Control of antral follicle development and selection in sheep and cattle.

The development of antral follicles in sheep and cattle is dependent on FSH, but large antral follicles can shift their gonadotrophic dependence from FSH to LH. The mechanisms that result in the selection of at least one ovulatory follicle from identical follicular cohorts, exposed to the same endocrine environment, still remain to be elucidated. The aim of this research was to extend in vitro results from the rodent to sheep and cattle and, using both in vivo and in vitro models, to identify factors that can enhance or attenuate the action of gonadotrophins in stimulating follicle development. Using sheep with ovarian autotransplants, we have obtained evidence to show that a number of factors inhibit ovarian function in vivo, whereas only insulin-like growth factor I (IGF-I) has a stimulatory effect. Further study of the mechanism of action of these factors at a cellular level has been made possible by the development of a serum-free granulosa cell culture system for both sheep and cattle that allows induction and maintenance of oestradiol production. Using this model system, we have confirmed many of the results from out studies in vivo and have shown that IGF-I and insulin interact at physiological concentrations to influence both cellular proliferation and oestradiol production. Overall, these data support the hypothesis that the physiological basis of follicle selection is the differential expression of factors that modulate the action of gonadotrophins on follicular cells at key points during the process of follicle development.