nitric oxide (NO ). Therefore , vaso-
constriction takes precedence over
vasodilation.
Erect penis
Wh en erotoge nic stimul i are pres-
ent the NANC neu rons fire and
release nitric oxide (NO) from their
termina ls. When NO is released,
it activ ates a n enzyme called
guanylate cyclase. This enzyme
co nverts guanylate tri phosphate
(GTP) to cyclic guanyosine mono-
phosphate (cGMP) and cau ses
the smooth muscle of the corporal
sinusoids to relax (vasodilatation).
The cave rnous sinusoids engorge
with blood and intracorpo ral pres-
sure in creases dramatically. This
compresses the ve nules through
whi ch bl ood exits the penis. Blood
is then trapped within t he penis
causing an erection.
Reproductive Behavior 241
can cause an erection. These extrinsic stim uli are called
erotogenic stimuli. As shown in Figure 11-4, these
stimuli cause afferent sensory nerves to fire. Their
tern1inals synapse with neurons in the so-called “behav-
ior center” in the hypotha lamus. These hypothalamic
neuro ns synapse with parasympathetic and sympathetic
efferent neurons that control peni le vascular smooth
muscle (vascular tone). The basic step s in the erecti le
process are outlined in Figure 11 -8.
Mounting postures and cha racteri sti cs of
copulatory behavior for various species are presented
in Figures 11 – I 0 and I I -l I. T he purpose of mounting
is fo r the male to position himself so that intromission
can occur. Introm iss ion is the successful entrance of
the penis into t he vagina. F ollowing intromission,
ejaculation takes place in r esponse to sensory stimula-
tion of the glans penis. T he time of ejaculation relative
to intromission varies significantly among species (See
F igures 11- 10, I 1- 11 and 11-12). F or examp le, in the
bull and the ram ejaculation occurs within one or two
seconds after intromission . In these species ejaculati on
is stimulated by the warm temperature of th e vagina.
Vag inal pressure is relatively unimportant in inducing
ejaculation in the ram and bull. In contrast, the boar
may have a sustained ejaculation for periods of up to 30
minutes. The stallio n has a mating duration of between
30 seconds and one minute. The llama and the dog are
perhaps the most sustained copulators with reports of
copulation occuring continually for up to 50 minutes.
Erection is caused by the firing of nonadrener-
gic, noncholonergic (NAN C) parasymp athetic neurons
that release nitric oxide (NO), a gas, fro m their ter-
minals. N itric oxide is the principal neurotransmitter
that “dr ives” the erecti le process. Nitric oxide causes
its effect by stimulating an enzyme, guanylate cyclase,
to convert gua ny late triphosphate (GTP ) to cyclic
guanosine monophosphate ( cGMP). Cyclic guanosine
monophosphate causes corporal smooth m uscle relax-
ation (vasodilation) and an erection results.
Under n onerotogen ic c o nditions, cGMP is
acted upon by PDE 5 (Phosphodie sterase 5) and this
enzyme promotes the conversion of cGMP to GM P.
This breakdown causes increased vascular tone result-
ing in outflow of blood fro m the corpora cavernosa
and loss of an erection. Si ldenafil blocks the action of
PDE5 thus prolonging the vasodi lation effect of cGMP
and an erection develops that can be maintained for a
sustained period of time. It should be emphasized tha t
w ithout nitric oxide production by the parasympathetic
nerve terminals Si ldenafil can have no effect because
nitric o xi de must be pr esent for cG MP to be produced.
The usual fla ccid state of the penis (contracted corporal
arteries) results from a to nic contraction of the arteria l
and corporal smooth m uscles mediated by sympa thetic
adrenergic neurons. Such vasoconstriction keep s pe-
nile blood flow to a minimum under no n-erotogenic
conditions.
When t he corp o ral smooth muscles relax
because of cGMP, the resistance to blood flow by the
penile arteri oles and corporal sinusoids decreases and
blood flow to the p enis triples or q uadruples w hen the
appropriate erotogenic stimul i are present. When an
erection occurs, the sinusoid pressure is so great that the
emissary veins are collapsed. Therefore, blood cannot
return through them because venous outflow is blocked.
Penile erectio n can be maintained fo r as long as vasodi-
lation of the corporal smooth muscle takes place. TI1ese
reactions are summarized in Figure l l-9 .
Ejaculation is a simple neural reflex
caused by:
• intromission
• stimulation of the glans penis
• forceful muscle contraction
E jaculation is defined as the reflex expulsion of
spermatozoa and semina l plasma from the male repro-
d uctive tract. The basic mechanism for ejaculation of
semen is quite s imilar among all mamm als. Expulsio n
of semen is the result of sensory stimulatio n, primarily
to the glans penis, that causes a series of coordinated
muscular contractions . Once intromissio n has been
achieved, reflex imp ulses ar e initiated . These neural
impu lses are derived mainly fro m sensory nerves in the
glans penis. Up on thresho ld stimulation, impu lses are
transmitted from the glans penis by way of the internal
pudendal nerve to the lumbosacral region of the spinal
cord (See F igure 11 -13 ). The sensory impulses resu lt
in fi ring of nerves in the spina l cord and the forcing
of semen into t he urethra is accompl ished by nerves
in the hypogastric p lexus that innervate the target
m uscles . Of primary importance for ejaculation are the
urethralis m uscle (that sun ounds the pelvic uretlu-a), the
ischiocavernosus and the bulbospongiosus muscles.
Copulatory behavior includes:
• mounting
• intromission
• ejaculation
F igure l l -13 summarizes the nerve pathways
r esulting in emission and ejaculation. It should be
emphasized that emission is defined as the movement
of seminal fl uids from the accessory sex glands into
the pelvic uretlu-a so they can mix with spennatozoa.
Emission occurs befor e and during ejaculation. In some
V
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I
240 Reproductive Behavior
Figure 11-9. Vascular and Biochemical Contra! of an Erection
(Modified from Korenman. 1998. Am. J. Med. 105.135.)
Su perficial
dorsal vei n
vein
· Erect Penis
Arte rl ol
inflo w
Internal
pudendal
— .
Circ umfl ex
vein
Emissory –+
Cavernosal
artery
Flaccid Penis
ve in
PDEs
+ Inhibi tion
Erect Penis
Sinusoid smooth
muscle relaxes
I ERElJoNI
Anatomy
The shaft of the penis co nsists of
tw o dorso-lateral co rpora cave r-
nosa and the corp us spongiosum.
Arteria l blood is supplied by the in-
ternal pudendal artery that supplies
the dorsal and deep cave rnosal ar-
teries. Corpo ral sinusoids are sup-
plied by helici ne arteries. The deep
dorsal vein and superficial dorsal
vein drain the erectil e tissues.
Flaccid penis
The sinuso ids are flattened be-
cause adrenergic nerves secrete
norepinephe rine that causes vaso-
constriction. Blood flow to the cav-
ernous tissue therefore is quite low
for the majority of the time. Since
no erotogenic sti muli a re p res-
ent, nonadrenergic noncholine rgic
(NANC) parasympathetic neurons
do not fire and thus do not release
nitric oxide (NO ). Therefore , vaso-
constriction takes precedence over
vasodilation.
Erect penis
Wh en erotoge nic stimul i are pres-
ent the NANC neu rons fire and
release nitric oxide (NO) from their
termina ls. When NO is released,
it activ ates a n enzyme called
guanylate cyclase. This enzyme
co nverts guanylate tri phosphate
(GTP) to cyclic guanyosine mono-
phosphate (cGMP) and cau ses
the smooth muscle of the corporal
sinusoids to relax (vasodilatation).
The cave rnous sinusoids engorge
with blood and intracorpo ral pres-
sure in creases dramatically. This
compresses the ve nules through
whi ch bl ood exits the penis. Blood
is then trapped within t he penis
causing an erection.
Reproductive Behavior 241
can cause an erection. These extrinsic stim uli are called
erotogenic stimuli. As shown in Figure 11-4, these
stimuli cause afferent sensory nerves to fire. Their
tern1inals synapse with neurons in the so-called “behav-
ior center” in the hypotha lamus. These hypothalamic
neuro ns synapse with parasympathetic and sympathetic
efferent neurons that control peni le vascular smooth
muscle (vascular tone). The basic step s in the erecti le
process are outlined in Figure 11 -8.
Mounting postures and cha racteri sti cs of
copulatory behavior for various species are presented
in Figures 11 – I 0 and I I -l I. T he purpose of mounting
is fo r the male to position himself so that intromission
can occur. Introm iss ion is the successful entrance of
the penis into t he vagina. F ollowing intromission,
ejaculation takes place in r esponse to sensory stimula-
tion of the glans penis. T he time of ejaculation relative
to intromission varies significantly among species (See
F igures 11- 10, I 1- 11 and 11-12). F or examp le, in the
bull and the ram ejaculation occurs within one or two
seconds after intromission . In these species ejaculati on
is stimulated by the warm temperature of th e vagina.
Vag inal pressure is relatively unimportant in inducing
ejaculation in the ram and bull. In contrast, the boar
may have a sustained ejaculation for periods of up to 30
minutes. The stallio n has a mating duration of between
30 seconds and one minute. The llama and the dog are
perhaps the most sustained copulators with reports of
copulation occuring continually for up to 50 minutes.
Erection is caused by the firing of nonadrener-
gic, noncholonergic (NAN C) parasymp athetic neurons
that release nitric oxide (NO), a gas, fro m their ter-
minals. N itric oxide is the principal neurotransmitter
that “dr ives” the erecti le process. Nitric oxide causes
its effect by stimulating an enzyme, guanylate cyclase,
to convert gua ny late triphosphate (GTP ) to cyclic
guanosine monophosphate ( cGMP). Cyclic guanosine
monophosphate causes corporal smooth m uscle relax-
ation (vasodilation) and an erection results.
Under n onerotogen ic c o nditions, cGMP is
acted upon by PDE 5 (Phosphodie sterase 5) and this
enzyme promotes the conversion of cGMP to GM P.
This breakdown causes increased vascular tone result-
ing in outflow of blood fro m the corpora cavernosa
and loss of an erection. Si ldenafil blocks the action of
PDE5 thus prolonging the vasodi lation effect of cGMP
and an erection develops that can be maintained for a
sustained period of time. It should be emphasized tha t
w ithout nitric oxide production by the parasympathetic
nerve terminals Si ldenafil can have no effect because
nitric o xi de must be pr esent for cG MP to be produced.
The usual fla ccid state of the penis (contracted corporal
arteries) results from a to nic contraction of the arteria l
and corporal smooth m uscles mediated by sympa thetic
adrenergic neurons. Such vasoconstriction keep s pe-
nile blood flow to a minimum under no n-erotogenic
conditions.
When t he corp o ral smooth muscles relax
because of cGMP, the resistance to blood flow by the
penile arteri oles and corporal sinusoids decreases and
blood flow to the p enis triples or q uadruples w hen the
appropriate erotogenic stimul i are present. When an
erection occurs, the sinusoid pressure is so great that the
emissary veins are collapsed. Therefore, blood cannot
return through them because venous outflow is blocked.
Penile erectio n can be maintained fo r as long as vasodi-
lation of the corporal smooth muscle takes place. TI1ese
reactions are summarized in Figure l l-9 .
Ejaculation is a simple neural reflex
caused by:
• intromission
• stimulation of the glans penis
• forceful muscle contraction
E jaculation is defined as the reflex expulsion of
spermatozoa and semina l plasma from the male repro-
d uctive tract. The basic mechanism for ejaculation of
semen is quite s imilar among all mamm als. Expulsio n
of semen is the result of sensory stimulatio n, primarily
to the glans penis, that causes a series of coordinated
muscular contractions . Once intromissio n has been
achieved, reflex imp ulses ar e initiated . These neural
impu lses are derived mainly fro m sensory nerves in the
glans penis. Up on thresho ld stimulation, impu lses are
transmitted from the glans penis by way of the internal
pudendal nerve to the lumbosacral region of the spinal
cord (See F igure 11 -13 ). The sensory impulses resu lt
in fi ring of nerves in the spina l cord and the forcing
of semen into t he urethra is accompl ished by nerves
in the hypogastric p lexus that innervate the target
m uscles . Of primary importance for ejaculation are the
urethralis m uscle (that sun ounds the pelvic uretlu-a), the
ischiocavernosus and the bulbospongiosus muscles.
Copulatory behavior includes:
• mounting
• intromission
• ejaculation
F igure l l -13 summarizes the nerve pathways
r esulting in emission and ejaculation. It should be
emphasized that emission is defined as the movement
of seminal fl uids from the accessory sex glands into
the pelvic uretlu-a so they can mix with spennatozoa.
Emission occurs befor e and during ejaculation. In some
V
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ks
.ir
‘
[ill
242 Reproductive Behavior
Figure 11-10. Characteristics of Copulation, Site of Seminal Deposition
and Number of Ejaculations to Satiation and Exhaustion in the Ram,
Bull, Stallion and Boar
Mating pair
Photos of:
Deposition Ejaculations Ejaculations
to Satiation to Exhaustion
1 to 2 sec- .8 to 1 ml e xt e r n a I
onds (1 pel- (.1 to 2ml) cervical os
1 to 3 sec- 3-5ml fornix vagina
onds (1 pel- (.5 to 12ml)
commences
that is ac-
companied by
somnolence)
75-120ml
200-250ml
external cer-
v ical as b ut
semen enters
uterus at high
pressu re
ce rvix and
uterus
10 30 to 40
20 60 to 80
3 2 0
3 8
Ram/Ewe-courtesy of Drs. G.S. Lewis and J.B. Taylor. U.S. Sheep Experimental Station http://p wa.ars.usda.go v/dubois!index
Bull/Cow-courtesy of Dr. L.S. Katz, Rutgers University
Stallion/Mare-courtesy of Dr. A. Tibary, Washington State University, College of Veterinary Medicine
Reproductive Behavior 243
Figure 11-11. Characteristics of Copulation , Site of Seminal Deposition
and Number of Ejaculations to Satiation and Exhaustion in the Camel
and Llama
Duration of Volume of Site of Average Maximum
Mating pair Copulation Ejaculate Semen Number of Number of
(Range) Deposition Ejaculations Ejacu lations
to Satiation to Exhaustion
6-20 minutes, 3-8m I Partly intrauter- 23 matings Data not
ex t ension of ine, partly intrac- in 24 hr available
neck, straining erv ic al , some
of the body, intravaginal
multiple ejacu-
l a t i ons per
copulation
20 – 30 min – 1-5ml intrauterine Data not Data not
u tes, bo d y ava ilable available
tremo rs an d
pelvic thrusts
(Photos courtesy o f Dr. A. Tibary, Washington State Un iversity, College of Veterinary Medicine)
species, such as the boar, stallion and dog, emission oc-
curs in a sequence resulting in an ej aculate that consists
of various fluid fractions (See Chapter I 2).
Postcopulatory behavior involves
refractivity and recovery.
Following ej aculation , all males experience a
refractory per iod before a second ejaculation can occur.
The length oftime of this refractory period depends on
several factors. These fac tors are ; degree of sexual
rest prior to copulation, age of the male, species of the
male, degree of fe ma le novelty and number of previous
ej aculations. The postcopulatory refractory period is
sometimes erroneously refeiTed to as sexual exhaustion.
The refractory period should be considered as part of
satiation rather than exhausti on. With natural service, it
is quite nonnal for a male to copulate repeatedly with the
same female. For example, a stall ion will breed a mare
in heat 5 to I 0 times during one estrus period. Rams are
noted to remate with the same ewe 4 to 5 tim es. Bulls
also remate w ith estrous cows repeatedly. In fact, it has
been noted in most species that if more than one fema le
is in heat at the same time, some males will generally
copulate preferentially with one and sometimes wi ll not
copulate with a second female. Boars nonnally serve
sows severa l times over a period of 1 to 2 days.
Sexual satiation refers to a condition in whi ch
fi1rther stimul i will not cause immediate responsive-
ness or motivation under a given set of stimulus con-
ditions. Restimu lation may occur after the refractory
period. F igures I I – 1 0 and I 1- I I compare the normal
number of ejaculations to satiety and the number of
ejacu lations to exhaustion among species. Exhaustion
is the condition whereby no further sexu a l behavior
can be induced even if sufficient stimuli are present.
As you can see fro m Figures 11 – 1 0 and 11-11 , there
is a large variation in the behav ioral reserves (the
behav ioral cap acity, or libido) among species. There
is also a large variation in libido within species. For
example, beef bulls have significantly lower behav-
ioral reserves than dairy bulls. While the factors that
contro l the degree of reproductive behavior among
males are poorly understood, they are almost certa inly
governed by genetic factors as well as environmental
fac tors.
Reproductive behavior can be
enhanced by:
• introducing novel stimulus animals
• changing stimulus settings
V
et
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oo
ks
.ir
‘
[ill
242 Reproductive Behavior
Figure 11-10. Characteristics of Copulation, Site of Seminal Deposition
and Number of Ejaculations to Satiation and Exhaustion in the Ram,
Bull, Stallion and Boar
Mating pair
Photos of:
Deposition Ejaculations Ejaculations
to Satiation to Exhaustion
1 to 2 sec- .8 to 1 ml e xt e r n a I
onds (1 pel- (.1 to 2ml) cervical os
1 to 3 sec- 3-5ml fornix vagina
onds (1 pel- (.5 to 12ml)
commences
that is ac-
companied by
somnolence)
75-120ml
200-250ml
external cer-
v ical as b ut
semen enters
uterus at high
pressu re
ce rvix and
uterus
10 30 to 40
20 60 to 80
3 2 0
3 8
Ram/Ewe-courtesy of Drs. G.S. Lewis and J.B. Taylor. U.S. Sheep Experimental Station http://p wa.ars.usda.go v/dubois!index
Bull/Cow-courtesy of Dr. L.S. Katz, Rutgers University
Stallion/Mare-courtesy of Dr. A. Tibary, Washington State University, College of Veterinary Medicine
Reproductive Behavior 243
Figure 11-11. Characteristics of Copulation , Site of Seminal Deposition
and Number of Ejaculations to Satiation and Exhaustion in the Camel
and Llama
Duration of Volume of Site of Average Maximum
Mating pair Copulation Ejaculate Semen Number of Number of
(Range) Deposition Ejaculations Ejacu lations
to Satiation to Exhaustion
6-20 minutes, 3-8m I Partly intrauter- 23 matings Data not
ex t ension of ine, partly intrac- in 24 hr available
neck, straining erv ic al , some
of the body, intravaginal
multiple ejacu-
l a t i ons per
copulation
20 – 30 min – 1-5ml intrauterine Data not Data not
u tes, bo d y ava ilable available
tremo rs an d
pelvic thrusts
(Photos courtesy o f Dr. A. Tibary, Washington State Un iversity, College of Veterinary Medicine)
species, such as the boar, stallion and dog, emission oc-
curs in a sequence resulting in an ej aculate that consists
of various fluid fractions (See Chapter I 2).
Postcopulatory behavior involves
refractivity and recovery.
Following ej aculation , all males experience a
refractory per iod before a second ejaculation can occur.
The length oftime of this refractory period depends on
several factors. These fac tors are ; degree of sexual
rest prior to copulation, age of the male, species of the
male, degree of fe ma le novelty and number of previous
ej aculations. The postcopulatory refractory period is
sometimes erroneously refeiTed to as sexual exhaustion.
