Page 493 - Anatomy and Physiology of Farm Animals, 8th Edition

478 / Anatomy and Physiology of Farm Animals

estrogen acts on granulosa cells to increase
FSH and LH receptors, and, together with
VetBooks.ir these gonadotrophins, it promotes further

granulosa cell replication, growth, and
secretion. The overall effect is that locally
produced estrogens promote the develop-
ment of the follicle from which they are
being produced. This is characterized as a
local positive feedback effect of the estro-
gens. This positive feedback effect is one
factor in the selection process that deter-
mines which of the developing follicles will
ultimately produce the ovum and ovulate.
A second factor is that circulating estro-
gens have a negative feedback effect on
FSH secretion from the adenohypophysis.
The decrease in FSH during this period
contributes to the atresia of more slowly
developing follicles. The selection process
also involves another follicular hormone,
inhibin, discussed later.
Estrogens from developing follicles are
also necessary to prepare the follicles and
the hypothalamic–adenohypophyseal axis
for ovulation. Within the ovary, estrogens
promote an increase in LH receptors in
Figure 27-2. Bovine oocyte shortly before ovula- thecal cells so that these cells increase their
tion. Oocyte is immediately surrounded by zona production of androgens and appropriately
pellucida (a). The granulosa cells immediately respond to LH at the time of ovulation.
adjacent to the zona can be distinguished as the
corona radiata (b) and the cumulus oophorus (c). Circulating estrogens are the endocrine
The cumulus appears to have separated from the signal of a maturing follicle ready for ovu-
membrana granulosa (e). The basement membrane lation and therefore promote an increase
of the follicle (f) and theca interna (g) are also visi- of GnRH release from the hypothalamus,
ble. Source: Dellmann and Eurell, 1998. Reproduced an increase in GnRH receptors on gonado-
with permission of John Wiley & Sons, Inc. trope cells, and synthesis of LH within
the adenohypophysis. It is the positive
feedback of estrogen that conditions the
(also known as vesicular or Graafian follicles) hypothalamic–adenohypophyseal axis so
when an antrum can be identified among that the short‐term, but very large LH
the granulosa cells (Figs. 27‐1 and 27‐3). release (termed LH surge) necessary for
Theca surrounding tertiary follicles have ovulation can be delivered (Fig. 27‐4)
two layers, the theca externa and theca Nonlitter‐bearing animals typically have
interna (Fig. 27‐1). The internal layer is one or two follicles per estrous cycle that
highly vascular and contains thecal cells develop faster and grow larger than the
with cellular characteristics of steroid‐pro- rest. These are dominant follicles. In pri-
ducing cells. The theca externa primary mates there is typically only one dominant
consists of connective tissue. follicle per estrous cycle, and it provides
The estrogen produced by the granulosa the ovum. The development of the domi-
cells acts as a paracrine agent on the devel- nant follicle is accelerated after the corpus
oping follicle and also enters the systemic luteum (discussed later) from the previous
circulation to affect other sites throughout estrous cycle has regressed (luteolysis).
the body (Figs. 27‐4 and 27‐5). Locally, the The phase of the estrous cycle in primates