Page 498 - Anatomy and Physiology of Farm Animals, 8th Edition

The Ovary and Estrous Cycles / 483

(A) (B)
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Figure 27-7. Ovary of a mare (A) and a cow (B) with corpora lutea (cl). The tunica albuginea (ta) of the
equine ovary is significantly thicker than that of the bovine, thus allowing the bovine corpus luteum to
bulge from the ovarian surface (crown). The probe is sitting in the ovulation fossa of the equine ovary and
two small follicles (f) have been indicated. A corpus albicans (ca) is evident in the bovine ovary. Source:
images courtesy of Library of Reproduction Images (LORI: http://lorimainsection.blogspot.ca).

and the role of the corpus luteum during and luteolysis occurs shortly thereafter
pregnancy are discussed in more detail in (Fig. 27‐4).
Chapter 28. Luteolysis can be induced in cattle by
The basic function of progesterone during administering analogs of PGF at any
2α
this part of the estrous cycle is to prepare for point in the estrous cycle as long as a
a pregnancy. Progesterone increases uterine corpus luteum is intact and functioning.
gland secretion and inhibits uterine motility The removal of the corpus luteum per-
to promote implantation and maintain mits rapid development of new follicles
any pregnancy (Fig. 27‐4). Progesterone also and ovulation in about 3 days. The use of
promotes mammary gland development. PGF to induce ovulation and estrus at
2α
High levels of progesterone act on the hypo- a predictable time is a management
thalamic–adenohypophyseal axis to inhibit tool to synchronize the estrus cycles of
further LH secretion. groups of animals.
If a successful pregnancy is not estab-
lished, the corpora lutea must undergo
regression (luteolysis) to permit the animal Phases of the Estrous Cycle
to continue the estrous cycle. The humoral
signals between the uterus and ovary that The basic pattern of the estrous cycle is the
initiate or inhibit luteolysis differ among same in all domestic animals, but some spe-
species. In most domestic species (mare, cies differences are found in specific parts of
cow, ewe, sow), prostaglandin F the cycle. Some specifics about relevant
2α
(PGF ) is the humoral signal used by the farm species are summarized in Table 27‐1.
2α
nonpregnant uterus to stimulate luteolysis. The estrous cycle may be divided into sev-
The nonpregnant uterus increases PGF eral phases based on behavioral changes or
2α
synthesis; releases are increased after ovu- structural changes in internal and external
lation at times appropriate for the species genitalia. These phases are called proestrus,
(e.g., 10 days for sows and 14 days for ewes); estrus, metestrus, and diestrus.