The refractory period should be considered as part of
satiation rather than exhausti on. With natural service, it
is quite nonnal for a male to copulate repeatedly with the
same female. For example, a stall ion will breed a mare
in heat 5 to I 0 times during one estrus period. Rams are
noted to remate with the same ewe 4 to 5 tim es. Bulls
also remate w ith estrous cows repeatedly. In fact, it has
been noted in most species that if more than one fema le
is in heat at the same time, some males will generally
copulate preferentially with one and sometimes wi ll not
copulate with a second female. Boars nonnally serve
sows severa l times over a period of 1 to 2 days.
Sexual satiation refers to a condition in whi ch
fi1rther stimul i will not cause immediate responsive-
ness or motivation under a given set of stimulus con-
ditions. Restimu lation may occur after the refractory
period. F igures I I – 1 0 and I 1- I I compare the normal
number of ejaculations to satiety and the number of
ejacu lations to exhaustion among species. Exhaustion
is the condition whereby no further sexu a l behavior
can be induced even if sufficient stimuli are present.
As you can see fro m Figures 11 – 1 0 and 11-11 , there
is a large variation in the behav ioral reserves (the
behav ioral cap acity, or libido) among species. There
is also a large variation in libido within species. For
example, beef bulls have significantly lower behav-
ioral reserves than dairy bulls. While the factors that
contro l the degree of reproductive behavior among
males are poorly understood, they are almost certa inly
governed by genetic factors as well as environmental
fac tors.
Reproductive behavior can be
enhanced by:
• introducing novel stimulus animals
• changing stimulus settings
V
et
B
oo
ks
.ir
244 Reproductive Behavior
Figure 11-12. Copulation in the Dog
First Stage Coitus
(1-2 min)
The Turn
(2-5 sec)
The male and female remain “tied” together be-
cause the bulbus glandis of the penis remains
engorged with blood after the turn. Contractions
of the muscles near the base of the penis prevent
venous outflow of blood from the bulbus glandis.
Also, the sphincter muscles of the vulva constrict
thus compressing the dorsal veins of the penis
preventing blood from leaving. (Figures modified from
Grandage. 1972. Vet. Rec. 91:141)
The vascu lature of the dog penis has been in-
jected with latex and the tissue dissolved away
leaving cast of the vascu lature. Red vessels
are arteries and the blue vessels are veins.
IL=IIeum, MCA=Medial Caudal Artery, LCA=Lateral
Caudal Artery, IS=Ischium , A=Ace tabulum ,
CS=Corpus Spong iousum, CC=Corpus Caver-
nosum, DPA=Dorsal Penile Artery, DPV=Dorsal
Penile Vein, OP=Os Penis, BG=Bulbus Glandis,
PLG=Pars Longa Glandis, PA=Prostatic A rtery,
I P=lnternal Pudendal Artery, IIA=InternallliacArtery
(Specimen courtesy of the W orthman Veterinary Anatom y Teach-
ing Museum, College of Veterinary Medicine, Washington State
University. Specimen prepared by Dr. R.P. Worthman)
First Stage Coitus
The male mounts the female in a manner typical
of a quadraped . The female holds the tail to one
side and the penis is introd uced into the vagina by
a few thrusting movements. This stage of copula-
tion lasts for only 1-2 minutes. The first and second
fractions of semen are ejaculated during the first
stage coitus.
The Turn
This is the transition between first stage and se c-
ond stage co itus. Shortly after ejaculation, the dog
dismounts and turns around while lifting one hind
leg over the bitch.
Second Stage Coitus
After the turn, the animals stand with their hind
quarters in contact and their heads facing opposite
directions. The third fraction of semen is ejaculated
during this stage. Second stage coitus may last
from 5-45 minutes. It is believed that the purpose
of second stage coitus is to encourage uterine
rather than vag inal insemination. Turning around
discourages detumescence of the penis and there-
fore maintains high intravaginal pressu re. The dog
steadily ejaculates up to 30-ml of seminal fluid that
is delivered through the cervix into the uterus. This
phenomenon tends to force the sperm-rich fraction
into the uterus. The copulatory behavior described
here is perfectly natural. Unfortunately this behavio r
is often interpreted as being unnatural and attempts
to break the “tie” are often made by the uninformed.
Such intervention com promises fertility because
delivery of semen to the uterus over a sustained
period of time is reduced.
Reproductive Behavior 245
Figure 11-13. Major Steps in Ejaculation
Afferent
Sensory stimulat io n of glans penis
(temperature and pr essure)
Int r omissio n
Reproductive Behavior and Spermatozoal
Output can be Manipulated
The degree of novelty of both the copulatmy
partner and the copulatmy environment can be of great
importance when managing reproductive behavior in
breeding males. U nder condi tions of artificial insemina-
tion, where repeated seminal collectio n is necessary to
maximize the harvest of spermatozoa, understanding the
influence of novelty and mating situations is impor tant.
The ” Coolidge Effect” is defined as the restoration of
mating behavior in mal es (that have reached sexual
satiation) when the origina l fema le is replaced by a
novel female . In other words, a sexually sati ated ma le
can be restimulated if exposed to a novel female. (For
derivation of the term “Cool idge E ffect” see Further
Phenomena for Fertility)
Semen collection in bull studs can occur as
frequently as 4 to 6 ejaculations per week. In o rder
for this collection frequency to be successful, the male
0
Sudden and pow erful contractio n of
urethralis, bulbospo ngiosus and
ischiocavernosus m uscles
0
Expulsion of semen
m ust first be sexu ally stimulated. Sexual stimula tion
is defin ed as the presentation of a stimulus situation that
w ill achieve mounting and ej acu lation. The purpose of
sexual stimulation is to o btain ejaculation or mating in
the shmtest time possible so that manpower involved
in managi ng the mating of animals can be minimized.
Ther e are three approaches used to re-induce sexual
stimul atio n in bulls u sed for artificia l insemination.
These approaches ar e: to introduce a novel stimulus
animal; to change the stim ulus setting; or both. Pre-
sentatio n of nove l stim ulus animals reinitiates sexual
behavior after sexual satiation in bulls (See F igure 11 –
14, “Novel Fema les”). A second approach to achieve
sexual stimulation after satiation is to present familiar
stimulus animals in new stimulus situations . In other
words, changing the location or setting has a stimulatmy
effect on the satiated male (See F igure 11- 14 “New Lo-
cation”). In cases where sexual stimulation is difficult
to achieve, presenting a novel stimulus animal, coupled
with changing locatio ns, often has positive effects.
V
et
B
oo
ks
.ir
244 Reproductive Behavior
Figure 11-12. Copulation in the Dog
First Stage Coitus
(1-2 min)
The Turn
(2-5 sec)
The male and female remain “tied” together be-
cause the bulbus glandis of the penis remains
engorged with blood after the turn. Contractions
of the muscles near the base of the penis prevent
venous outflow of blood from the bulbus glandis.
Also, the sphincter muscles of the vulva constrict
thus compressing the dorsal veins of the penis
preventing blood from leaving. (Figures modified from
Grandage. 1972. Vet. Rec. 91:141)
The vascu lature of the dog penis has been in-
jected with latex and the tissue dissolved away
leaving cast of the vascu lature. Red vessels
are arteries and the blue vessels are veins.
IL=IIeum, MCA=Medial Caudal Artery, LCA=Lateral
Caudal Artery, IS=Ischium , A=Ace tabulum ,
CS=Corpus Spong iousum, CC=Corpus Caver-
nosum, DPA=Dorsal Penile Artery, DPV=Dorsal
Penile Vein, OP=Os Penis, BG=Bulbus Glandis,
PLG=Pars Longa Glandis, PA=Prostatic A rtery,
I P=lnternal Pudendal Artery, IIA=InternallliacArtery
(Specimen courtesy of the W orthman Veterinary Anatom y Teach-
ing Museum, College of Veterinary Medicine, Washington State
University. Specimen prepared by Dr. R.P. Worthman)
First Stage Coitus
The male mounts the female in a manner typical
of a quadraped . The female holds the tail to one
side and the penis is introd uced into the vagina by
a few thrusting movements. This stage of copula-
tion lasts for only 1-2 minutes. The first and second
fractions of semen are ejaculated during the first
stage coitus.
The Turn
This is the transition between first stage and se c-
ond stage co itus. Shortly after ejaculation, the dog
dismounts and turns around while lifting one hind
leg over the bitch.
Second Stage Coitus
After the turn, the animals stand with their hind
quarters in contact and their heads facing opposite
directions. The third fraction of semen is ejaculated
during this stage. Second stage coitus may last
from 5-45 minutes. It is believed that the purpose
of second stage coitus is to encourage uterine
rather than vag inal insemination. Turning around
discourages detumescence of the penis and there-
fore maintains high intravaginal pressu re. The dog
steadily ejaculates up to 30-ml of seminal fluid that
is delivered through the cervix into the uterus. This
phenomenon tends to force the sperm-rich fraction
into the uterus. The copulatory behavior described
here is perfectly natural. Unfortunately this behavio r
is often interpreted as being unnatural and attempts
to break the “tie” are often made by the uninformed.
Such intervention com promises fertility because
delivery of semen to the uterus over a sustained
period of time is reduced.
Reproductive Behavior 245
Figure 11-13. Major Steps in Ejaculation
Afferent
Sensory stimulat io n of glans penis
(temperature and pr essure)
Int r omissio n
Reproductive Behavior and Spermatozoal
Output can be Manipulated
The degree of novelty of both the copulatmy
partner and the copulatmy environment can be of great
importance when managing reproductive behavior in
breeding males. U nder condi tions of artificial insemina-
tion, where repeated seminal collectio n is necessary to
maximize the harvest of spermatozoa, understanding the
influence of novelty and mating situations is impor tant.
The ” Coolidge Effect” is defined as the restoration of
mating behavior in mal es (that have reached sexual
satiation) when the origina l fema le is replaced by a
novel female . In other words, a sexually sati ated ma le
can be restimulated if exposed to a novel female. (For
derivation of the term “Cool idge E ffect” see Further
Phenomena for Fertility)
Semen collection in bull studs can occur as
frequently as 4 to 6 ejaculations per week. In o rder
for this collection frequency to be successful, the male
0
Sudden and pow erful contractio n of
urethralis, bulbospo ngiosus and
ischiocavernosus m uscles
0
Expulsion of semen
m ust first be sexu ally stimulated. Sexual stimula tion
is defin ed as the presentation of a stimulus situation that
w ill achieve mounting and ej acu lation. The purpose of
sexual stimulation is to o btain ejaculation or mating in
the shmtest time possible so that manpower involved
in managi ng the mating of animals can be minimized.
Ther e are three approaches used to re-induce sexual
stimul atio n in bulls u sed for artificia l insemination.
These approaches ar e: to introduce a novel stimulus
animal; to change the stim ulus setting; or both. Pre-
sentatio n of nove l stim ulus animals reinitiates sexual
behavior after sexual satiation in bulls (See F igure 11 –
14, “Novel Fema les”). A second approach to achieve
sexual stimulation after satiation is to present familiar
stimulus animals in new stimulus situations . In other
words, changing the location or setting has a stimulatmy
effect on the satiated male (See F igure 11- 14 “New Lo-
cation”). In cases where sexual stimulation is difficult
to achieve, presenting a novel stimulus animal, coupled
with changing locatio ns, often has positive effects.
V
et
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ks
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II
246 Reproductive Behavior
Figure 11-14. Introduction of Novel Females and a Change of
Locations has a Positive Effect on Mounting Behavior
(Hypothetical examples, not experimental data)
Familiar Female
A familia r female may stimulate a
lllllllll —— “——lll — ——–” —– —-1
bull to mount about 12 times in an 8
112 Mounts I hour period.
SS= sexual satiation
I I I I I I I I I
0 I 2 3 4 5 6 7 8
Time (h)
Familiar Female and New Location Bulls can be restimulated to mount
+ New location + New location (after satiation) by changing the
! lllllllll — ·;.II IIlli — Jiilll stimulus setting (new location) . This induces more total mounts (18 liB Mounts I mounts) than the familia r female (12 I I mounts).
0 I 2 3 4 5 6 7 8
Time(h)
Novel Females
When the novel females (1-5) are
introduced after a period of sexual
llillllll Ji IIlli _,[ 1111 — – I satiation, mounting behavior is stimulated beyond that realized with change of location and exposure to a sing le familiar female (24 mounts
I I I I I I I I I
0 I 2 3 4 5 6 7
Time (h)
There has been little research conducted on the
effect of introducing novel animals upon stimulation of
mounting behavior in the female. However, it has been
shown that dairy cows will mount novel cows with a
greater frequency than they do familiar cows. As you
might expect, the effect of novelty is confounded with
the stage of the cycle.
Sexual preparation prolongs sexual
stimulation and increases
spermatozoa per ejaculation.
In order to maximize the output of spermatozoa
per ejaculate, sexual preparation is necessary. Sexual
preparation is extending the period of sexual stimula-
tion beyond that needed for mounting and ejaculation.
8
vs. 18 and 12 respectively).
Sexual preparation pro longs the precopulatory stage of
reproductive behavior. The purpose of sexual prepara-
tion is to collect semen containing the greatest possible
number of spemmtozoa per ejaculation. Figure ll-15
illustrates the phys iologic me chanisms believed to be
responsible for enhancing spermatozoal numbers in
the ejaculate. Three approaches are used to sexually
prepare a male. These are: false-mounting, restraint
and false-mounting plus restraint.
Sexual preparation may include:
• false-mounting
• restraint
• false-mounting plus restraint
Reproductive Behavior 24 7
Figure 11-15. Major Steps in Sexual Preparation Resulting in Transport
of Spermoatozoa from the Tail of the Epididymis into the Pelvic Urethra
Sensory stimulation
(optic, olfactory, tactile and aud itory)
Affe re nt
Transport of spermatozoa
into an ejaculatory position
• Stim ulation o f nerves in the
supraoptic and paraventricular nuclei
0
Contractions of smooth muscle in distal tail of
epididymis and ductus deferens
[ill
I
V
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II
246 Reproductive Behavior
Figure 11-14. Introduction of Novel Females and a Change of
Locations has a Positive Effect on Mounting Behavior
(Hypothetical examples, not experimental data)
Familiar Female
A familia r female may stimulate a
lllllllll —— “——lll — ——–” —– —-1
bull to mount about 12 times in an 8
112 Mounts I hour period.
SS= sexual satiation
I I I I I I I I I
0 I 2 3 4 5 6 7 8
Time (h)
Familiar Female and New Location Bulls can be restimulated to mount
+ New location + New location (after satiation) by changing the
! lllllllll — ·;.II IIlli — Jiilll stimulus setting (new location) . This induces more total mounts (18 liB Mounts I mounts) than the familia r female (12 I I mounts).
0 I 2 3 4 5 6 7 8
Time(h)
Novel Females
When the novel females (1-5) are
introduced after a period of sexual
llillllll Ji IIlli _,[ 1111 — – I satiation, mounting behavior is stimulated beyond that realized with change of location and exposure to a sing le familiar female (24 mounts
I I I I I I I I I
0 I 2 3 4 5 6 7
Time (h)
There has been little research conducted on the
effect of introducing novel animals upon stimulation of
mounting behavior in the female. However, it has been
shown that dairy cows will mount novel cows with a
greater frequency than they do familiar cows. As you
might expect, the effect of novelty is confounded with
the stage of the cycle.
Sexual preparation prolongs sexual
stimulation and increases
spermatozoa per ejaculation.
In order to maximize the output of spermatozoa
per ejaculate, sexual preparation is necessary. Sexual
preparation is extending the period of sexual stimula-
tion beyond that needed for mounting and ejaculation.
8
vs. 18 and 12 respectively).
Sexual preparation pro longs the precopulatory stage of
reproductive behavior. The purpose of sexual prepara-
tion is to collect semen containing the greatest possible
number of spemmtozoa per ejaculation. Figure ll-15
illustrates the phys iologic me chanisms believed to be
responsible for enhancing spermatozoal numbers in
the ejaculate. Three approaches are used to sexually
prepare a male. These are: false-mounting, restraint
and false-mounting plus restraint.
Sexual preparation may include:
• false-mounting
• restraint
• false-mounting plus restraint
Reproductive Behavior 24 7
Figure 11-15. Major Steps in Sexual Preparation Resulting in Transport
of Spermoatozoa from the Tail of the Epididymis into the Pelvic Urethra
Sensory stimulation
(optic, olfactory, tactile and aud itory)
Affe re nt
Transport of spermatozoa
into an ejaculatory position
• Stim ulation o f nerves in the
supraoptic and paraventricular nuclei
0
Contractions of smooth muscle in distal tail of
epididymis and ductus deferens
[ill
I
V
et
B
oo
ks
.ir
248 Reproductive Behavior
False mounting consists of manually deviat-
ing the penis during a mount so that intromission can-
not occur. If intromission does not occur, ejaculation
usually does not occur. Restraint prevents the male
from mounting even though he wishes to do so. Gen-
erally, restraint is for two to tlu·ee minutes within two
or three feet of the stimulus animal. A combination of
false mounting and restraint will result in the greatest
improvement of spemmtozoal output.
In dairy bulls, the recommended procedures
for sexual preparation are: one false mount followed
by two minutes of restraint, followed by two additional
false mounts before each ejaculation. In beef bulls,
sexual preparation involves three false mounts with no
restraint. In general, beef bulls have lower behavioral
reserves (libido) than dairy bulls and thus have a less
rigorous sexual preparation regimen.
While sexual preparation is taking place, re-
lease of oxytocin from the posterior pituitary occurs.
Oxytocin causes contraction of the smooth musculature
surrounding the tail of the epididymis and the ductus
deferens. These contractions transport spem1atozoa
from the tail of the epididymis into the duchts deferens
and eventually into the pelvic urethra. Once spem1 gain
entrance into the pelvic urethra, they begin to mix with
secretions from the accessory sex glands.
Homosexual-like Behavior
Homosexual-like behavior is common among
domestic animals and is particularly common in cattle.
The tenn homosexuality implies a sexual preference for
same-sex partners. In animals, there is not a preference,
but rather indiscriminate orientation or same-sex di-
rected behavior. Thus, an alternative term that is appli-
cable to sub-primate animals would be homosexual-like
behavior. Cows and bulls exhibit strong homosexual-
like behavior. Similar behavior is seen in sheep and
dogs and to a Jesser extent in swine and horses. Such
behavior has profound usefulness for detecting cattle
in estrus. When a female stands to be mounted by
another cow, this alerts the management team that
the cow is in estrus and artificial insemination can be
performed. A favorite question of managers and stu-
dents of reproductive physiology alike is, “What is the
evolutionary advantage of animals displaying this kind
of behavior?” While a definitive answer is not known,
two theories exist to explain female-female mounting
behavior in cattle.
The first explanation theorizes that cows
mounting each other provide a visual signal that attracts
a bull to the cow in estrus. In other words, when a bull
sees cows mounting each other he will investigate and
if the cow is in standing estrus, he will breed her.
