Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

III. Physiology of Reproductive Hormones in the Female
645
Concentration
Progesterone
Prostaglandin
LH
FIGURE 21-4 Schematic presentation of the
preovulatory events in general applicable to the
cow, doe, ewe, and sow. These endocrine events
also occur in the mare, but in this species, the
release of LH occurs over a considerably longer
time period.
Estradiol
4 3 2 1 0 1 2
Days before and after LH peak
Cow
Doe
Ewe
Sow
Mare
Queen
Bitch
CL life span
Estrus
Ovulation
Day of cycle
CL life span
Estrus
Ovulation
Day of cycle
CL life span
Estrus
Ovulation
Day of cycle
CL life span
Estrus
Ovulation
Day of cycle
CL life span
Estrus
Ovulation
Day of cycle
CL life span
Estrus
Ovulation
Day of cycle
1 3 5 7 9 11 13 15 17 19 21 1 3 5 7 9 11 13 15 17 19 21 1 3 5......
1 3 5 7 9 11 13 15 17 1 3 5 7 9 11 13 15 17 1 3 5......
1 3 5 7 9 11 13 15 17 19 21 1 3 5 7 9 11 13 15 17 19 21 1 3 5......
1 3 5 7 9 11 13 15 17 19 21 1 3 5 7 9 11 13 15 17 19 21 1 3 5......
No coitus
Coitus - nonpregnant
1 3 5 7 9......... 21 1 3 5 7 9 1 3 5 7 9 ...............35 37 39 41 1 3 5 7...
1 3 5 7 9 11............................................................................50 – 80
FIGURE 21-5 Comparative data on the
duration of estrus, time of ovulation, and
duration of corpus luteum function in the
cow, ewe, sow, mare, queen, and bitch.
Modified from Stabenfeldt (1974).
function of GnRH is to elicit sexual receptivity. Thus, the
onset of the preovulatory LH surge and sexual receptivity
is coordinated via GnRH synthesis and release.
Following ovulation, a CL is formed under the influence
of pituitary gonadotropins. In most species, LH is
the major luteotropin, though prolactin is thought to play a
role in sheep and in rodents. If pregnancy does not ensue,
the CL regresses, which permits the estrus cycle to be
repeated. This well-timed sequence occurs repetitively at
set intervals if not interrupted by pregnancy.
A summary of estrus cycle activity in common domestic
animal species is given in Figure 21-5 . Most of the
domestic species have evolved from seasonally breeding
ancestors. In mammals, photoperiod, via the circadian
melatonin profile, synchronizes endogenous circannual
rhythms ( Lincoln et al. , 2003 ). The mare, ewe, doe, and
queen undergo cyclic ovarian activity only during the
breeding season in response to seasonal changes in photoperiod.
In contrast, cattle, pigs, and dogs can reproduce
the entire year, although vestiges of seasonality have been
demonstrated (e.g., Andersson et al. , 1998 ).
Estrus cycle length is approximately 21 days in the
cow, doe, mare, and sow and 17 days in the ewe. The bitch
has a much longer estrus cycle, the luteal phase often being
between 50 to 80 days in duration. The interval between
cyclic ovarian activities in the bitch is extended even further
by the occurrence of a 4- to 5-month anestrus period
following regression of the CL. The cat is an induced ovulator
requiring coital stimulation for ovulation and thus for
CL formation. In the absence of coitus, follicles develop
every 15 to 20 days, with follicular growth and regression
occupying 5 to 7 days of each period. An ovulatory, nonfertile
mating results in the formation of CL that persists
for approximately 35 days and a syndrome termed pseudopregnancy
in the cat.
B . Control of the Corpus Luteum
The regression of the CL (luteolysis) is a key event that
is responsible for the well-timed estrus cyclicity seen in
most domestic species. The importance of the uterus in
the control of the life span of the CL has been documented