Page 496 - Anatomy and Physiology of Farm Animals, 8th Edition

The Ovary and Estrous Cycles / 481

Ovulation levels are decreasing and progesterone
levels are increasing at ovulation. Under
VetBooks.ir In mature follicles just prior to ovulation, the influence of the LH surge, granulosa

cells also acquire the ability to synthesize
ova are usually seen surrounded by a halo
of granulosa cells (cumulus) that are con- prostaglandins, thromboxanes, and leu-
tinuous with granulosa cells lining the kotrienes. These agents induce a local
fluid‐filled antrum (Fig. 27‐2). The large, response similar to inflammation that will
thin‐walled follicles bulge from the ovarian weaken the wall of the follicle and promote
surface (Fig. 26‐3). The primary oocyte, its rupture.
which remains in an arrested stage of mei-
osis during follicular development, under- Spontaneous and Reflex Ovulators
goes the first meiotic division to produce a
secondary oocyte and the first polar body The LH surge and ovulation occur in most
just before ovulation in most species (see domestic species (mare, cow, ewe, and
Chapter 28). The first polar body is sow) independent of copulation, and these
extruded from the ovary with the second- species are spontaneous ovulators. In
ary oocyte. In the mare, this first meiotic these species, the preovulatory increase in
division occurs just after ovulation. estrogens from developing follicles is the
primary event that brings about ovulation.
Luteinizing Hormone Surge The female animals of some species (cat,
rabbit, ferret, mink, camel, llama, and
alpaca) usually require copulation for ovu-
In most species, LH release from the ade- lation. These are induced ovulators. In
nohypophysis increases sevenfold to ten- these species, the final preovulatory surge
fold during the 24 hours prior to ovulation of GnRH, and subsequent LH surge, is
and then rapidly decreases as plasma levels apparently dependent on a neural reflex
return to preovulatory levels. This short‐ elicited by vaginal stimulation. Induced
term change in LH release is the LH surge. ovulators have characteristic estrous
While most species follow the general cycles and follicular development, but
endocrine pattern depicted in Figure 27‐4, mature follicles regress if copulation does
it is important to recognize the sex steroid not occur.
feedback loops that initiate these changes
in hormone concentrations. While the
peak of the LH surge in the mare occurs 24 Seasonal Transition
hours after ovulation, the rise in LH is
dependent on changes in the hypophy- Most domestic species are seasonal breed-
seal–adenohypophysis axis and an increase ers. The onset of seasonal estrus and the
in adenohypophysis content of LH induced length of gestation align with the availabil-
by the rapid rise in estrogens secreted by ity of favorable weather and nutrient avail-
large, mature follicles. The extremely high ability. For example, horses are long‐day
levels of LH promote the final develop- breeders, coming into estrus in spring;
ment of the primary oocyte and its pro- sheep are short‐day breeders, with sea-
gress through the first meiotic division, sonal estrus in the fall; however, foaling
thus preparing the oocyte for ovulation. and lambing season is in the spring for
Granulosa cells also respond to the LH both species as a result of different gesta-
surge by transforming from estrogen‐pro- tional lengths. Changes in photoperiod
ducing cells to progesterone‐producing result in changes in the pulsatile release of
cells. This is part of luteinization, the GnRH. The subsequent increases in FSH
transformation of granulosa cells to luteal and LH initiate the first ovulation of the
cells (cells of a corpus luteum). This pro- season. In seasonal breeders, the period
cess begins prior to ovulation, so estrogen prior to the first ovulation is referred to as