The second theory explaining the evolution of
homosexual-like behavior among cows involves inad-
vertent genetic selection by man fo r this behavior. It has
been proposed that cattle of European descent were se-
lected by humans for their estrous behavior. In Medieval
Europe, cattle husbandry involved the use of a few cows
by each peasant farmer for three purposes: dra ft, milk
and meat. Peasant f.·mners could not afford to maintain a
bull for breeding purposes since the bulls gave no milk,
gave birth to no calves and had obnoxious behavior that
made them unsuitable for everyday management. In
addition, most bulls apparently were owned by wealthy
land holders who probably controlled the breeding, as
well as the financial aspects of cattle management. Since
most cows were k ept in groups without intact males, the
herdsmen needed some sign to tell him when his cows
should be bred. Obviously, the cow that showed the most
intense mounting behavior was the one most likely to be
observed by the peasant and most likely to be bred by
the nobleman’s bull. Those that showed little mount-
ing behavior did not become pregnant in a reasonable
amount of time. This theory suggests that cows with a
high degree of mounting behavior were inadvertently
selected because they were noticed by man and offered
a greater opporhmity to become pregnant. T hus, this
behavioral trait was transmitted to their offspring.
Artificial Insemination Requires an
Understanding of Reproductive Behavior
and Physiology
Th ere are two fundamental ways to collect
semen from the male. The prefe rred method utilizes
an artificial vagina or a device that simulates vaginal
conditions of a female in estrus. The second method
relies on electrical stimulation of the accessory sex
glands and the pelvic urethra and this method is called
electroejaculation. Electroejaculation is generally used
in males of high genetic value that cannot physically
perform mounting and ejaculation. In the beef industry,
electroejaculation is used in range bulls.
Typical artificial vaginas for domestic animals
are shown are Figure 11-I 6. In general, artificial vaginas
consist of an outer casing fashioned of reinforced rubber
and a liner that is generally made of rubber that can be
lubricated. Tempera hire and pressure are controlled by
the water that is p laced between the casing and the liner.
One end of the artificial vagina is attached to a funnel-
like cone that in tum is attached to a collection vessel,
usually a nonbreakable graduated test tube.
From a behavioral perspective, males that are
to be collected with an artificial vagina need some form
of training. Males with previous sexual experience will
readily mount a surrogate animal (artificial animal or
“dummy”). The degree to which animals will mount
Reproductive Behavior 249
Figure 11-16. Artificial Vaginas for Various Animals
Outer casing
l — Warm water
Rubbe r
liner – Warm water
Rubber
co llection
funne l
I
-· ; – .–………
The typical artificia l vagina consists of
a sturdy outer casing, a rubber liner, a
chamber fi ll ed with warm water, a rubber
collection funnel and a collection tube.
tube
The artificial vag ina for the stallion consists of a
leather outer casing (C) equiped with a port to
drain water (arrow). The collection vessel (CV)
and the protective covering (PC) are shown. Ide-
ally, ej aculation takes place in the collection cone
(CC) so that most of the semen will drain directly
into the collection vessel. (Artific ial vagina courtesy of
Northwest Equine Reproduction Laboratory, University of Idaho,
www.avs.uidaho.edu/nerl)
The artificial vagina for the bull consists of a black
casing (C), a rubber liner (RL) a collection cone
(CC) and a collection vessel (CV). Wate r is placed
between the casing and the liner. The proper tem-
peratu re is critical for successful ejaculation in the
bull . Wh ile not shown in the photograph a protec-
tive covering is placed over the cone and collection
vessel to prevent cold shock of the semen.
The artificial vagina for the boar consists of a bulb
that can apply pressure to the artificial vag ina. High
pressure is obligatory fo r stimulation of the glans
pen is and ej aculation in the boar. The artificial
vagi na for the boar also consists of an outer casing
(C), a liner (L) and a protective covering (PC) that
houses the collection vessel. (Photograph courtes y of
MinitO b Germany, www.minitilb.de)
The artificial vagina for coll ection of semen from
rams and bucks consists of a rubber casing (C)
with a valve (arrow) through which water can be
added or emptied, a rubber liner and a collection
vessel (CV). The protective covering (PC) is shown
above the artificial vagina. (Photograph courtesy of
MinitOb Germany, www.minitilb.de)
V
et
B
oo
ks
.ir
248 Reproductive Behavior
False mounting consists of manually deviat-
ing the penis during a mount so that intromission can-
not occur. If intromission does not occur, ejaculation
usually does not occur. Restraint prevents the male
from mounting even though he wishes to do so. Gen-
erally, restraint is for two to tlu·ee minutes within two
or three feet of the stimulus animal. A combination of
false mounting and restraint will result in the greatest
improvement of spemmtozoal output.
In dairy bulls, the recommended procedures
for sexual preparation are: one false mount followed
by two minutes of restraint, followed by two additional
false mounts before each ejaculation. In beef bulls,
sexual preparation involves three false mounts with no
restraint. In general, beef bulls have lower behavioral
reserves (libido) than dairy bulls and thus have a less
rigorous sexual preparation regimen.
While sexual preparation is taking place, re-
lease of oxytocin from the posterior pituitary occurs.
Oxytocin causes contraction of the smooth musculature
surrounding the tail of the epididymis and the ductus
deferens. These contractions transport spem1atozoa
from the tail of the epididymis into the duchts deferens
and eventually into the pelvic urethra. Once spem1 gain
entrance into the pelvic urethra, they begin to mix with
secretions from the accessory sex glands.
Homosexual-like Behavior
Homosexual-like behavior is common among
domestic animals and is particularly common in cattle.
The tenn homosexuality implies a sexual preference for
same-sex partners. In animals, there is not a preference,
but rather indiscriminate orientation or same-sex di-
rected behavior. Thus, an alternative term that is appli-
cable to sub-primate animals would be homosexual-like
behavior. Cows and bulls exhibit strong homosexual-
like behavior. Similar behavior is seen in sheep and
dogs and to a Jesser extent in swine and horses. Such
behavior has profound usefulness for detecting cattle
in estrus. When a female stands to be mounted by
another cow, this alerts the management team that
the cow is in estrus and artificial insemination can be
performed. A favorite question of managers and stu-
dents of reproductive physiology alike is, “What is the
evolutionary advantage of animals displaying this kind
of behavior?” While a definitive answer is not known,
two theories exist to explain female-female mounting
behavior in cattle.
The first explanation theorizes that cows
mounting each other provide a visual signal that attracts
a bull to the cow in estrus. In other words, when a bull
sees cows mounting each other he will investigate and
if the cow is in standing estrus, he will breed her.
The second theory explaining the evolution of
homosexual-like behavior among cows involves inad-
vertent genetic selection by man fo r this behavior. It has
been proposed that cattle of European descent were se-
lected by humans for their estrous behavior. In Medieval
Europe, cattle husbandry involved the use of a few cows
by each peasant farmer for three purposes: dra ft, milk
and meat. Peasant f.·mners could not afford to maintain a
bull for breeding purposes since the bulls gave no milk,
gave birth to no calves and had obnoxious behavior that
made them unsuitable for everyday management. In
addition, most bulls apparently were owned by wealthy
land holders who probably controlled the breeding, as
well as the financial aspects of cattle management. Since
most cows were k ept in groups without intact males, the
herdsmen needed some sign to tell him when his cows
should be bred. Obviously, the cow that showed the most
intense mounting behavior was the one most likely to be
observed by the peasant and most likely to be bred by
the nobleman’s bull. Those that showed little mount-
ing behavior did not become pregnant in a reasonable
amount of time. This theory suggests that cows with a
high degree of mounting behavior were inadvertently
selected because they were noticed by man and offered
a greater opporhmity to become pregnant. T hus, this
behavioral trait was transmitted to their offspring.
Artificial Insemination Requires an
Understanding of Reproductive Behavior
and Physiology
Th ere are two fundamental ways to collect
semen from the male. The prefe rred method utilizes
an artificial vagina or a device that simulates vaginal
conditions of a female in estrus. The second method
relies on electrical stimulation of the accessory sex
glands and the pelvic urethra and this method is called
electroejaculation. Electroejaculation is generally used
in males of high genetic value that cannot physically
perform mounting and ejaculation. In the beef industry,
electroejaculation is used in range bulls.
Typical artificial vaginas for domestic animals
are shown are Figure 11-I 6. In general, artificial vaginas
consist of an outer casing fashioned of reinforced rubber
and a liner that is generally made of rubber that can be
lubricated. Tempera hire and pressure are controlled by
the water that is p laced between the casing and the liner.
One end of the artificial vagina is attached to a funnel-
like cone that in tum is attached to a collection vessel,
usually a nonbreakable graduated test tube.
From a behavioral perspective, males that are
to be collected with an artificial vagina need some form
of training. Males with previous sexual experience will
readily mount a surrogate animal (artificial animal or
“dummy”). The degree to which animals will mount
Reproductive Behavior 249
Figure 11-16. Artificial Vaginas for Various Animals
Outer casing
l — Warm water
Rubbe r
liner – Warm water
Rubber
co llection
funne l
I
-· ; – .–………
The typical artificia l vagina consists of
a sturdy outer casing, a rubber liner, a
chamber fi ll ed with warm water, a rubber
collection funnel and a collection tube.
tube
The artificial vag ina for the stallion consists of a
leather outer casing (C) equiped with a port to
drain water (arrow). The collection vessel (CV)
and the protective covering (PC) are shown. Ide-
ally, ej aculation takes place in the collection cone
(CC) so that most of the semen will drain directly
into the collection vessel. (Artific ial vagina courtesy of
Northwest Equine Reproduction Laboratory, University of Idaho,
www.avs.uidaho.edu/nerl)
The artificial vagina for the bull consists of a black
casing (C), a rubber liner (RL) a collection cone
(CC) and a collection vessel (CV). Wate r is placed
between the casing and the liner. The proper tem-
peratu re is critical for successful ejaculation in the
bull . Wh ile not shown in the photograph a protec-
tive covering is placed over the cone and collection
vessel to prevent cold shock of the semen.
The artificial vagina for the boar consists of a bulb
that can apply pressure to the artificial vag ina. High
pressure is obligatory fo r stimulation of the glans
pen is and ej aculation in the boar. The artificial
vagi na for the boar also consists of an outer casing
(C), a liner (L) and a protective covering (PC) that
houses the collection vessel. (Photograph courtes y of
MinitO b Germany, www.minitilb.de)
The artificial vagina for coll ection of semen from
rams and bucks consists of a rubber casing (C)
with a valve (arrow) through which water can be
added or emptied, a rubber liner and a collection
vessel (CV). The protective covering (PC) is shown
above the artificial vagina. (Photograph courtesy of
MinitOb Germany, www.minitilb.de)
V
et
B
oo
ks
.ir
250 Reproductive Behavior
Figure 11-17. Surrogate Stimulus Animals for Semen Collection
“Phantom” for Stallion Semen Collection
In general, males of most species can be trained
to mount and ejaculate using surrogate stimulus
animals. A surrogate stimulus animal provides
ease of cleaning and minimizes the risk of injury
and disease transmission . Further, surrogate
stimulus animals do not require feed, hous-
ing and labor for maintenance as does a live
stimulus animal. The use of artificial stimulus
animals req uires previous training of the male.
Once the male has been trained he will gener-
ally mount the “dummy” readily. The size can be
adjusted easily to accomodate various males.
Mobile surrogate stimulus animals are used fo r
collection of semen in bulls because the location
can be changed with ease.
The surrogate stimulus animal used to collect semen from the stallion is generally referred to as a “phantom”. The “phan-
tom” contains a biting belt (arrow) to provide the stallion with a surface to bite during mounting thus provid ing a means
for natural behavior. All of the devices shown have a built-in artificial vagina in which the temperature and pressure can
be controlled. (Photographs courtesy of Mini tOb Germany, www.minitDb.de)
dummies depends on the amount of training provided.
A surrogate stimulus animal provides the advantage of
safety, reduced expense and they can be designed to
accomodate males of various stature. The disadvantage
of using surrogate stimulus animal is that changing lo-
cations and teasers is difficult. Figure 11-17 illustrates
examples of surrogate animals for semen collection.
Sometimes it is diffi cult to train animals to
mount either a stimulus animal or a surrogate stimulus
animal. In this event, semen can be collected by plac-
ing a condom-like structure inside the vagina of the
female in estrus. When the male mounts the female
and ejaculates , the semen is deposited inside the vessel.
Such techniques are valuable when animals have not
been adequately trained.
The design of an artificial vagina
should accomplish the following:
• provide a suitable environment for
stimulation of the glans penis
• provide an environment that prevents
damage to the penis
• provide an environment that maxi-
mizes sperm recove1y and minimizes
sperm insult
Further
PHENOMENA
for Fertility
One day President and Mrs. Coolidge were
visiting a government farm. Soon after
their arrival they were taken off on sepa-
rate tours. Wizen M rs. Coolidge passed
the chicken pens, she paused to ask the
man in charge if the rooster copulated
more than once each day. “Dozens of
times,” was the reply. “Please tell that to
the President, ” Mrs. Coolidge requested.
Wizen the President passed the pens am/
was told about the rooste1; he asked, “Same
h en eve1y day?” “Oft no, Mr. President,
a different one each time. ” The President
nodded slowly and then said, “Please tell
that to Mrs. Coolidge.”
The praying mantis has mwsual reproduc-
tive As soon as the male mounts
the female and accomplishes intromission,
the f emale bites his head off. She imme-
diately eats the top half of his body while
intromission is still taking place. The rea-
son for this behavior is because ejaculation
is permanently inhibited in the male and
can take place only after the head has been
removed. It is not known whether the slang
phrase “bite-your-head-off’ was derived
from this behavior.
Roman snails shoot love darts at one an-
other before copulation to determine if they
are both members of the same species.
Some male insects (certain flies and mos-
quitoes) have evolved mmsual adaptations
to ins me that their genetics will be passed
on. Males have a sharp, specialized penis
that can enter a pupa. The male insemi-
nates the rmbom female.
When a grey squirrel comes into estrus, up
to a dozm males noisily chase her through
the trees. This chase is necessary, because
the female will not ovulate without it.
Reproductive Behavior 251
To mate, the queen bee leaves the hive and
p elforms a mating flight in an area where
drones are congregated. The fastest dron e
is the first to copulate with the queen. Copu-
lation is a11 hz-jfight event that lasts from
1 to 3 seconds. Wizen the copulating bees
separate, the entire male genitalia is ripped
from the male and stays with the queen. The
male soon dies am/ another male will then
mate with the queen. Up to 17 matings in
one mati11g flight have been observed.
Females of some species are quite choosy
about who gets to fertilize their eggs. In
these cases, mate choice is determined by
nuptial gifts presented by the male. Th e
female black-tipped hangffy accepts nuptial
gifts in the form of food in exchange for
copulation. Wizen edible food is presented
by the male, the duration of copu/atio11 is
depende11t on the size of the gift. If the gift
is small and can be consumed in 5 minutes
or less, the female will not allow mating. If
the gift is large (cannot be consumed in 20
minutes), the female will allow mating to
take place. If the gift provides a meal of
only 12 minutes she will/eave the gift-giver
prematurely and seek another gift-giver as
a mate.
Satin bowerbirds build their nests only with
blue objects. Males gather blue flo wers, pen
caps, berries and ribbons and arrange them
under bushes or in other cozy spots. If a
female “likes” what she sees, she will choose
the nest’s decorator as Iter mate.
A male newt begins his courtship by jump-
ing on the back of the female and rubbing
his jaw against h er snout. This releases
a scent that drives the female newt “crazy
with desire. ”
When female rhinoceri are in heat they will
run away from a male, then suddenly tum
and fight him horn-to-horn, sometimes for
longer than a day. Only if he is fit enough to
pursue will she submit. There are no “wimp
genes” in the rhinocerous gene pool.
[li[]
I
V
et
B
oo
ks
.ir
250 Reproductive Behavior
Figure 11-17. Surrogate Stimulus Animals for Semen Collection
“Phantom” for Stallion Semen Collection
In general, males of most species can be trained
to mount and ejaculate using surrogate stimulus
animals. A surrogate stimulus animal provides
ease of cleaning and minimizes the risk of injury
and disease transmission . Further, surrogate
stimulus animals do not require feed, hous-
ing and labor for maintenance as does a live
stimulus animal. The use of artificial stimulus
animals req uires previous training of the male.
Once the male has been trained he will gener-
ally mount the “dummy” readily. The size can be
adjusted easily to accomodate various males.
Mobile surrogate stimulus animals are used fo r
collection of semen in bulls because the location
can be changed with ease.
The surrogate stimulus animal used to collect semen from the stallion is generally referred to as a “phantom”. The “phan-
tom” contains a biting belt (arrow) to provide the stallion with a surface to bite during mounting thus provid ing a means
for natural behavior. All of the devices shown have a built-in artificial vagina in which the temperature and pressure can
be controlled. (Photographs courtesy of Mini tOb Germany, www.minitDb.de)
dummies depends on the amount of training provided.
A surrogate stimulus animal provides the advantage of
safety, reduced expense and they can be designed to
accomodate males of various stature. The disadvantage
of using surrogate stimulus animal is that changing lo-
cations and teasers is difficult. Figure 11-17 illustrates
examples of surrogate animals for semen collection.
Sometimes it is diffi cult to train animals to
mount either a stimulus animal or a surrogate stimulus
animal. In this event, semen can be collected by plac-
ing a condom-like structure inside the vagina of the
female in estrus. When the male mounts the female
and ejaculates , the semen is deposited inside the vessel.
Such techniques are valuable when animals have not
been adequately trained.
The design of an artificial vagina
should accomplish the following:
• provide a suitable environment for
stimulation of the glans penis
• provide an environment that prevents
damage to the penis
• provide an environment that maxi-
mizes sperm recove1y and minimizes
sperm insult
Further
PHENOMENA
for Fertility
One day President and Mrs. Coolidge were
visiting a government farm. Soon after
their arrival they were taken off on sepa-
rate tours. Wizen M rs. Coolidge passed
the chicken pens, she paused to ask the
man in charge if the rooster copulated
more than once each day. “Dozens of
times,” was the reply. “Please tell that to
the President, ” Mrs. Coolidge requested.
Wizen the President passed the pens am/
was told about the rooste1; he asked, “Same
h en eve1y day?” “Oft no, Mr. President,
a different one each time. ” The President
nodded slowly and then said, “Please tell
that to Mrs. Coolidge.”
The praying mantis has mwsual reproduc-
tive As soon as the male mounts
the female and accomplishes intromission,
the f emale bites his head off. She imme-
diately eats the top half of his body while
intromission is still taking place. The rea-
son for this behavior is because ejaculation
is permanently inhibited in the male and
can take place only after the head has been
removed. It is not known whether the slang
phrase “bite-your-head-off’ was derived
from this behavior.
Roman snails shoot love darts at one an-
other before copulation to determine if they
are both members of the same species.
Some male insects (certain flies and mos-
quitoes) have evolved mmsual adaptations
to ins me that their genetics will be passed
on. Males have a sharp, specialized penis
that can enter a pupa. The male insemi-
nates the rmbom female.
When a grey squirrel comes into estrus, up
to a dozm males noisily chase her through
the trees. This chase is necessary, because
the female will not ovulate without it.
Reproductive Behavior 251
To mate, the queen bee leaves the hive and
p elforms a mating flight in an area where
drones are congregated. The fastest dron e
is the first to copulate with the queen. Copu-
lation is a11 hz-jfight event that lasts from
1 to 3 seconds. Wizen the copulating bees
separate, the entire male genitalia is ripped
from the male and stays with the queen. The
male soon dies am/ another male will then
mate with the queen. Up to 17 matings in
one mati11g flight have been observed.
