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 possible on a knowledge of physiological, rather than anatomical, changes taking place during follicle development. The model is therefore functional, rather than descriptive, and consists of five classes of follicles that have been defined by their dependency and sensitivity to gonadotrophins. These classes are: primordial, committed, gonadotrophin-responsive, gonadotrophin-dependent and ovulatory. The model is an attempt to encourage discussion and to promote the integration of morphological models of folliculogenesis with recent advances in the molecular endocrinology of the ovarian follicle. Two hypotheses for the mechanisms that determine ovulation rate are developed in light of the model. In the first, multiple ovulation results when the viability of gonadotropin-dependent follicles is enhanced. In the second, multiple ovulation is caused by increasing the number of gonadotrophin-responsive follicles available for further development; this results from the increasing rate of folliculogenesis and the throughput of follicles. The final 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 gonadotrophins, immunity to both oestrogens and androgens, and immunity to inhibin are discussed.