Page 495 - Anatomy and Physiology of Farm Animals, 8th Edition

480 / Anatomy and Physiology of Farm Animals

Granulosa
cells
VetBooks.ir Nucleus Oocyte Oocyte Follicular
cell

Zona pellucida
Zona
pellucida
Membrana granulosa
Granulosa cells FSH
Developing theca
Membrana granulosa
LH-R LH
FSH-R

Theca
interna
Theca Granulosa cell Theca cell
externa
Blood vessels

Cumulus
oophorus
LH-R FSH-R

Theca
interna
Theca Granulosa cell Theca cell
externa
Figure 27-5. Growth and maturation of a primary, secondary and tertiary follicle, demonstrating the
two cell mechanism endocrine regulation of follicular estrogen synthesis. The conversion of testosterone
to estrogen by aromatase in granulosa cells is dependent on testosterone synthesis in the thecal cells. FSH
receptors (FSH‐R) are only expressed on granulosa cells, and LH receptors (LH‐R) are only present on
theca cells. Source: Carlson, 2009. Reproduced with permission of Elsevier.

In all species, a follicular development large domestic animal is considered to have
occurs in “waves” such that follicular devel- overlapping follicular and luteal phases.
opment begins with a cohort of domestic Inhibins are peptide hormones secreted
animals that typically have only one or two by granulosa cells of developing follicles.
offspring per pregnancy, large dominant Circulating levels of inhibins increase with
follicles may develop while a corpus luteum follicular development, and inhibins have
remains intact. These dominant follicles may a negative feedback effect on FSH release
or may not ovulate. Dominant follicles that do from the adenohypophysis. By this means,
not ovulate undergo atresia if a corpus luteum a developing dominant follicle can suppress
remains intact because progesterone pro- the development of competing follicles in
duction by the corpus luteum prevents the nonlitter‐bearing animals. In litter‐bearing
LH surge. Waves of follicular development animals, the combined negative feedback
will often succeed each other so that another effect of inhibins from multiple follicles can
dominant follicle begins to develop rapidly so suppress other follicles to prevent litter
that ovulation can occur soon after luteolysis. sizes from becoming inappropriately large.
In many species, including cattle, dominant Inhibins from developing follicles apparently
follicles may develop while a corpus luteum do not suppress LH secretion necessary for
remains intact; thus the estrous cycle of a ovulation.