A MODEL FOR FOLLICLE SELECTION AND THE DETERMINATION OF OVULATION RATE IN THE EWE

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.