Page 492 - Anatomy and Physiology of Farm Animals, 8th Edition

The Ovary and Estrous Cycles / 477

Basement membrane Theca externa
Zona
VetBooks.ir Granulosa cells pellucida Theca interna
Oocyte
Granulosa cells
Antrum
(a)
(b) Zona pellucida
(c)
Oocyte

(d)

Released ovum

Corpus luteum
(e)
Figure 27-1. Sagittal section of an ovary showing the origin, growth, and ovulation of a follicle and a
corpus luteum developing at the site of a follicle that has ovulated. a, Primary follicle; b, growing follicle;
c, Graafian or tertiary follicle; d, ovulation; e, corpus luteum. Source: reprinted with permission of Wiley‐
Blackwell from Reece, WO. Duke’s Physiology of Domestic Animals. 13th Ed. 2015.

enlarged and is surrounded by a developing GnRH do not result in the continuous
granulosa. A theca, consisting of layers of release of FSH and LH.
stromal cells immediately surrounding the The granulosa and theca of secondary
granulosa, also first develops late during follicles develop cellular receptors for
the secondary follicle stage. FSH and LH, respectively, and become
responsive to these hormones (Fig. 27-5).
Hormones and Follicular Development From this point, the coordinated effects of
FSH and LH are both needed for normal
Gonadotropin‐releasing hormone (GnRH) follicular development. Under the influ-
is released from the hypothalamus to pro- ence of LH, thecal cells proliferate and
mote the release of both FSH and LH from produce androgens (androstenedione and
gonadotrope cells of the adenohypophysis. testosterone) that diffuse into the granu-
The release of GnRH can be modulated losa. FSH promotes further granulosa cell
by steroid (estradiol and progesterone) proliferation, the development of cellular
and peptide (inhibin) hormones from the enzymes necessary for the conversion of
ovary, but its basal release is determined by androgens to estrogens (estradiol), and the
neural inputs to the hypothalamus. The secretion of several other paracrine agents
basal release of GnRH is pulsatile, for this necessary for follicular development. The
is the type of release seen even in animals cellular secretions accumulate among the
that have had their gonads removed. The granulosa cells, and ultimately a fluid‐filled
pulsatile nature is physiologically impor- cavity (antrum) can be identified. The
tant, because continuous infusions of developing follicles are tertiary follicles