Females of some species are quite choosy
about who gets to fertilize their eggs. In
these cases, mate choice is determined by
nuptial gifts presented by the male. Th e
female black-tipped hangffy accepts nuptial
gifts in the form of food in exchange for
copulation. Wizen edible food is presented
by the male, the duration of copu/atio11 is
depende11t on the size of the gift. If the gift
is small and can be consumed in 5 minutes
or less, the female will not allow mating. If
the gift is large (cannot be consumed in 20
minutes), the female will allow mating to
take place. If the gift provides a meal of
only 12 minutes she will/eave the gift-giver
prematurely and seek another gift-giver as
a mate.
Satin bowerbirds build their nests only with
blue objects. Males gather blue flo wers, pen
caps, berries and ribbons and arrange them
under bushes or in other cozy spots. If a
female “likes” what she sees, she will choose
the nest’s decorator as Iter mate.
A male newt begins his courtship by jump-
ing on the back of the female and rubbing
his jaw against h er snout. This releases
a scent that drives the female newt “crazy
with desire. ”
When female rhinoceri are in heat they will
run away from a male, then suddenly tum
and fight him horn-to-horn, sometimes for
longer than a day. Only if he is fit enough to
pursue will she submit. There are no “wimp
genes” in the rhinocerous gene pool.
[li[]
I
V
et
B
oo
ks
.ir
252 Reproductive Behavior
During courtship the female balloon fly
will eat the male if given the chance. To
achieve copulation and keep from getting
eaten, the male will present the female with
a balloon-shaped cocoon as a “present”.
Unwrapping this “present” keeps the female
occupied long enough for the male to mate
her and fly off.
When box turtles copulate, the male mounts
the female and remains in an upright posi-
tion in order to facilitate insemination. The
pair may remain in this position for hours
to ensure adequate insemination. A t the
conclusion of the event the female will sud-
denly move away, sometimes causing the
male to fall p recariously on his hack where
he may remain until his death if h e can’t
right himself.
Most frogs and toads copulate in the dark.
They are often so eager to mate that the
male will try to momzt any thing that passes
by. They have been observed keeping a .firm
grip on strange objects and even other small
animals in the hope that they might turn out
to he females.
Th e long neck of the giraffe plays an im-
portant role in their reproductive
First the male samples the urine to
ascertain whether she is in estrus. If so, the
two giraffes then indulge in a form of sexual
preparation by entwining and rubbing their
necks together. Physiologically, this behav-
ior is like a false-mount and no doubt causes
the release of oxytocin that moves sperm
in the distal tail of the epididymis into an
ejaculatory position.
The p ressure within the penis of the bull at
the time of ejaculation is equivalent to 10
times the pressure within a normal vehicle
tire.
Key References
A lbright, J.L., and C.W. Arave. 1997. The Behaviour
o(Cattle. CAB International, Wellingford, UK. ISBN
0- 851 99-1 96-3.
Cra ig, J. V. 198 1. Domestic Animal Behavior: causes
and implications (or animal care and management.
Prentice-Ha ll, Inc . New Jersey. ISBN 0-13-2 18339-
0.
Evans, H.E. 1993. Anatomv o( the Do[, 3rd
Edition. W.B .Saunders Co. Philadelphia. ISBN 0-721 6-
3200-9.
Grandage, J. 1972. “The erect dog pe nis: a paradox of
flexibl e rig idity.” Vet Rec: 9 1: 14 1- 147.
Hart, Benjamin L. 1985. The Behavior o[Domestic
Animals. W.H. Freeman and Co., New York. ISBN
0-7 167-1595-3.
Houpt, K.A . 1998. Domestic Animal Behavior for
Veterinarians and Animal Scientists. 3rd Edition. Iowa
State Uni versity Press, ISBN 0-8138-1061 -2.
Katz, L. S. and T.J. McDonald. 1992. “Sexual Behavior
of farm animals” in Repoduction in Far m A ni mals :
Science, App lic ation and Mode ls . Theriogenology
38:240-254.
Korenman, S.G. 1998 . “New ins ights into erectile dys-
function : a practical approach.” Am. J. Med. 105:135-
144.
Signoret, J.P. and J. Balthazart. 1993 “Sexual behavior”
in Reproduction in Mammals and Man . C. Thibault,
M.C. Levasseur and R.I-I.F. I-I under, eds. Ell ipses, Paris.
ISBN 2-72 98-9354-7.
Tibary, A. and A. Anouassi. 1997. Theriogenolorsy in
Camelidae. United Arab Emirates. Ministry of Cul-
ture and Infor mation. Publ ication authorization N o.
3849/ 1116. ISBN 9981 – 801 -32- 1.
Reproductive Behavior 253
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252 Reproductive Behavior
During courtship the female balloon fly
will eat the male if given the chance. To
achieve copulation and keep from getting
eaten, the male will present the female with
a balloon-shaped cocoon as a “present”.
Unwrapping this “present” keeps the female
occupied long enough for the male to mate
her and fly off.
When box turtles copulate, the male mounts
the female and remains in an upright posi-
tion in order to facilitate insemination. The
pair may remain in this position for hours
to ensure adequate insemination. A t the
conclusion of the event the female will sud-
denly move away, sometimes causing the
male to fall p recariously on his hack where
he may remain until his death if h e can’t
right himself.
Most frogs and toads copulate in the dark.
They are often so eager to mate that the
male will try to momzt any thing that passes
by. They have been observed keeping a .firm
grip on strange objects and even other small
animals in the hope that they might turn out
to he females.
Th e long neck of the giraffe plays an im-
portant role in their reproductive
First the male samples the urine to
ascertain whether she is in estrus. If so, the
two giraffes then indulge in a form of sexual
preparation by entwining and rubbing their
necks together. Physiologically, this behav-
ior is like a false-mount and no doubt causes
the release of oxytocin that moves sperm
in the distal tail of the epididymis into an
ejaculatory position.
The p ressure within the penis of the bull at
the time of ejaculation is equivalent to 10
times the pressure within a normal vehicle
tire.
Key References
A lbright, J.L., and C.W. Arave. 1997. The Behaviour
o(Cattle. CAB International, Wellingford, UK. ISBN
0- 851 99-1 96-3.
Cra ig, J. V. 198 1. Domestic Animal Behavior: causes
and implications (or animal care and management.
Prentice-Ha ll, Inc . New Jersey. ISBN 0-13-2 18339-
0.
Evans, H.E. 1993. Anatomv o( the Do[, 3rd
Edition. W.B .Saunders Co. Philadelphia. ISBN 0-721 6-
3200-9.
Grandage, J. 1972. “The erect dog pe nis: a paradox of
flexibl e rig idity.” Vet Rec: 9 1: 14 1- 147.
Hart, Benjamin L. 1985. The Behavior o[Domestic
Animals. W.H. Freeman and Co., New York. ISBN
0-7 167-1595-3.
Houpt, K.A . 1998. Domestic Animal Behavior for
Veterinarians and Animal Scientists. 3rd Edition. Iowa
State Uni versity Press, ISBN 0-8138-1061 -2.
Katz, L. S. and T.J. McDonald. 1992. “Sexual Behavior
of farm animals” in Repoduction in Far m A ni mals :
Science, App lic ation and Mode ls . Theriogenology
38:240-254.
Korenman, S.G. 1998 . “New ins ights into erectile dys-
function : a practical approach.” Am. J. Med. 105:135-
144.
Signoret, J.P. and J. Balthazart. 1993 “Sexual behavior”
in Reproduction in Mammals and Man . C. Thibault,
M.C. Levasseur and R.I-I.F. I-I under, eds. Ell ipses, Paris.
ISBN 2-72 98-9354-7.
Tibary, A. and A. Anouassi. 1997. Theriogenolorsy in
Camelidae. United Arab Emirates. Ministry of Cul-
ture and Infor mation. Publ ication authorization N o.
3849/ 1116. ISBN 9981 – 801 -32- 1.
Reproductive Behavior 253
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The Puerperium & Lactation
Early Embryogenesis &
Maternal Recognition of Pregnancy
Ovulation & Fertilization
Cyclicity
Regulation of
Reproduction
Tract Function
Puberty
Prenatal
Development
Spermatogenesis
Regulat ion of
Reproduction
Tract Function
Puberty
Prenatal
Development
Take Home M essage
Gestation is the period of time that a female is pregnant. During gestation, the
placenta forms a major organ of pregnancy that provides an inteJface for metabolic
exchange between the dam and the f etus. Placentas are described mmplwlogically
according to the distribution of villi on the chorionic smface ami the degree of separa..:.
tion between matemal and fetal blood. The placenta is also an endocrine organ that
secretes hormones responsible for: 1) maintenance of pregnancy; 2) stimulation of the
matemal mammmy gland and 3) ensures fetal growth. Parturition is brought about
by secretion of fetal corticoitls and requires removal of the progesterone block. Par-
turition consists of three stages. They are: 1) initiation of myometrial contractions;
2) expulsion ofthe f etus and 3) e.:\:pulsion oftlzefetalmembranes.
T he word gestation literally means “the act
of carry ing or being carried”. Thus, gestation means
the action or process of carrying or being carried ‘ in
the uterus between conception and birth . · G est ation
and pregnancy are synonymous and thus, gestation
length means the length of pregnancy. Attachment
of the conceph1s to form an intimate, but temporary,
relationship w ith the uterus is an evolutionary step that
provides s ignifi cant advantage to the conceph1s . The
phenomenon of intrauterine development ensures that
the deve loping conceptus wi ll receive adequate nutri-
tion and protection duri ng its development. In contrast,
lower fon11S of animals lay eggs (oviparous) . The
surviva l of potential offspring of oviparous ani ma ls
is jeopardized because the fema le cannot completely
protect the eggs from environmental and predatmy dan-
ger. Thus, fro m an evolutionary perspective, eutherian
mammals (mammals with a placenta), are ” equipped”
w ith an in-utero protection mechanis m that is highly
successful after the placenta is formed.
The final prepartum steps of
reproduction are:
• formation of a placenta
• acquisition of endocrine function of
the placenta
• initiation of parturition
The term implantation is often used to mean
attachment of the placental membranes to the endo-
metrium in most animals. Achmlly, true impla ntation
is a phenomenon in humans in which the conceptus
“buries” itself into the uterine endometrium. The con-
ceptus temporarily disappears beneath the surface. In
most other species, the conceph 1s does not truly implant,
but rather attaches to the en dometrial surface and never
disappears from the luminal compartment.
The placenta is an organ of metabol ic inter-
change between the conceph1s and the dam. It is also
an endocrine organ. The placenta is composed of a fetal
component derived fi·om the chorion and a maternal
component derived from modificati ons of the uter ine
endometri um . The discrete reg ions of contact between
the chorion and the endometr ium form sp ecific zones
of metabolic exchange . The p lacenta also produces
a variety of hormones . This endocrine fu nction is
important for the maintenance of pregnancy and the
induction of parh1rition.
Parturition (giving birth to young) is t he step
in the reproductive process that immediately precedes
lactation, uterine repa ir and return to cyclicity. It is
ini tiated by the fe h1 s and involves a complex cascade
of endocr ine events that promote myometrial co ntrac-
tions, dilation of the cervix, expuls ion of the fe h1s and
expulsion o f the extraembryonic membranes.
Placentas Have Different Distributions
of Chorionic Villi
As you have learned in the previous chapter, the
conceptus cons ists of the embryo and the extraembry-
onic membranes (amnion , allantois and chorion) . T he
chorion is the fetal contribution to the p lacenta . The
funct ional uni t of the fetal p lacenta is the chorionic
villus . The chorionic villus is an ” exchange apparatus”
and provides increased sur face area so that exchange
is maximized. C horionic villi are sma ll, fi nger- like
projections that are on the surface of the chorion. These
tiny villi protrude aw ay from the chor ion toward the
uterine endometrium. Placentas ar e classified according
to the distrib ution of chorionic v ill i on their surfaces,
V
et
B
oo
ks
.ir
The Puerperium & Lactation
Early Embryogenesis &
Maternal Recognition of Pregnancy
Ovulation & Fertilization
Cyclicity
Regulation of
Reproduction
Tract Function
Puberty
Prenatal
Development
Spermatogenesis
Regulat ion of
Reproduction
Tract Function
Puberty
Prenatal
Development
Take Home M essage
Gestation is the period of time that a female is pregnant. During gestation, the
placenta forms a major organ of pregnancy that provides an inteJface for metabolic
exchange between the dam and the f etus. Placentas are described mmplwlogically
according to the distribution of villi on the chorionic smface ami the degree of separa..:.
tion between matemal and fetal blood. The placenta is also an endocrine organ that
secretes hormones responsible for: 1) maintenance of pregnancy; 2) stimulation of the
matemal mammmy gland and 3) ensures fetal growth. Parturition is brought about
by secretion of fetal corticoitls and requires removal of the progesterone block. Par-
turition consists of three stages. They are: 1) initiation of myometrial contractions;
2) expulsion ofthe f etus and 3) e.:\:pulsion oftlzefetalmembranes.
T he word gestation literally means “the act
of carry ing or being carried”. Thus, gestation means
the action or process of carrying or being carried ‘ in
the uterus between conception and birth . · G est ation
and pregnancy are synonymous and thus, gestation
length means the length of pregnancy. Attachment
of the conceph1s to form an intimate, but temporary,
relationship w ith the uterus is an evolutionary step that
provides s ignifi cant advantage to the conceph1s . The
phenomenon of intrauterine development ensures that
the deve loping conceptus wi ll receive adequate nutri-
tion and protection duri ng its development. In contrast,
lower fon11S of animals lay eggs (oviparous) . The
surviva l of potential offspring of oviparous ani ma ls
is jeopardized because the fema le cannot completely
protect the eggs from environmental and predatmy dan-
ger. Thus, fro m an evolutionary perspective, eutherian
mammals (mammals with a placenta), are ” equipped”
w ith an in-utero protection mechanis m that is highly
successful after the placenta is formed.
The final prepartum steps of
reproduction are:
• formation of a placenta
• acquisition of endocrine function of
the placenta
• initiation of parturition
The term implantation is often used to mean
attachment of the placental membranes to the endo-
metrium in most animals. Achmlly, true impla ntation
is a phenomenon in humans in which the conceptus
“buries” itself into the uterine endometrium. The con-
ceptus temporarily disappears beneath the surface. In
most other species, the conceph 1s does not truly implant,
but rather attaches to the en dometrial surface and never
disappears from the luminal compartment.
The placenta is an organ of metabol ic inter-
change between the conceph1s and the dam. It is also
an endocrine organ. The placenta is composed of a fetal
component derived fi·om the chorion and a maternal
component derived from modificati ons of the uter ine
endometri um . The discrete reg ions of contact between
the chorion and the endometr ium form sp ecific zones
of metabolic exchange . The p lacenta also produces
a variety of hormones . This endocrine fu nction is
important for the maintenance of pregnancy and the
induction of parh1rition.
Parturition (giving birth to young) is t he step
in the reproductive process that immediately precedes
lactation, uterine repa ir and return to cyclicity. It is
ini tiated by the fe h1 s and involves a complex cascade
of endocr ine events that promote myometrial co ntrac-
tions, dilation of the cervix, expuls ion of the fe h1s and
expulsion o f the extraembryonic membranes.
Placentas Have Different Distributions
of Chorionic Villi
As you have learned in the previous chapter, the
conceptus cons ists of the embryo and the extraembry-
onic membranes (amnion , allantois and chorion) . T he
chorion is the fetal contribution to the p lacenta . The
funct ional uni t of the fetal p lacenta is the chorionic
villus . The chorionic villus is an ” exchange apparatus”
and provides increased sur face area so that exchange
is maximized. C horionic villi are sma ll, fi nger- like
projections that are on the surface of the chorion. These
tiny villi protrude aw ay from the chor ion toward the
uterine endometrium. Placentas ar e classified according
to the distrib ution of chorionic v ill i on their surfaces,
V
et
B
oo
ks
.ir
294 Placentation, Gestation and Parturition
giving each placental type a disti nct anatomical appear-
ance. Placentas may also be classifi ed by number of
tissue layers separating maternal and feta l blood.
Placentas are classified acc01·ding to the
distribution of chorionic viili. These
classifications are:
• diffuse
• zonary
• discoid
• cotyledonary
The diffi.tse placenta of the pig has a velvet-like
surface with many closely spaced chorionic villi that are
distributed over the entire surface of the chorion (See
Figure 14-1 ). Initial attachment occurs around day 12
and is well established by day 18 to 20 after ovulation
(See Chapter 13) .
Diffuse placentas have uniform
distribution of chorionic villi that
cover the swface of the chorion.
Example= pig
T he mare placenta is also classified as diffuse,
however it is characterized by having many specia lized
“microzones” of chorionic villi known as microcoty-
ledons (See Figure 14-1 ). These microcotyledons are
microscopically discrete regions at the fetal-maternal
interface . As in the pig, they are also distributed over
the entire chorionic surface.
The mare placenta also conta ins unique tran-
sitory structures known as endometrial cups. These
are discrete areas that range from a few millimeters
to several centimeters in diameter. The endometria l
cups are of both trophoblastic and endometrial origin.
There are 5 to I 0 endometrial cups distributed over the
surface of the placenta (See Figure 14-6). Endometrial
cups produce equine chorionic gonadotropin (eCG)
and develop between days 35 and 60 of pregnancy.
Following day 60, the endometria l cups are sloughed
into the uterine lumen and ar e no longer fu nctional.
Attachment of the conceptus to the endometrium is
initiated at about day 24 and becomes well established
by 36 to 38 days (See Ch apter 13).
Zonary placentas have a band-like
zone of chorionic villi.
Example = dogs and cats
The zonary placenta (found in dogs and cats)
includes a prominent region of exchange that fonns
a broad zone around the chorion near t he middle of
the conceptus (See F igure 14-2). A second regio n
consists of a highly pigmented ring at either e nd of the
central zone. This pigmented zone consists of s mall
hematomas (blood clots). The pigmented zone is also
refetTed to as the paraplacenta and is thought to be
important in iron transport from the dam to the fe hts.
The function of this zone is not well understood. A
third region is the transparent zone on the distal ends
of the chorion that has poor vascularity. This zone may
be invo lved in absorption of materials directly fro m the
uterine lumen.
Discoid placentas form a
regionalized disc.
Example = rodents and primates
The discoid placenta (See Figure 14-2) is fo und
in rodents and primates. It is characterized by having
one or two distinct adj acent discs. These discs contain
chorionic vi lli that in terface w ith the endometrium
and provide the region fo r gas, nutrient and metabolic
waste exch ange .
Cotyledonary placentas have nu-
merous, discrete button-like
structures called cotyledons.
Example = ruminants
Ruminants have a cotyledonary placenta (See
Figure 14-3). A cotyled on is defined as a placental
unit of trophoblastic ori gin cons isting of abundant
blood vessels and connective tissue. In sheep, there
are between 90 and 100 cotyledons distributed across
the s urface of the chorion and, in cattle, 70 to 120
cotyledons have been observed. The placentome (point
of interface) in the cotyledonary placenta consists of
a fetal cotyledon contributed by the chorion a nd a
maternal cotyledon , orig inating f rom the caruncular
regions of the u terus. At about day 16 in sheep and
day 25 in cattle the chorion initiates attachment to the
cm·uncles of the uterus. Prior to this time the placenta
is essentially diffi.tse. During the formation of the
placentomes, chorionic v illi protrude into crypts in the
caruncular tissue. This relationship .lli not implantation
but an anatomically specialized forn1 of attachment.
Attachment is well estab lished by day 30 in ewes and
day 40 in cows (See Chapter 13).
