Rj. Scaramuzzi et al., A MODEL FOR FOLLICLE SELECTION AND THE DETERMINATION OF OVULATION RATE IN THE EWE, Reproduction, fertility and development, 5(5), 1993, pp. 459-478
A model for folliculogenesis is proposed that is based as far as possi
ble on a knowledge of physiological, rather than anatomical, changes t
aking place during follicle development. The model is therefore functi
onal, rather than descriptive, and consists of five classes of follicl
es that have been defined by their dependency and sensitivity to gonad
otrophins. These classes are: primordial, committed, gonadotrophin-res
ponsive, gonadotrophin-dependent and ovulatory. The model is an attemp
t to encourage discussion and to promote the integration of morphologi
cal models of folliculogenesis with recent advances in the molecular e
ndocrinology of the ovarian follicle. Two hypotheses for the mechanism
s that determine ovulation rate are developed in light of the model. I
n the first, multiple ovulation results when the viability of gonadotr
ophin-dependent follicles is enhanced. In the second, multiple ovulati
on is caused by increasing the number of gonadotrophin-responsive foll
icles available for further development; this results from the increas
ing rate of folliculogenesis and the throughput of follicles. The fina
l section of this paper examines how these two hypothetical mechanisms
, which are not mutually exclusive, appear to account for most of the
known genetical and environmental effects on ovulation rate of sheep.
In particular, the effects of nutrition, genotype, exogenous gonadotro
phins, immunity to both oestrogens and androgens, and immunity to inhi
bin are discussed.