In the cow, the placentom es fo rm a convex
structure, whi le in the ewe they are concave (See
Figure 14- 3). Dur ing gestation, the cotyle dons will
increase many-fold in diameter. In fact, cotyledons in
the cow near the end of gestation may measure 5 to
6 centimeters in diameter. Such growth provides
enormou s surface area to support p lacental transfer
of nutrients from the dam and metabolic wastes from
the fetus.
Placental Classification by Microscopic
Appearance is Based on the Number of
Placental Layers that Separate the Fetal
Blood from the Maternal Blood
The nomenclature for describing placental in-
timacy is derived by first descri bing the tiss ues of the
maternal placenta in the prefix of the word. The tissues
of the feta l placenta constitute the suffix. Exchange can
occur through as many as six tissue layers and as few
as three . The name of the prefix and suffix of each type
of placenta changes depending on the number of tissue
layers that exist.
Prefix =maternal side Suffix =fetal side
“epithelia” “chorial”
epitheliochorial
Placentation, Gestation and Parturition 295
cells originate from trophoblast cells and are thotwht
to be fanned continuously throughout gestation. Bi-
nucleate giant cells constitute around 20% of the fetal
placenta. Duri ng development, the binucleate giant
cells migrate from the chorionic epithelium and invade
the endometrial epi t helium (See Figure 14-4) . The
binucleate giant cells are believed to transfer complex
molecules from the fetal to the maternal p lacenta.
There is evidence that they secrete placen t al lactogen.
Also, these cells secrete pregnancy specific protein
B (PSPB) that are also called pregnancy associated
g lycoproteins (PAG) . T hese proteins are unique to
pregnancy in ruminants. The binucleate giant cells
are also important sites of steroidogenes is, secreting
progesterone and estradiol. These cells will no doubt
emerge as increasingly important “players” in the func –
tion of the ruminant placenta with further research.
I Endotheliochorial = 5 layers I
The endothelioch orial placenta is character-
ized as having complete erosion of the endometrial
epithelium and underlying interstitium. T hus, maternal
capillaries are directly exposed to epithelial cells of the
chorion (See Fig ure 14-5). The chorionic epi the lium
packs around the vessels on the materna l side. Note in
Figure 14-5 that this type of placenta is more intimate
I Epitheliochorial 6 layers I than the epitheliochoriat p lacenta because the en dome– _ trial epithelium no longer exists. Dogs and cats possess ..__ _ _ ____ _ _ _ _____ ____ _. endothe liochorial placentation.
The epitheliochoria l pl acenta (See Figure
14-5) is the least intimate among the placental types.
In the epithel iochorial placenta, both the endometrial
epithelium (maternal side) and ep ithelium of the chori-
onic villi are intact. In other words, there is a complete
intact layer of ep ithe lium in both the maternal and feta l
components. The epitheliochorial placenta is found in
the sow and the mare. Recall that the placentas of the
sow and the mare are diffitse and v illi occupy a large
proportion of the surface area of the chorion.
Ruminants also have an epitheli ochorial pla-
centa. However, the endometria l epithelium transiently
erodes and then regrows, causing intennittent exposure
of the maternal capillaries to the chorionic epithelium.
This type of placenta has been tenned syndesmocho-
rial.
In addition to the feature of partial erosion
of the endometrial epithelium, a unique cell type is
fo und in the ruminant placenta. These cells are called
binucleate g iant cells. As their name implies, they are
characterized as being quite large and have two nuclei.
Binucleate giant cells appear at about day 14 in the
sheep and between days 18 and 20 in the cow. These
I Hemochorial = 3 layers I
The h emochorial plac enta (See Figure 14-5)
is characterized as having the chorionic epithelium in
direct apposition to materna l pools ofblood. Thus, nu-
trients and gases are exchanged directly from maternal
blood and must move tlu-ough only tlu-ee tissue layers.
This high ly inti mate relationship is found in primates
and rodents (See Figure 14-5).
The Placenta Regulates the Exc hange
Between the Fetus a nd Dam
Placental exchange in vo lves a num ber of
mechanisms found in other tissues. These are simple
diffusion, facilitated diffusion and active t r a nsport.
Gases and water pass from high to low concentrations
by simple diffusion. The p lacenta contains act ive
transport pumps for sodium and potassium, as well as
calcium. Glucose and other metabolically important
materials such as amino acids are transported by fac ili-
tated di ffusion uti lizing specific carrier molecules.
141
V
et
B
oo
ks
.ir
294 Placentation, Gestation and Parturition
giving each placental type a disti nct anatomical appear-
ance. Placentas may also be classifi ed by number of
tissue layers separating maternal and feta l blood.
Placentas are classified acc01·ding to the
distribution of chorionic viili. These
classifications are:
• diffuse
• zonary
• discoid
• cotyledonary
The diffi.tse placenta of the pig has a velvet-like
surface with many closely spaced chorionic villi that are
distributed over the entire surface of the chorion (See
Figure 14-1 ). Initial attachment occurs around day 12
and is well established by day 18 to 20 after ovulation
(See Chapter 13) .
Diffuse placentas have uniform
distribution of chorionic villi that
cover the swface of the chorion.
Example= pig
T he mare placenta is also classified as diffuse,
however it is characterized by having many specia lized
“microzones” of chorionic villi known as microcoty-
ledons (See Figure 14-1 ). These microcotyledons are
microscopically discrete regions at the fetal-maternal
interface . As in the pig, they are also distributed over
the entire chorionic surface.
The mare placenta also conta ins unique tran-
sitory structures known as endometrial cups. These
are discrete areas that range from a few millimeters
to several centimeters in diameter. The endometria l
cups are of both trophoblastic and endometrial origin.
There are 5 to I 0 endometrial cups distributed over the
surface of the placenta (See Figure 14-6). Endometrial
cups produce equine chorionic gonadotropin (eCG)
and develop between days 35 and 60 of pregnancy.
Following day 60, the endometria l cups are sloughed
into the uterine lumen and ar e no longer fu nctional.
Attachment of the conceptus to the endometrium is
initiated at about day 24 and becomes well established
by 36 to 38 days (See Ch apter 13).
Zonary placentas have a band-like
zone of chorionic villi.
Example = dogs and cats
The zonary placenta (found in dogs and cats)
includes a prominent region of exchange that fonns
a broad zone around the chorion near t he middle of
the conceptus (See F igure 14-2). A second regio n
consists of a highly pigmented ring at either e nd of the
central zone. This pigmented zone consists of s mall
hematomas (blood clots). The pigmented zone is also
refetTed to as the paraplacenta and is thought to be
important in iron transport from the dam to the fe hts.
The function of this zone is not well understood. A
third region is the transparent zone on the distal ends
of the chorion that has poor vascularity. This zone may
be invo lved in absorption of materials directly fro m the
uterine lumen.
Discoid placentas form a
regionalized disc.
Example = rodents and primates
The discoid placenta (See Figure 14-2) is fo und
in rodents and primates. It is characterized by having
one or two distinct adj acent discs. These discs contain
chorionic vi lli that in terface w ith the endometrium
and provide the region fo r gas, nutrient and metabolic
waste exch ange .
Cotyledonary placentas have nu-
merous, discrete button-like
structures called cotyledons.
Example = ruminants
Ruminants have a cotyledonary placenta (See
Figure 14-3). A cotyled on is defined as a placental
unit of trophoblastic ori gin cons isting of abundant
blood vessels and connective tissue. In sheep, there
are between 90 and 100 cotyledons distributed across
the s urface of the chorion and, in cattle, 70 to 120
cotyledons have been observed. The placentome (point
of interface) in the cotyledonary placenta consists of
a fetal cotyledon contributed by the chorion a nd a
maternal cotyledon , orig inating f rom the caruncular
regions of the u terus. At about day 16 in sheep and
day 25 in cattle the chorion initiates attachment to the
cm·uncles of the uterus. Prior to this time the placenta
is essentially diffi.tse. During the formation of the
placentomes, chorionic v illi protrude into crypts in the
caruncular tissue. This relationship .lli not implantation
but an anatomically specialized forn1 of attachment.
Attachment is well estab lished by day 30 in ewes and
day 40 in cows (See Chapter 13).
In the cow, the placentom es fo rm a convex
structure, whi le in the ewe they are concave (See
Figure 14- 3). Dur ing gestation, the cotyle dons will
increase many-fold in diameter. In fact, cotyledons in
the cow near the end of gestation may measure 5 to
6 centimeters in diameter. Such growth provides
enormou s surface area to support p lacental transfer
of nutrients from the dam and metabolic wastes from
the fetus.
Placental Classification by Microscopic
Appearance is Based on the Number of
Placental Layers that Separate the Fetal
Blood from the Maternal Blood
The nomenclature for describing placental in-
timacy is derived by first descri bing the tiss ues of the
maternal placenta in the prefix of the word. The tissues
of the feta l placenta constitute the suffix. Exchange can
occur through as many as six tissue layers and as few
as three . The name of the prefix and suffix of each type
of placenta changes depending on the number of tissue
layers that exist.
Prefix =maternal side Suffix =fetal side
“epithelia” “chorial”
epitheliochorial
Placentation, Gestation and Parturition 295
cells originate from trophoblast cells and are thotwht
to be fanned continuously throughout gestation. Bi-
nucleate giant cells constitute around 20% of the fetal
placenta. Duri ng development, the binucleate giant
cells migrate from the chorionic epithelium and invade
the endometrial epi t helium (See Figure 14-4) . The
binucleate giant cells are believed to transfer complex
molecules from the fetal to the maternal p lacenta.
There is evidence that they secrete placen t al lactogen.
Also, these cells secrete pregnancy specific protein
B (PSPB) that are also called pregnancy associated
g lycoproteins (PAG) . T hese proteins are unique to
pregnancy in ruminants. The binucleate giant cells
are also important sites of steroidogenes is, secreting
progesterone and estradiol. These cells will no doubt
emerge as increasingly important “players” in the func –
tion of the ruminant placenta with further research.
I Endotheliochorial = 5 layers I
The endothelioch orial placenta is character-
ized as having complete erosion of the endometrial
epithelium and underlying interstitium. T hus, maternal
capillaries are directly exposed to epithelial cells of the
chorion (See Fig ure 14-5). The chorionic epi the lium
packs around the vessels on the materna l side. Note in
Figure 14-5 that this type of placenta is more intimate
I Epitheliochorial 6 layers I than the epitheliochoriat p lacenta because the en dome– _ trial epithelium no longer exists. Dogs and cats possess ..__ _ _ ____ _ _ _ _____ ____ _. endothe liochorial placentation.
The epitheliochoria l pl acenta (See Figure
14-5) is the least intimate among the placental types.
In the epithel iochorial placenta, both the endometrial
epithelium (maternal side) and ep ithelium of the chori-
onic villi are intact. In other words, there is a complete
intact layer of ep ithe lium in both the maternal and feta l
components. The epitheliochorial placenta is found in
the sow and the mare. Recall that the placentas of the
sow and the mare are diffitse and v illi occupy a large
proportion of the surface area of the chorion.
Ruminants also have an epitheli ochorial pla-
centa. However, the endometria l epithelium transiently
erodes and then regrows, causing intennittent exposure
of the maternal capillaries to the chorionic epithelium.
This type of placenta has been tenned syndesmocho-
rial.
In addition to the feature of partial erosion
of the endometrial epithelium, a unique cell type is
fo und in the ruminant placenta. These cells are called
binucleate g iant cells. As their name implies, they are
characterized as being quite large and have two nuclei.
Binucleate giant cells appear at about day 14 in the
sheep and between days 18 and 20 in the cow. These
I Hemochorial = 3 layers I
The h emochorial plac enta (See Figure 14-5)
is characterized as having the chorionic epithelium in
direct apposition to materna l pools ofblood. Thus, nu-
trients and gases are exchanged directly from maternal
blood and must move tlu-ough only tlu-ee tissue layers.
This high ly inti mate relationship is found in primates
and rodents (See Figure 14-5).
The Placenta Regulates the Exc hange
Between the Fetus a nd Dam
Placental exchange in vo lves a num ber of
mechanisms found in other tissues. These are simple
diffusion, facilitated diffusion and active t r a nsport.
Gases and water pass from high to low concentrations
by simple diffusion. The p lacenta contains act ive
transport pumps for sodium and potassium, as well as
calcium. Glucose and other metabolically important
materials such as amino acids are transported by fac ili-
tated di ffusion uti lizing specific carrier molecules.
141
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296 Placentation, Gestation and Parturition
Figure 14-1. The Diffuse Placenta
Sow
r
Endometrium
=—
The diffuse placenta of the sow consists of
many chorionic villi distributed over the entire
surface of the chorion. They penetrate into
the endometrium forming the fetal-maternal
interface. Vessels from each chorionic vil-
lus merge and eventually form large vessels
that enter the umbilical cord. A= Allantois,
AC= Allantochorion, AM= Amnioni c Cavity,
E= Endometrium, M= Myometrium
Mare
Endometrium
Myometrium
The diffuse placenta of the mare consists
of many microcotyledons distributed over
the entire surface of the chorion. These mi-
crocotyledons are the site of fetal-maternal
exchange. A= Allantois, AC= Allantochorion,
AM= Amnionic Cavity, E= Endometrium,
M= Myometrium, YS= Yolk Sac
Placentation, Gestation and Parturition 297
Figure 14-2. The Zonary and Discoid Placentas
AC
YS …..-“‘
PZ
Bitch
The zonary placenta consists of three distinct zones;
a transfer zone (TZ), a pigmented zone (PZ) and a
relatively nonvascular zone , the allantochorion (AC).
In the zonary placenta, a band of tissue forms around
the conceptus where nutrient transfer occurs. The
pigmented zone (PZ) or paraplacenta represents local
regions of maternal hemorrhage and necrosis.
A= Allantois, AC= Allantochorion, A M= Amn ionic Cavity,
E= Endometrium, M= Myometrium, YS= Yolk Sac
Primates
The d iscoid placenta consists of a round patch of chori-
onic tissue that forms the fetal-maternal interface. Ves-
sels from the exchange zone merge to form the umbilical
vessels that supply the fetus with blood. The vasculature
of the chorion (within the disc) is immersed in pools of
blood where metabolic exchange takes place.
A= Allantois, AC = Allantochorion ,
AM= Amnionic Cavity, E = Endometrium,
EZ = Exchange Zone, M = Myometrium
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296 Placentation, Gestation and Parturition
Figure 14-1. The Diffuse Placenta
Sow
r
Endometrium
=—
The diffuse placenta of the sow consists of
many chorionic villi distributed over the entire
surface of the chorion. They penetrate into
the endometrium forming the fetal-maternal
interface. Vessels from each chorionic vil-
lus merge and eventually form large vessels
that enter the umbilical cord. A= Allantois,
AC= Allantochorion, AM= Amnioni c Cavity,
E= Endometrium, M= Myometrium
Mare
Endometrium
Myometrium
The diffuse placenta of the mare consists
of many microcotyledons distributed over
the entire surface of the chorion. These mi-
crocotyledons are the site of fetal-maternal
exchange. A= Allantois, AC= Allantochorion,
AM= Amnionic Cavity, E= Endometrium,
M= Myometrium, YS= Yolk Sac
Placentation, Gestation and Parturition 297
Figure 14-2. The Zonary and Discoid Placentas
AC
YS …..-“‘
PZ
Bitch
The zonary placenta consists of three distinct zones;
a transfer zone (TZ), a pigmented zone (PZ) and a
relatively nonvascular zone , the allantochorion (AC).
In the zonary placenta, a band of tissue forms around
the conceptus where nutrient transfer occurs. The
pigmented zone (PZ) or paraplacenta represents local
regions of maternal hemorrhage and necrosis.
A= Allantois, AC= Allantochorion, A M= Amn ionic Cavity,
E= Endometrium, M= Myometrium, YS= Yolk Sac
Primates
The d iscoid placenta consists of a round patch of chori-
onic tissue that forms the fetal-maternal interface. Ves-
sels from the exchange zone merge to form the umbilical
vessels that supply the fetus with blood. The vasculature
of the chorion (within the disc) is immersed in pools of
blood where metabolic exchange takes place.
A= Allantois, AC = Allantochorion ,
AM= Amnionic Cavity, E = Endometrium,
EZ = Exchange Zone, M = Myometrium
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298 Placentation, Gestation and Parturition
Figure 14-3. The Cotyledonary Placenta
Convex
(cow, giraffe)
In the photograph above, the fetal membranes and
the feta l cotyledons (FC) can be visualized . The
membrane labeled AC is the allantochorion. The
umbilical cord, (UC-arrow) of the fetus receives
blood vessels (BV) from the fetal cotyled ons (FC).
Glycogen plaques (GP) can be visualized on the
surface of the chorion and the amnion. These
plaques are localized squamous proliferations
called verrucae.
Concave
(sheep, goat)
The cotyledonary placenta is characterized by numerous “button-like” structures distributed across the surface
of the chorion . These are called fetal cotyledons. When they jo in with the maternal caruncle they form a
placentome. Aconvex cotyledon becomes covered with the chorion. Many finger-like villi (red) ori ginating
from the chorionic tissue protrude toward the lumen of the uterus. In the concave c otyledon , the chorionic
tissue pushes inward, forming a concave interface between the cho rio n and the maternal caruncle.
Placentation, Gestation and Parturition 299
Figure 14-3. The Cotyledonary Placenta
The diag ram in the upper left illustrates the distribution
of the extraembryonic membranes prior to comp lete at-
tachment. The extraembryonic membranes consist of the
amnion (blue sac), yolk sac (YS) and the allantois (A).
Even though the fetus is located in one uterine horn, the
cho rion invades the contralateral uterine horn a nd form s
placentomes.
Cow
Some fetal cotyledons (FC) have been partially separated
from maternal cotyledons (MC). The chorion (C) is th e
outer fetal membrane. Arrows indicate the border of the
amnion (A) . The myometrium (M) is indicated by the ar-
rows . Notice that the fe tal cotyledon (FC) is attached to
the surface of the caruncle creating a convex cotyledon.
E= Endometrium
Ew e-A
The chorion ca n be seen entering the pla centome (P).
The chorionic stalk (CS) conta ins the fetal vascu lature.
Ew e-8
)
A portion of the chorion has been incised so that the fetal
vasculature can be visualized clearly. The fetal vessels
(arrow) and chorion ic tissue “push” into the caruncu lar
tissue forming a concave cotyledon. A set of arteries (A)
a nd veins (V) emerge from each cotyledon and eventu ally
merge in the umbilica l cord (UC). P= Placentoma
Ewe-C
A concave placentoma is clearly visible. The chorionic
stalk is draped over the needle holder. Notice the vesse ls
(arrows) within the chorion ic tissue. T he reddish-beige
tissue is the maternal cotyledon (MC) that is covered by
th e allantochorion. The dark tiss ue in the center (arro ws)
is the fetal component of the placentome.
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298 Placentation, Gestation and Parturition
Figure 14-3. The Cotyledonary Placenta
Convex
(cow, giraffe)
In the photograph above, the fetal membranes and
the feta l cotyledons (FC) can be visualized . The
membrane labeled AC is the allantochorion. The
umbilical cord, (UC-arrow) of the fetus receives
blood vessels (BV) from the fetal cotyled ons (FC).
Glycogen plaques (GP) can be visualized on the
surface of the chorion and the amnion. These
plaques are localized squamous proliferations
called verrucae.
Concave
(sheep, goat)
The cotyledonary placenta is characterized by numerous “button-like” structures distributed across the surface
of the chorion . These are called fetal cotyledons. When they jo in with the maternal caruncle they form a
placentome. Aconvex cotyledon becomes covered with the chorion. Many finger-like villi (red) ori ginating
from the chorionic tissue protrude toward the lumen of the uterus. In the concave c otyledon , the chorionic
tissue pushes inward, forming a concave interface between the cho rio n and the maternal caruncle.
Placentation, Gestation and Parturition 299
Figure 14-3. The Cotyledonary Placenta
The diag ram in the upper left illustrates the distribution
of the extraembryonic membranes prior to comp lete at-
tachment. The extraembryonic membranes consist of the
amnion (blue sac), yolk sac (YS) and the allantois (A).
Even though the fetus is located in one uterine horn, the
cho rion invades the contralateral uterine horn a nd form s
placentomes.
Cow
Some fetal cotyledons (FC) have been partially separated
from maternal cotyledons (MC). The chorion (C) is th e
outer fetal membrane. Arrows indicate the border of the
amnion (A) . The myometrium (M) is indicated by the ar-
rows . Notice that the fe tal cotyledon (FC) is attached to
the surface of the caruncle creating a convex cotyledon.
E= Endometrium
Ew e-A
The chorion ca n be seen entering the pla centome (P).
The chorionic stalk (CS) conta ins the fetal vascu lature.
Ew e-8
)
A portion of the chorion has been incised so that the fetal
vasculature can be visualized clearly. The fetal vessels
(arrow) and chorion ic tissue “push” into the caruncu lar
tissue forming a concave cotyledon. A set of arteries (A)
a nd veins (V) emerge from each cotyledon and eventu ally
merge in the umbilica l cord (UC). P= Placentoma
Ewe-C
A concave placentoma is clearly visible. The chorionic
stalk is draped over the needle holder. Notice the vesse ls
(arrows) within the chorion ic tissue. T he reddish-beige
tissue is the maternal cotyledon (MC) that is covered by
th e allantochorion. The dark tiss ue in the center (arro ws)
is the fetal component of the placentome.
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300 Placentation, Gestation and Parturition
Glucose is the major source of energy for the
fetus. The majority of glucose is derived from the ma-
temal circulation. Near the end of gestation, glucose
consumption by the fetus is exceptionally high and
can lead to a metabolic drain of glucose away from the
dam. Such a glucose drain favors the development of
ketosis in the dam. Ketosis results from the metabo-
lism of body fat that generate ketones for energy when
glucose is limited. Periparturient ketosis is common
in dairy cows where postpartum metabolic demands
are exceptionally high because of high milk produc-
tion. Some materials cannot be transported across the
placenta. With the exception of some immunoglobu-
lins, matemal proteins do not cross the placental banier.
Immunoglobulins can be transported from the matemal
to the fetal side in a hemochorial or an endotheliochorial
placenta. However, the fetus synthesizes the majority
of its own proteins from amino acids contributed by
the dam. Nutritionally-based lipids do not cross the
placenta. Instead, the placenta hydrolyzes triglycer-
ides and matemal phospholipids and synthesizes new
lipid materials to be used by the fetus. Large peptide
honnones such as thyroid stimulating hom1one, adrenal
cortical stimulating hormone, growth honnone, insu-
lin and glucagon do not cross the placenta. Smaller
molecular weight hormones such as steroids, thyroid
hormone and the catecholamines (epinephrine and
norepinephrine) cross the placenta with relative ease.
Vitamins and minerals are transfened to the fetus at
variable rates. Fat soluble vitamins do not cross the
placenta with ease, while water soluble vitamins (Band
K) pass across the placenta w ith relative ease. Nutrients
are also transferred by pinocytosis and phagocytosis.
Areolae from the chorion form over the openings of
the uterine glands and are thought to absorb secretions
from these glands.
Of significant importance is the ability of the
placenta to transfer toxic and potentially pathogenic ma-
terials. Many toxic substances easily cross the placental
banier. These include ethyl alcohol, lead, phosphorus
and mercmy . Also, opiate drugs and numerous common
phmmaceuticals such as barbiturates and antibiotics can
cross the placental banier. Some substances may be
highly teratogenic . Teratogenic means inducing ab-
normal development (birth defects). The se substances
include LSD, amphetamines, lithium, diethylstilbestrol
and thalidom ide. It is well documented that these ma-
terials induce abnormal embtyonic development and
cause serious birth defects.
It is known that a wide range of microorgan-
isms can contaminate the fetus . Viruses can cross the
placental banier with ease and thus many viral diseases
can be transmitted from the dam to the fetus. Such
human diseases as German measles, Herpes virus and
HIV can be transmitted from the pregnant mother to the
fetus. Bacteria such as syphilis can also be transmitted
to the fetus.
Figure 14-4. The Migration of Binucleate Giant Cells
in the Ruminant Placenta
r::
Fetal
(chorion)
Maternal
(endometrial
epithelium)
Basement membrane
Maternal circulation
Binucleate giant cells
(BNGC) migrate from
the chorion to the en-
dometrial epithelium
in ruminants. These
cells are thought to se-
crete placental lactogen
and pregnancy specific
protein B.
(www. biotracking. com)
Placentation, Gestation and Parturition 301
Figure 14-5. Placental Classification Based on Separati on
Between Fetal and Maternal Blood Supplies
Fetal
(chorion)
Maternal
(endometrial
epithelium)
Fetal
(chori on)
Fetal
(chorion)
Maternal
(endometrium)
Epitheliochorial
Endotheliochorial
Hemochorial
Epithel iochorial
(pigs, horses and rumi nants)
6. Chorionic cap ill aries
5. Chorionic interstitium
4. Chorionic epithelium
3. Endometrial epith elium
2. Endometrial interstitium
1. Endometrial capillaries
Endotheliochorial
(dogs and cats)
5. Chorionic cap illaries
4. C horionic interstitium
3. Chorionic epithelium
2. Endometrial interstitium
1. Endometri al capillaries
Hemochorial
(primates and rodents)
3. Chorionic capillaries
2. Chorionic interstitium
1. Chorionic epithelium
RBC= Red blood cell
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300 Placentation, Gestation and Parturition
Glucose is the major source of energy for the
fetus. The majority of glucose is derived from the ma-
temal circulation. Near the end of gestation, glucose
consumption by the fetus is exceptionally high and
can lead to a metabolic drain of glucose away from the
dam. Such a glucose drain favors the development of
ketosis in the dam. Ketosis results from the metabo-
lism of body fat that generate ketones for energy when
glucose is limited. Periparturient ketosis is common
in dairy cows where postpartum metabolic demands
are exceptionally high because of high milk produc-
tion. Some materials cannot be transported across the
placenta. With the exception of some immunoglobu-
lins, matemal proteins do not cross the placental banier.
Immunoglobulins can be transported from the matemal
to the fetal side in a hemochorial or an endotheliochorial
placenta. However, the fetus synthesizes the majority
of its own proteins from amino acids contributed by
the dam. Nutritionally-based lipids do not cross the
placenta. Instead, the placenta hydrolyzes triglycer-
ides and matemal phospholipids and synthesizes new
lipid materials to be used by the fetus. Large peptide
honnones such as thyroid stimulating hom1one, adrenal
cortical stimulating hormone, growth honnone, insu-
lin and glucagon do not cross the placenta. Smaller
molecular weight hormones such as steroids, thyroid
hormone and the catecholamines (epinephrine and
norepinephrine) cross the placenta with relative ease.
Vitamins and minerals are transfened to the fetus at
variable rates. Fat soluble vitamins do not cross the
placenta with ease, while water soluble vitamins (Band
K) pass across the placenta w ith relative ease. Nutrients
are also transferred by pinocytosis and phagocytosis.
Areolae from the chorion form over the openings of
the uterine glands and are thought to absorb secretions
from these glands.
Of significant importance is the ability of the
placenta to transfer toxic and potentially pathogenic ma-
terials. Many toxic substances easily cross the placental
banier. These include ethyl alcohol, lead, phosphorus
and mercmy . Also, opiate drugs and numerous common
phmmaceuticals such as barbiturates and antibiotics can
cross the placental banier. Some substances may be
highly teratogenic . Teratogenic means inducing ab-
normal development (birth defects). The se substances
include LSD, amphetamines, lithium, diethylstilbestrol
and thalidom ide. It is well documented that these ma-
terials induce abnormal embtyonic development and
cause serious birth defects.
It is known that a wide range of microorgan-
isms can contaminate the fetus . Viruses can cross the
placental banier with ease and thus many viral diseases
can be transmitted from the dam to the fetus. Such
human diseases as German measles, Herpes virus and
HIV can be transmitted from the pregnant mother to the
fetus. Bacteria such as syphilis can also be transmitted
to the fetus.
Figure 14-4. The Migration of Binucleate Giant Cells
in the Ruminant Placenta
r::
Fetal
(chorion)
Maternal
(endometrial
epithelium)
Basement membrane
Maternal circulation
Binucleate giant cells
(BNGC) migrate from
the chorion to the en-
dometrial epithelium
in ruminants. These
cells are thought to se-
crete placental lactogen
and pregnancy specific
protein B.
(www. biotracking. com)
Placentation, Gestation and Parturition 301
Figure 14-5. Placental Classification Based on Separati on
Between Fetal and Maternal Blood Supplies
Fetal
(chorion)
Maternal
(endometrial
epithelium)
Fetal
(chori on)
Fetal
(chorion)
Maternal
(endometrium)
Epitheliochorial
Endotheliochorial
Hemochorial
Epithel iochorial
(pigs, horses and rumi nants)
6. Chorionic cap ill aries
5. Chorionic interstitium
4. Chorionic epithelium
3. Endometrial epith elium
2. Endometrial interstitium
1. Endometrial capillaries
Endotheliochorial
(dogs and cats)
5. Chorionic cap illaries
4. C horionic interstitium
3. Chorionic epithelium
2. Endometrial interstitium
1. Endometri al capillaries
Hemochorial
(primates and rodents)
3. Chorionic capillaries
2. Chorionic interstitium
1. Chorionic epithelium
RBC= Red blood cell
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302 Placentation, Gestation and Parturition
The Placenta is a Major Endocrine Organ
During Pregnancy
In addition to serv ing as a metabolic exchange
organ, the placenta serves as a transitory endocrine or-
gan. Hormones from the placenta gain access to both
the fetal and the matemal circulation.
The placenta secretes hormones that can:
• stimulate ovarian function
• maintain pregnancy
• influence fetal growth
• stimulate mammary function
• assist in parturition
The placenta of the mare produces a gonado-
tropin called equine chorionic gonadotropin (eCG).
Equine chorionic gonadotropin is also called pregnant
mare’s serum gonadotropin (PMSG). Equine cho-
rionic gonadotropin is produced by the endometrial
cups of the placenta. Endomeh·ial cups are a transient
placental endocrine gland. They begin producing eCG
at the time of attachment of the conceptus to the endo-
metrium. The relationship between the fom1ation of
the endometrial cups in the mare and the synthesis of
eCG is presented in Figure 14-6. As you can see, the
production of eCG is closely related to the weight of
the endometrial cups.
Equine chorionic gonadotropin acts as a lu-
teotropin and provides a stimulus for maintenance of the
primary cm·pus luteum . The primary corpus luteum
in the mare is defined as the corpus luteum fom1ed
from the ovulated follicle. In addition, eCG is respon-
sible for controlling the formation and maintenance of
supplementary (accessory) corpora lutca. As eCG
increases, the pregnant mare will often ovulate, thus
generating accessory corpora lutea. The eCG-induced
ovulations occur between days 40 and 70 of preg-
nancy. Luteinization (promoted by eCG) also occurs
in antral follicles that do not ovulate. Thus, eCG has a
significant positive impact on the ability of the ovary
to produce progesterone. Indeed, if one examines the
progesterone profile, it can be seen that there is a close
relationship between the concentrations of proges-
terone and the production of accessory corpora lutea
(See Figure 14-7).
In addition to its luteotropic action, eCG has
powerful FSH-like actions when administered to fe-
males of other species. In fact, eCG will cause marked
follicular development in most species. It is used com-
monly to induce superovulation where embryo transfer
is performed (cow, sheep, rabbit). In mares, however,
eCG does not exert significant FSH-like action.
–‘E
Db c .._,
l!l u
Cll
Figure 14-6. Production of
Equine Chorionic Gonadotropin
(eGG) is Closely Related to the
Weight of the
Endometrial Cups
(Modified from Ginther,
Reproductive Biolog v of the Mare )
175 10
ISO 9
125 I 0
I
I
I 100 I 7
I
I
75 I 6 I
I
I
50 I 5
4 25 I —-
40 60 80 100 120 14 0 16 0 18 0 200
Days of Gestation
Endometrial cups (EC) are seen here in
a U-shaped configuration. The fetus (F)
is surrounded by the amnion (not visible).
The membrane indicated by arrows is the
allantochorion . This specimen was re-
moved from a mare at 50 days of gestation.
(Photograph courtesy of Dr. O.J. Gi nther from Reproductive
Biology of the Mare. 2nd Ed.)
,…,
Ill a.
:I u
iii
‘i: …
Ill
E
0
‘C c w ….
0
…..
J:
Placentation, Gestation and Parturition 303
Figure 14-7. Luteal Progesterone Output During the First Half
of Gestation in the Mare
(Modified from Ginther, Reproductive Biologv of the Mare)
Progesterone { P4) from the primary corpus
luteum in creases rapidly after ovulation and
then decreases (hatched region) . Without
eCG , P4 woul d continue to decrease {dashed
line) and th e pregnancy would terminate.
Ill c
0 -:p
ns
:1.. ns -4J c c
:1.. Cl)
Cl) u
-4J c ns 0 :ru
Cl) Cl)
> c ·.p 0
ns :1..
Q) Cl) -4J cc: Ill Cl)
b.O
0
:1..
Q.
‘ ,,
Upon stimulation by eCG , th e primary CL is
stimulated and P4 in the maternal blood again
increases. If eCG were not produced , P4
would continue to decrease (dashed line).
As eCG continues to increase, accessory CL
develop and P4 increases until about day 100.
After day 100 , the placenta assumes the
major P4 producing ro le.
0 30 60 90 120 ISO 180 2 10 240 270
Days of Gestation
Figure 14-8. The Production of hCG and Progesterone During
Gestation in the Pregnant Woman
Human chorio nic gon ado trop in peaks at about 2. 5
months of gestation and then declines. T his period of
time is critical for ma intena nce of pregna ncy because
the corpus luteum as sumes primary responsibility for
progesterone secretion.
Ovarian P4
hCG
2 3 4
At about 2 .5 to 3 months of the placenta
begins to assume the primary responsibility for proges-
terone s ecretion and continues this role until the time of
parturition . hCG increases slightly between months 6
and 9 because of the increased placental mass.
Parturition
Placental P4
5 6 7 8 9
Months of Gestation
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302 Placentation, Gestation and Parturition
The Placenta is a Major Endocrine Organ
During Pregnancy
In addition to serv ing as a metabolic exchange
organ, the placenta serves as a transitory endocrine or-
gan. Hormones from the placenta gain access to both
the fetal and the matemal circulation.
The placenta secretes hormones that can:
• stimulate ovarian function
• maintain pregnancy
• influence fetal growth
• stimulate mammary function
• assist in parturition
The placenta of the mare produces a gonado-
tropin called equine chorionic gonadotropin (eCG).
Equine chorionic gonadotropin is also called pregnant
mare’s serum gonadotropin (PMSG). Equine cho-
rionic gonadotropin is produced by the endometrial
cups of the placenta. Endomeh·ial cups are a transient
placental endocrine gland. They begin producing eCG
at the time of attachment of the conceptus to the endo-
metrium. The relationship between the fom1ation of
the endometrial cups in the mare and the synthesis of
eCG is presented in Figure 14-6. As you can see, the
production of eCG is closely related to the weight of
the endometrial cups.
Equine chorionic gonadotropin acts as a lu-
teotropin and provides a stimulus for maintenance of the
primary cm·pus luteum . The primary corpus luteum
in the mare is defined as the corpus luteum fom1ed
from the ovulated follicle. In addition, eCG is respon-
sible for controlling the formation and maintenance of
supplementary (accessory) corpora lutca. As eCG
increases, the pregnant mare will often ovulate, thus
generating accessory corpora lutea. The eCG-induced
ovulations occur between days 40 and 70 of preg-
nancy. Luteinization (promoted by eCG) also occurs
in antral follicles that do not ovulate. Thus, eCG has a
significant positive impact on the ability of the ovary
to produce progesterone. Indeed, if one examines the
progesterone profile, it can be seen that there is a close
relationship between the concentrations of proges-
terone and the production of accessory corpora lutea
(See Figure 14-7).
In addition to its luteotropic action, eCG has
powerful FSH-like actions when administered to fe-
males of other species. In fact, eCG will cause marked
follicular development in most species. It is used com-
monly to induce superovulation where embryo transfer
is performed (cow, sheep, rabbit). In mares, however,
eCG does not exert significant FSH-like action.
–‘E
Db c .._,
l!l u
Cll
Figure 14-6. Production of
Equine Chorionic Gonadotropin
(eGG) is Closely Related to the
Weight of the
Endometrial Cups
(Modified from Ginther,
Reproductive Biolog v of the Mare )
175 10
ISO 9
125 I 0
I
I
I 100 I 7
I
I
75 I 6 I
I
I
50 I 5
4 25 I —-
40 60 80 100 120 14 0 16 0 18 0 200
Days of Gestation
Endometrial cups (EC) are seen here in
a U-shaped configuration. The fetus (F)
is surrounded by the amnion (not visible).
The membrane indicated by arrows is the
allantochorion . This specimen was re-
moved from a mare at 50 days of gestation.
(Photograph courtesy of Dr. O.J. Gi nther from Reproductive
Biology of the Mare. 2nd Ed.)
,…,
Ill a.
:I u
iii
‘i: …
Ill
E
0
‘C c w ….
0
…..
J:
Placentation, Gestation and Parturition 303
Figure 14-7. Luteal Progesterone Output During the First Half
of Gestation in the Mare
(Modified from Ginther, Reproductive Biologv of the Mare)
Progesterone { P4) from the primary corpus
luteum in creases rapidly after ovulation and
then decreases (hatched region) . Without
eCG , P4 woul d continue to decrease {dashed
line) and th e pregnancy would terminate.
Ill c
0 -:p
ns
:1.. ns -4J c c
:1.. Cl)
Cl) u
-4J c ns 0 :ru
Cl) Cl)
> c ·.p 0
ns :1..
Q) Cl) -4J cc: Ill Cl)
b.O
0
:1..
Q.
‘ ,,
Upon stimulation by eCG , th e primary CL is
stimulated and P4 in the maternal blood again
increases. If eCG were not produced , P4
would continue to decrease (dashed line).
As eCG continues to increase, accessory CL
develop and P4 increases until about day 100.
After day 100 , the placenta assumes the
major P4 producing ro le.
0 30 60 90 120 ISO 180 2 10 240 270
Days of Gestation
Figure 14-8. The Production of hCG and Progesterone During
Gestation in the Pregnant Woman
Human chorio nic gon ado trop in peaks at about 2. 5
months of gestation and then declines. T his period of
time is critical for ma intena nce of pregna ncy because
the corpus luteum as sumes primary responsibility for
progesterone secretion.
Ovarian P4
hCG
2 3 4
At about 2 .5 to 3 months of the placenta
begins to assume the primary responsibility for proges-
terone s ecretion and continues this role until the time of
parturition . hCG increases slightly between months 6
and 9 because of the increased placental mass.
Parturition
Placental P4
5 6 7 8 9
Months of Gestation
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14
304 Placentation, Gestation and Parturition
The second major gonadotropin of placental
origin is human chorionic gonadotropin (hCG). This
hormone is not only found in the human but in many
other primates. Often hCG (and eCG) may simply be
referred to as “CG”. It originates from the trophoblas-
tic cells of the chorion and is secreted as soon as the
blastocyst hatches from the zona pellucida. Human
chorionic gonadotropin can be detected in the blood
and urine of the pregnant woman as early as days 8 to
1 0 of gestation. It increases rapidly in the urine of the
pregnant woman, reaching a maximum value at about
2.5 months (See Figure 14-8). Its presence in the urine
constitutes the basis for over-the-counter pregnancy
diagnosis kits.
The primary role of hCG during early preg-
nancy is to provide a luteotropic stimulus for the
ovulatory corpus luteum as it transitions into the CL of
pregnancy. Luteal LH receptors also bind hCG resulting
in sustained progesterone production. Administration
of hCG to non-primate females can cause ovulation.
In fact, hCG is used commonly to induce ovulation in
superovulation protocols.
The Placenta Secretes Progesterone
and Estrogens
Progesterone is obligatory for early embry-
onic development because it provides the stimulus for
elevated secretion by the endometrial glands. High
progesterone is also responsible for the so-called ” pro-
gesterone block” that inhibits myometrial contractions.
Progesterone increases in the blood of the pregnant
female and peaks at different stages of gestation for
different species. The absolute levels of progesterone
also vary significantly among species (See Figure 14-9).
While progesterone is always produced by the corpus
luteum in early pregnancy, the role of the corpus luteum
in maintenance of pregnancy varies among species. In
some species (ewe, mare and woman), the corpus luteum
is not needed for the entire gestational period because
the placenta takes over production of progesterone. For
example, in the ewe the corpus 1uteum is responsible
for initial production of progesterone, but the placenta
assumes responsibility for its production after only 50
days of gestation (See Table 14-1 ). In other species
(sow or rabbit), lutectomy (surgical removal of corpora
lutea) will terminate pregnancy regardless of when this
occurs during gestation. Lutectomy in the cow up to
8 months of gestation will result in abortion. It should
be pointed out that even though the placenta takes over
for the corpus luteum of pregnancy, the corpus luteum
secretes progesterone throughout gestation.
In addition to progesterone, estradiol also is an
important product of the placenta, particularly during
the last part of gestation. In fact, the peak of estradiol
in most species signals the early preparttu·ient period.
The profiles of estradiol during gestation are presented
in the subsequent section on parhrrition.
Cea·tain Placental Hormones
Stimulate Mammaa·y Function of the Dam
and Fetal Growth
The placenta is known to produce a polypep-
tide hom1one known as placental lactogen that is also
ca lled somatomammotropin. Placental lactogens
have been found in rats, mice, sheep, cows and humans.
They are believed to be similar to grow th hormone, thus
promoting the growth of the fehts. Placental lactogen
also stimulates the mammary gland (lactogen ic) of the
dam. The degree to which fetal somatotropic (growth)
versus lactogenic effects occur depends on the species
(See Figure 14-10). For example, in the ewe ovine
placental lactogen (oPL) has a more potent lactogenic
activity than somatotropic activity. A simi lar condition
exists in humans, but not in the cow. Placentallactogens
have been shtdied most intensely in the ewe. They are
produced and secreted by the binucleate g iant cells of
the placenta. The secretory products of the binucleate
cells are transferred into the maternal circulation.
It is hypothesized that the sire may have an
effect on the degree to which the feht s can produce
placental lactogen. Such an effect could cause elevated
concentrations of placental lactogen by the ferns . In-
creased placental lactogen secretion would cause
enhanced stimulation of the maternal manunary gland
and thus promote elevated milk production. This theory
suggests that it might be possible for the sire to influence
fe tal placental lactogen and enhance milk production in
the dam. This sire-on-fetus-hypothesis has not been
teste d critically, but could hold promise for the genetic
improvement in dairy, beef cattle and goats.
Pl acental relaxin is secreted in humans, mares,
cats, dogs, pigs, rabbits and monkeys . Its function is to
cause softening and “relaxation” ofthe pelvic ligaments
to facilitate expulsion o f the ferns. The stimulus for
relaxin secretion is not known. Relaxin is not present
in the bovine placenta during any stage of gestation. It
is likely (with the exception of the rabbit) that relaxin,
during the time of parrnrition, originates from both the
ovary and the placenta. The role of relaxin is therefore
questionab le in the cow. Maternal blood re laxin levels
are the basis for a commercial pregnancy diagnostic test
at about 30 days of gestation in the bitch.
Placentation, Gestation and Parturition 305
Figure 14-9. Progesterone Profiles in Various Pregnant Females
so (P = Parturition ) -E 40 -..
00
1: 30 –
“C 20
0
0
iil 10
t e 2 3 4 Months of Gestation
“1 – 100 E -..
00
1: – 20
“C
0
..5!
al 10
®
t e 2 4 6 Months of Gestation 8 10 II
14-1. Length and Time of Placental Takeover for Progesterone Production in
Vanous Spec1es
SPECIES
Alpaca
Bitch
Camel
Cow
Ewe
Goat
Llama
Mare
Queen
Rabbit
Sow
Woman
GESTATION
LENGTH
11.4 mo
2 mo (65 days)
12.3 mo
9 mo
5 rna
5 mo
11.3 mo
11 mo
2 mo (65 days)
1 mo
3.8 mo
9mo
TIME OF PLACENTAL
TAKEOVER
11.4 mo (none)
2mo (none)
12.3 mo (none)
6-8 mo
50 days
5 mo (none)
11.3 mo (none)
70 days
2 mo (none)
1 mo (none)
3.8 mo (none)
60-70 days
14
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14
304 Placentation, Gestation and Parturition
The second major gonadotropin of placental
origin is human chorionic gonadotropin (hCG). This
hormone is not only found in the human but in many
other primates. Often hCG (and eCG) may simply be
referred to as “CG”. It originates from the trophoblas-
tic cells of the chorion and is secreted as soon as the
blastocyst hatches from the zona pellucida. Human
chorionic gonadotropin can be detected in the blood
and urine of the pregnant woman as early as days 8 to
1 0 of gestation. It increases rapidly in the urine of the
pregnant woman, reaching a maximum value at about
2.5 months (See Figure 14-8). Its presence in the urine
constitutes the basis for over-the-counter pregnancy
diagnosis kits.
The primary role of hCG during early preg-
nancy is to provide a luteotropic stimulus for the
ovulatory corpus luteum as it transitions into the CL of
pregnancy. Luteal LH receptors also bind hCG resulting
in sustained progesterone production. Administration
of hCG to non-primate females can cause ovulation.
In fact, hCG is used commonly to induce ovulation in
superovulation protocols.
The Placenta Secretes Progesterone
and Estrogens
Progesterone is obligatory for early embry-
onic development because it provides the stimulus for
elevated secretion by the endometrial glands. High
progesterone is also responsible for the so-called ” pro-
gesterone block” that inhibits myometrial contractions.
Progesterone increases in the blood of the pregnant
female and peaks at different stages of gestation for
different species. The absolute levels of progesterone
also vary significantly among species (See Figure 14-9).
While progesterone is always produced by the corpus
luteum in early pregnancy, the role of the corpus luteum
in maintenance of pregnancy varies among species. In
some species (ewe, mare and woman), the corpus luteum
is not needed for the entire gestational period because
the placenta takes over production of progesterone. For
example, in the ewe the corpus 1uteum is responsible
for initial production of progesterone, but the placenta
assumes responsibility for its production after only 50
days of gestation (See Table 14-1 ). In other species
(sow or rabbit), lutectomy (surgical removal of corpora
lutea) will terminate pregnancy regardless of when this
occurs during gestation. Lutectomy in the cow up to
8 months of gestation will result in abortion. It should
be pointed out that even though the placenta takes over
for the corpus luteum of pregnancy, the corpus luteum
secretes progesterone throughout gestation.
In addition to progesterone, estradiol also is an
important product of the placenta, particularly during
the last part of gestation. In fact, the peak of estradiol
in most species signals the early preparttu·ient period.
The profiles of estradiol during gestation are presented
in the subsequent section on parhrrition.
Cea·tain Placental Hormones
Stimulate Mammaa·y Function of the Dam
and Fetal Growth
The placenta is known to produce a polypep-
tide hom1one known as placental lactogen that is also
ca lled somatomammotropin. Placental lactogens
have been found in rats, mice, sheep, cows and humans.
They are believed to be similar to grow th hormone, thus
promoting the growth of the fehts. Placental lactogen
also stimulates the mammary gland (lactogen ic) of the
dam. The degree to which fetal somatotropic (growth)
versus lactogenic effects occur depends on the species
(See Figure 14-10). For example, in the ewe ovine
placental lactogen (oPL) has a more potent lactogenic
activity than somatotropic activity. A simi lar condition
exists in humans, but not in the cow. Placentallactogens
have been shtdied most intensely in the ewe. They are
produced and secreted by the binucleate g iant cells of
the placenta. The secretory products of the binucleate
cells are transferred into the maternal circulation.
It is hypothesized that the sire may have an
effect on the degree to which the feht s can produce
placental lactogen. Such an effect could cause elevated
concentrations of placental lactogen by the ferns . In-
creased placental lactogen secretion would cause
enhanced stimulation of the maternal manunary gland
and thus promote elevated milk production. This theory
suggests that it might be possible for the sire to influence
fe tal placental lactogen and enhance milk production in
the dam. This sire-on-fetus-hypothesis has not been
teste d critically, but could hold promise for the genetic
improvement in dairy, beef cattle and goats.
Pl acental relaxin is secreted in humans, mares,
cats, dogs, pigs, rabbits and monkeys . Its function is to
cause softening and “relaxation” ofthe pelvic ligaments
to facilitate expulsion o f the ferns. The stimulus for
relaxin secretion is not known. Relaxin is not present
in the bovine placenta during any stage of gestation. It
is likely (with the exception of the rabbit) that relaxin,
during the time of parrnrition, originates from both the
ovary and the placenta. The role of relaxin is therefore
questionab le in the cow. Maternal blood re laxin levels
are the basis for a commercial pregnancy diagnostic test
at about 30 days of gestation in the bitch.
Placentation, Gestation and Parturition 305
Figure 14-9. Progesterone Profiles in Various Pregnant Females
so (P = Parturition ) -E 40 -..
00
1: 30 –
“C 20
0
0
iil 10
t e 2 3 4 Months of Gestation
“1 – 100 E -..
00
1: – 20
“C
0
..5!
al 10
®
t e 2 4 6 Months of Gestation 8 10 II
14-1. Length and Time of Placental Takeover for Progesterone Production in
Vanous Spec1es
SPECIES
Alpaca
Bitch
Camel
Cow
Ewe
Goat
Llama
Mare
Queen
Rabbit
Sow
Woman
GESTATION
LENGTH
11.4 mo
2 mo (65 days)
12.3 mo
9 mo
5 rna
5 mo
11.3 mo
11 mo
2 mo (65 days)
1 mo
3.8 mo
9mo
TIME OF PLACENTAL
TAKEOVER
11.4 mo (none)
2mo (none)
12.3 mo (none)
6-8 mo
50 days
5 mo (none)
11.3 mo (none)
70 days
2 mo (none)
1 mo (none)
3.8 mo (none)
60-70 days
14
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306 Placentation, Gestation and Parturition
Figure 14-10. Placental Lactogen in Blood Near
Termination of Gestation
(From Martal in Reproduction in Man and Mammals)
Woman
4000_1
‘5:b c – 600 c
CIJ
Q
Ewe
0 Somatotropic activity
0 Lactogenic activity
!)0
0 …. u
Ill
…J
iii …. c
CIJ u
Ill
0::
500
400
300
200
100 Cow
0
270 120 ISO 270
Day of Gestation
Parturition is a Complex Cascade of
Physiologic Events
Rat
12
The fetus triggers the onset of parh1rition by
initiating a cascade of complex endocrine/biochemical
events. The fetal hypothalamo-pihlitary-adrenal axis is
obligatory for the initiation of parturition. During the
conclusion of gestation, fetal mass approaches the in-
herent space limitations of the uterus. This space limita-
tion has been considered by some to be the stimulus that
causes adrenal corticotropin (ACTH) to be secreted
by the fetal pih1itary. The fetal pituitary then stimulates
secretion of adrenal corticoids from the fetal adrenal
cortex. The elevation of fetal corticoids initiates a
cascade of events that cause dramatic changes in the
endocrine condition of the dam. These endocrine
changes cause two major events to occur: 1) removal of
the myometrial “progesterone block,” enabling myome-
trial contractions to begin and 2) increased reproductive
tract secretions, particularly by the cervix.
The three stages of parturition are:
• I: initiation of myometrial
contractions (removal ofprogesterone
block)
• II: expulsion of the fetus
• III: expulsion of the fetal mebranes
Placental lactogen has both
lactogenic actions and soma-
totrophic actions. The lac-
togenic activity of placental
lactogen promotes mammary
function in the dam, while
the somatotropic activity
promotes fetal growth .
Removal of the ” progesterone block” oc curs
because fetal cortisol promotes the synthes is of three
enzymes that convert progesterone to estradiol. The
conversion pathway is illustrated in Figure 14-11.
Progesterone, that is high at the placental interface,
is converted to 17a-hydroxyprogesterone by the en-
zyme !?a-hydroxylase. Fetal cortisol also triggers
the enzyme 17-20 desmolase to convert 17a-hydroxy-
progesterone to androstenedione. Androstenedione is
converted to estrogen by activation of an aromatase
enzyme. This involves aromatization of the A ring of
the steroid and removal of the 19 carbon. The conver-
sion of progesterone to estradiol accounts, at least in
part, for the dramatic drop in progesterone and dramatic
elevation of estradiol. The relationship between pro-
gesterone and estradiol during gestation is presented
in Figure 14-12.
In addition to converting progesterone to es-
h·adiol, feta l corticoids also cause the placenta to syn-
thesize PGF2a. . The synthesis of PGF 2a helps abo lish
the “progesterone block.” As both estradiol and prosta-
glandin become elevated, the myometrium becomes in-
creasingly more active and begins to display noticeable
contractions. Also, PGF 2a causes the CL of pregnancy
to regress, facilitating the decline in progesterone. The
drop in progesterone in some species is brought about
both by the conversion of progesterone into estradiol
and by the luteolytic process brought about by PGF2a·
Endocrine events associated with parhrrition are sum-
marized in Figures 14-13 and 14-14.
The fetus initiates Stage I of parturition.
Figure 14-11. Conversion of
Progesterone to Estradiol as
Parturition Nears
Corticoids from the fetus activate 17 a-hydroxylase,
17-20 desmolase and aro matase that co nvert
progesterone to estradio l. T h is c o nve rsio n
removes the “progeste rone block” to myometrial
activity.
17 a Hydroxyprogesterone
Androstenedione
CHJ
I
)
JJ-SD
CH1
I
· ‘ 1’
0
1 17: 20 I
l I Arom otase I
o)D'”
OH
. As the pressure inside the uterus continues to
mcrease, the feh1s in the cow, mare and ewe rotates so
the fi·ont feet and head are pos itioned to the poste-
of the dam (See Figure 14-15). Such a rotation is
tmportant to insure a proper delivery. If the fetus fails
to position itself correctly, dystocia (d ifficu lt birth)
may occur.
. As the levels of estradiol increase, coupled
With the e l_evation in levels of PGF2 a , the contracting
begms to push the fetus toward the cervix, ap-
plymg pressure to the cervix. T he endocrine events
that pro?1ote the firs t stage of parturition (dilation of
the cervtx and entry of the fe h1s into the cervical canal)
are summarized in Figure 14-14.
Pressure ?n the cervix brought about by in-
myometnal contractions activates pressure-
sens ttl_ve neurons located in the cervix that synapse in
the spmal cord and evenhmlly synapse with oxytocin
Ill c
0
‘.P
1.’: …,
c
Q)
v c
0 u
N w
“‘C c
Rl
Placentation, Gestation and Parturition 307
Figure 14-12. Estradiol and
Progesterone Profiles During
Gestation in the Mare, Cow,
Woman, Ewe and Sow
(P = Parturition )
Mare
I Woman I
p
I Sow I
t 10 20 30 40 so e Weel
1i
r:x:
Figure 14-13. Relative
Hormone Profiles in the Cow
During the Periparturient Period
Estrogens
I Prostaglandin
-I 0 -B -6 -4 -3 -2 -I 0 I 2 3 4 5
t Parturition
Days
Note that as fetal
cortisol levels rise,
P4 levels fall.
In most species, expulsion of the fetal mem-
branes quickly follows expulsion of the fetu s. Expulsion
of the fetal membranes requires that the chorionic villi
become dislodged from the crypts of the matemal side
of the placenta. This release of the chorionic villi is
believed to be brought about by powerful vasoconstric-
tion of arteries in the villi. Vasoconstriction reduces
pressure and thus allows the villi to be released from
the crypts. Obviously in some fonns of placentation,
there must be some maternal vasoconsh·iction. For ex-
ample, in animals that have hemochorial placentation,
matemal blood is adjacent to the feta l placenta. Thus,
if vasoconstriction does not occur on the matemal side,
hemorrhage is likely.
The duration of parhlrition is variable among
species and this variation is summarized in Table 14-2.
Extension beyond what is considered to be the normal
upper-end duration of parturition constitutes a difficult
birth (dystocia). Such prolonged parturition can result
in serious complications to both the fetus and the dam.
Placentation, Gestation and Parturition 309
Figure 14-14. Cascade of Events Prompted by Fetal Cortisol
f t FETAL ACTH
f /I Fetal cortisol j \
Placental P4 Relaxin enzymes [!iJ I PGF2a I …,.I
.———–. t / t ….._____+ -l
I Luteolysis t Secretion by
<;;?tract
Lubrication
t Myometrial
contractions
I+ Pressure
f
t Cervical
stimulation
t Oxytocin
t
Maximum
pressure
Pelvic
ligament
stretching
V
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14
308 Placentation, Gestation and Parturition
producing neurons in the hypothalamus (See Figure
14- I 5). Oxytocin, released into the systemic circula-
tion, acts to facilitate the myomeh·ial contractility
initiated by estradiol and by PGF2u· As the pressure
against the cervix continues to increase, so does the
oxytocin secretion, and thus the force of conh·action of
the myometrial smooth muscle begins to peak. When
this occurs, the fetus enters the cervical canal and the
first stage of parturition is complete.
Expulsion of fetus (Stage II) requires
strong myometrial and abdominal
muscle contractions.
Another important hormone involved in suc-
cessful parhrrition is relaxin. Relaxin is a glycopro-
tein that is produced by either the corpus luteum or the
placenta, depending upon the species. The synthesis
of relaxin is stimulated by PGF2a · Relaxin causes a
softening of the connective tissue in the cervix and
promotes elasticity of the pelvic ligaments. Thus,
this hormone prepares the birth canal by loosening
the supportive tissues so that passage of the fehts can
occur with relative ease.
One of the dramatic effects of estradiol
elevation prior to parturition is that it initiates secre-
tory activity of the reproductive tract in general and
particularly the cervix. As estradiol increases, the
cervix and vagina begin to produce mucus. This
mucus washes out the cervical seal of pregnancy
and thoroughly lubricates the cervical canal and the
vagina. Mucus reduces friction and enables the fetus
to exit the reproductive tract with relative ease. As
myometrial contractions continue to increase, the
feet and head of the fehts begin to put pressure on the
fetal membranes. When the pressure reaches a certain
level, the membranes rupture, with subsequent loss of
amniotic and allantoic fluid. This fluid also serves to
lubricate the birth canal. As the fetus enters the birth
canal, it becomes hypoxic (deprived of adequate levels
of oxygen). This hypoxia promotes fetal movement
that, in tum, promotes further myometrial contrac-
tion. This positive feedback system creates a set of
conditions where the time of parhtrition is reduced
because an increased strength of contraction follows
fetal movement. In a sense, the fehts is controlling
its exit from the uterus. The uterine contractions are
accompanied by abdominal muscle contractions of the
dam that further aid in expulsion of the fetus.
VI
1:
0
"" "'"' 1:
Ql
u
1:
0 u
Ql
1:
0
E
"" 0 :r:
Ql
>
1i
r:x:
Figure 14-13. Relative
Hormone Profiles in the Cow
During the Periparturient Period
Estrogens
I Prostaglandin
-I 0 -B -6 -4 -3 -2 -I 0 I 2 3 4 5
t Parturition
Days
Note that as fetal
cortisol levels rise,
P4 levels fall.
In most species, expulsion of the fetal mem-
branes quickly follows expulsion of the fetu s. Expulsion
of the fetal membranes requires that the chorionic villi
become dislodged from the crypts of the matemal side
of the placenta. This release of the chorionic villi is
believed to be brought about by powerful vasoconstric-
tion of arteries in the villi. Vasoconstriction reduces
pressure and thus allows the villi to be released from
the crypts. Obviously in some fonns of placentation,
there must be some maternal vasoconsh·iction. For ex-
ample, in animals that have hemochorial placentation,
matemal blood is adjacent to the feta l placenta. Thus,
if vasoconstriction does not occur on the matemal side,
hemorrhage is likely.
The duration of parhlrition is variable among
species and this variation is summarized in Table 14-2.
Extension beyond what is considered to be the normal
upper-end duration of parturition constitutes a difficult
birth (dystocia). Such prolonged parturition can result
in serious complications to both the fetus and the dam.
Placentation, Gestation and Parturition 309
Figure 14-14. Cascade of Events Prompted by Fetal Cortisol
f t FETAL ACTH
f /I Fetal cortisol j \
Placental P4 Relaxin enzymes [!iJ I PGF2a I …,.I
.———–. t / t ….._____+ -l
I Luteolysis t Secretion by
<;;?tract
Lubrication
t Myometrial
contractions
I+ Pressure
f
t Cervical
stimulation
t Oxytocin
t
Maximum
pressure
Pelvic
ligament
stretching
V
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oo
ks
.ir
31 0 Placentation, Gestation and Parturition
Figure Pressure on the Cervix Causes Oxytocin Release and
Subsequent Myometrial Contractions
As the fetus moves through the
birth canal , elevated pressure
on the cervix stimulates sensory
neurons. A neural pathway ter-
minates in the paraventricular nu-
cleus (PVN) and causes
to be secreted from the postenor
pituitary lobe. Oxytocin
contraction of the myometnum .
Afferent
neurons
H ypothalamu s
Diffic ulties in parturi tion usually occur in the second
stage (expulsion of the fetus). One cause of dystocia
is excessive size of the fetus. Fetal size is controlled
by both the dam and the sire. In primiparous dams, it
is always advisab le to breed females to a male of small
body size so that fetal size does not exceed the ability
of the fe male to give birth successfully.
A second cause of dystocia is failure of proper
feta l rotation. About 5% of all births in cattle are char-
acterized by abnormal positioning of the fetus during
p ar turition . Such abnormal positioning results in di f-
ficult births a nd sometimes impossib le presentations/
positions that require caesarean section.
A third cause of dystocia is multiple births in
monotoco us species. Tw ins generally cause dystocia.
This is because: 1) both twins may be presented simul-
taneously, 2) the first fetus is positioned abnommlly and
therefore blocks the second or 3) the uterus becomes
fat igued by difficult and sustained contractions. A dis-
cussion of obstetrical procedures used to correct these
problems is beyond the scope of this book, but c .... atfbe
researched by consulting the appropriate references at
the conclusion of this chapter.
Placentation, Gestation and Parturition 311
Expulsion of fetal membranes
(Stage III) requires myometrial
contractions.
Myometrial contractions continue after expul-
sion of the fe tus although they are not as strong. These
contractions are responsible for expell ing the p lacenta.
T he ti me required for expulsion of the placenta varies
significantly among species. This variation is presented
in Table 14-2. Retention of the fetal membranes (also
referred to as "retained placenta"), is not uncommon in
ruminants, especially dairy cows. This condition will
occur in 5- 15% of parturitions in healthy dairy cows.
The underlying cause of retained placenta appears to
be that placental connective tissue is not enzymatically
degraded by cotyledonary proteolytic enzymes. Thus,
fetal cotyledons remain attached to matemal cotyledons.
Retained placenta is rare is mares, sows, bitches and
queens .
Table 14-2. Stages and Duration of Parturition Among Various Species
Stage I Stage II Stage III
(Mllometrial Contractions/ (Fetal (Fetal Membrane
Cervical Dilation)
Alpaca 2 to 6h 5 to 90 min 45 to 180 min
Bitch 6 to 12h 6h (24h in large litters) most placentas pass with
neonate or within 15 min
of birth
Camel 3 to 48h 5 to 45 min 40 min
Cow 2 to 6h 30 to 60 min 6 to 12h
Ewe 2 to 6h 30 to 120 min 5 to 8h
Llama 2 to 6h 5 to 90 min 45 to 180 min
Mare 1 to 4h 12 to 30 min 1h
Sow 2 to 12h 150 to 180 min 1 to 4h
Queen 4 to 42h 4 kittens/litter, most placentas pass with
30-60 min/kitten neonate
Woman 8+h 2h 1h or less
14
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oo
ks
.ir
31 0 Placentation, Gestation and Parturition
Figure Pressure on the Cervix Causes Oxytocin Release and
Subsequent Myometrial Contractions
As the fetus moves through the
birth canal , elevated pressure
on the cervix stimulates sensory
neurons. A neural pathway ter-
minates in the paraventricular nu-
cleus (PVN) and causes
to be secreted from the postenor
pituitary lobe. Oxytocin
contraction of the myometnum .
Afferent
neurons
H ypothalamu s
Diffic ulties in parturi tion usually occur in the second
stage (expulsion of the fetus). One cause of dystocia
is excessive size of the fetus. Fetal size is controlled
by both the dam and the sire. In primiparous dams, it
is always advisab le to breed females to a male of small
body size so that fetal size does not exceed the ability
of the fe male to give birth successfully.
A second cause of dystocia is failure of proper
feta l rotation. About 5% of all births in cattle are char-
acterized by abnormal positioning of the fetus during
p ar turition . Such abnormal positioning results in di f-
ficult births a nd sometimes impossib le presentations/
positions that require caesarean section.
A third cause of dystocia is multiple births in
monotoco us species. Tw ins generally cause dystocia.
This is because: 1) both twins may be presented simul-
taneously, 2) the first fetus is positioned abnommlly and
therefore blocks the second or 3) the uterus becomes
fat igued by difficult and sustained contractions. A dis-
cussion of obstetrical procedures used to correct these
problems is beyond the scope of this book, but c .... atfbe
researched by consulting the appropriate references at
the conclusion of this chapter.
Placentation, Gestation and Parturition 311
Expulsion of fetal membranes
(Stage III) requires myometrial
contractions.
Myometrial contractions continue after expul-
sion of the fe tus although they are not as strong. These
contractions are responsible for expell ing the p lacenta.
T he ti me required for expulsion of the placenta varies
significantly among species. This variation is presented
in Table 14-2. Retention of the fetal membranes (also
referred to as "retained placenta"), is not uncommon in
ruminants, especially dairy cows. This condition will
occur in 5- 15% of parturitions in healthy dairy cows.
The underlying cause of retained placenta appears to
be that placental connective tissue is not enzymatically
degraded by cotyledonary proteolytic enzymes. Thus,
fetal cotyledons remain attached to matemal cotyledons.
Retained placenta is rare is mares, sows, bitches and
queens .
Table 14-2. Stages and Duration of Parturition Among Various Species
Stage I Stage II Stage III
(Mllometrial Contractions/ (Fetal (Fetal Membrane
Cervical Dilation)
Alpaca 2 to 6h 5 to 90 min 45 to 180 min
Bitch 6 to 12h 6h (24h in large litters) most placentas pass with
neonate or within 15 min
of birth
Camel 3 to 48h 5 to 45 min 40 min
Cow 2 to 6h 30 to 60 min 6 to 12h
Ewe 2 to 6h 30 to 120 min 5 to 8h
Llama 2 to 6h 5 to 90 min 45 to 180 min
Mare 1 to 4h 12 to 30 min 1h
Sow 2 to 12h 150 to 180 min 1 to 4h
Queen 4 to 42h 4 kittens/litter, most placentas pass with
30-60 min/kitten neonate
Woman 8+h 2h 1h or less
14
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14
312 Placentation, Gestation and Parturition
Further
PHENOMENA
for Fertility
The term "caesarean" was derived from the
false notion that Julius Caesar was born by
removing him from his mother through an
incision in the abdominal and uterine wall.
His family name, Caesar was derived from
the belief that Julius' ancestors (centuries
before him) were hom in such a way. The
name Caesar is derived from the Latin word
"caesus" that means "to cut". The name also
fits the way Julius died.
In a number of teleost fishes (fishes with a
more or less ossified skeleton) the female
incubates the eggs in her mouth and in some
species the male does the same. The term
"keep your mouth shut" has a special meaning
in this species.
In pipe fishes and sea horses the female lays
her eggs in a brood pouch of the male and he
is responsible for gestation. In fact, several
females may lay eggs in one male's brood
pouch. The brood pouch offers a special
environment for developing offspring and is
under the control of prolactin.
Lampreys (a predatory eel) build nests in
sandy bottomed sh·eams. They assemble rock
walls to slow the water running over the nest.
At spawning, they stir up the sand that sticks
to the eggs. The sand weights the eggs and
prevents them from floating downstream. It
also reduces predation. This is mwtherform
of attachment that enables successful embryo-
genesis.
Infant kangaroos in their mother's pouches
nurse from two nipples, and two babies of
different ages commonly nurse at the same
time. So, the mother kangaroo produces two
kinds of milk- on one side, fully rich for the
younger and 011 the other side, a sort of skim
for the elder.
The most prolific mammal in existence is the
tiny J'Odent known as the multimammate rat.
One female is capable of producing up to 120
offspring a year if conditions are favorable.
This is because she has 24 teats, the most of
any female mammal. It is rare that all of them
are used but when they are a multimammate
population explosion catt occur.
The female Egyptian spiny mouse acts as a
midwife to other females. She bites through the
umbilical cord and licks the neonates while the
mother continues to deliver the litter.
The female African elephant has a gestation
period of 1.8 years. The calf weighs about
300 pounds at birth and nurses for about three
years.
Durittg the 19th Century, adultery was so
feared that the chastity belt was invented. Such
belts were devices that were locked around the
genitalia to prevent copulation. It has
been recorded that afaitliful wife locked into a
chastity belt discovered that she was pregnant
some months after her husband had left 011 a
crusade. Her husband had the o11ly key. Her
pregnancy progressed and eventually the vil-
lage blacksmith had to be called in to remove
the chastity belt.
During the Middle Ages, prostitution was
considered to he an honest and essential pro-
fession. This was because prostitution was
considered as a means to prevellt adulte1y,
homosexual behavior and masturbation. The
Church actually condoned prostitution for tlzis
reason.
The Mayans believed in a maize god. Since
corn was a nutritional staple for these people,
they revered it and believed that corn was
symbolic of both the male am/female. From a
nutritional perspective they believed that corn
was nurturing like a breast and that
each individual kernel had powerful fertilizing
capabilities like spermatozoa. Once the seeds
were planted in the earth and the mature com
was produced, the cob represented the penis
and the husk represented the vagina. Thus, the
ear of com was also symbolic of copulation.
Kev References
Arthur, G.H., D.E. Noakes, H. Pearson and T.J. Parkin-
son. 1996. Veterinarv Reproduction and Obstetrics.
7th Edition. W.B. Saunders Co. Phi lade lphia. ISBN
0-7020-1 758-X.
Catchpole, H.R. 199 1. " Hormona l mechanisms in
pregnancy and parturition" in Rep roduction in Domestic
Animals. 4 th Edition. P.T. Cupps, ed., Academic Press,
San Diego. ISBN 0-1 2-196575-9.
Flood, P.F. 199 I. "The deve lopment of the conceptus
and its relation ship to the uterus" in Reproduction in
Domestic Animals . 4th Edition. P.T. Cupps, ed., Aca-
demic Press, San Diego. ISBN 0-1 2-1965 75-9.
Fuchs, A.R. and M.J. Fields . 1999. "Parturition, no.!Jhtl-
man mammals" in Encvclopedia o(Reproducilon: Vol.
3 p703-7 I 6. Knobil , E. and J.D . Nei ll, eds . Academi c
Press, San Diego. ISBN 0- 12-227023- 1.
Ginther, O J . 1992. Reproductive Biology o{the Mare.
2nd Edition. Equiservices, Cross Plains, WI . Library of
Congress Catalog No. 9 1-075595 .
Johnston, S.D. M.V. R oot, Kustritz and P.N. S. O lson.
200 I. Canine and Feline Theriogenologv. W.B. Saun-
ders, Phi ladelphia. ISBN 0-72 16- 5607-2.
Morrow, D.A. 1986. Current Therapy in Theriogenol-
2nd Edition. W.B. Saunders Co. Philadelphia.
ISBN 0-7216-6580-2.
Mossman, H.W. 1987. Vertebrate Fetal Membranes.
Rutgers University Press, New Brunsw ick. ISBN
0-8135-1132-1.
Thibault, C. , M.C. Levasseur and R.H. F. Hunter.eds.
I 993. Reproduction in Man and Mam mals. Ellips es,
Paris. ISBN 2-7298-9354-7.
Placentation, Gestation and Parturition 313
14
V
et
B
oo
ks
.ir
14
312 Placentation, Gestation and Parturition
Further
PHENOMENA
for Fertility
The term "caesarean" was derived from the
false notion that Julius Caesar was born by
removing him from his mother through an
incision in the abdominal and uterine wall.
His family name, Caesar was derived from
the belief that Julius' ancestors (centuries
before him) were hom in such a way. The
name Caesar is derived from the Latin word
"caesus" that means "to cut". The name also
fits the way Julius died.
In a number of teleost fishes (fishes with a
more or less ossified skeleton) the female
incubates the eggs in her mouth and in some
species the male does the same. The term
"keep your mouth shut" has a special meaning
in this species.
In pipe fishes and sea horses the female lays
her eggs in a brood pouch of the male and he
is responsible for gestation. In fact, several
females may lay eggs in one male's brood
pouch. The brood pouch offers a special
environment for developing offspring and is
under the control of prolactin.
Lampreys (a predatory eel) build nests in
sandy bottomed sh·eams. They assemble rock
walls to slow the water running over the nest.
At spawning, they stir up the sand that sticks
to the eggs. The sand weights the eggs and
prevents them from floating downstream. It
also reduces predation. This is mwtherform
of attachment that enables successful embryo-
genesis.
Infant kangaroos in their mother's pouches
nurse from two nipples, and two babies of
different ages commonly nurse at the same
time. So, the mother kangaroo produces two
kinds of milk- on one side, fully rich for the
younger and 011 the other side, a sort of skim
for the elder.
The most prolific mammal in existence is the
tiny J'Odent known as the multimammate rat.
One female is capable of producing up to 120
offspring a year if conditions are favorable.
This is because she has 24 teats, the most of
any female mammal. It is rare that all of them
are used but when they are a multimammate
population explosion catt occur.
The female Egyptian spiny mouse acts as a
midwife to other females. She bites through the
umbilical cord and licks the neonates while the
mother continues to deliver the litter.
The female African elephant has a gestation
period of 1.8 years. The calf weighs about
300 pounds at birth and nurses for about three
years.
Durittg the 19th Century, adultery was so
feared that the chastity belt was invented. Such
belts were devices that were locked around the
genitalia to prevent copulation. It has
been recorded that afaitliful wife locked into a
chastity belt discovered that she was pregnant
some months after her husband had left 011 a
crusade. Her husband had the o11ly key. Her
pregnancy progressed and eventually the vil-
lage blacksmith had to be called in to remove
the chastity belt.
During the Middle Ages, prostitution was
considered to he an honest and essential pro-
fession. This was because prostitution was
considered as a means to prevellt adulte1y,
homosexual behavior and masturbation. The
Church actually condoned prostitution for tlzis
reason.
The Mayans believed in a maize god. Since
corn was a nutritional staple for these people,
they revered it and believed that corn was
symbolic of both the male am/female. From a
nutritional perspective they believed that corn
was nurturing like a breast and that
each individual kernel had powerful fertilizing
capabilities like spermatozoa. Once the seeds
were planted in the earth and the mature com
was produced, the cob represented the penis
and the husk represented the vagina. Thus, the
ear of com was also symbolic of copulation.
Kev References
Arthur, G.H., D.E. Noakes, H. Pearson and T.J. Parkin-
son. 1996. Veterinarv Reproduction and Obstetrics.
7th Edition. W.B. Saunders Co. Phi lade lphia. ISBN
0-7020-1 758-X.
Catchpole, H.R. 199 1. " Hormona l mechanisms in
pregnancy and parturition" in Rep roduction in Domestic
Animals. 4 th Edition. P.T. Cupps, ed., Academic Press,
San Diego. ISBN 0-1 2-196575-9.
Flood, P.F. 199 I. "The deve lopment of the conceptus
and its relation ship to the uterus" in Reproduction in
Domestic Animals . 4th Edition. P.T. Cupps, ed., Aca-
demic Press, San Diego. ISBN 0-1 2-1965 75-9.
Fuchs, A.R. and M.J. Fields . 1999. "Parturition, no.!Jhtl-
man mammals" in Encvclopedia o(Reproducilon: Vol.
3 p703-7 I 6. Knobil , E. and J.D . Nei ll, eds . Academi c
Press, San Diego. ISBN 0- 12-227023- 1.
Ginther, O J . 1992. Reproductive Biology o{the Mare.
2nd Edition. Equiservices, Cross Plains, WI . Library of
Congress Catalog No. 9 1-075595 .
Johnston, S.D. M.V. R oot, Kustritz and P.N. S. O lson.
200 I. Canine and Feline Theriogenologv. W.B. Saun-
ders, Phi ladelphia. ISBN 0-72 16- 5607-2.
Morrow, D.A. 1986. Current Therapy in Theriogenol-
2nd Edition. W.B. Saunders Co. Philadelphia.
ISBN 0-7216-6580-2.
Mossman, H.W. 1987. Vertebrate Fetal Membranes.
Rutgers University Press, New Brunsw ick. ISBN
0-8135-1132-1.
Thibault, C. , M.C. Levasseur and R.H. F. Hunter.eds.
I 993. Reproduction in Man and Mam mals. Ellips es,
Paris. ISBN 2-7298-9354-7.
Placentation, Gestation and Parturition 313
14
V
et
B
oo
ks
.ir