Folliculogenesis in the sheep as influenced by breed, season and oestrous cycle.

In the sheep the total duration of folliculogenesis, i.e. from the start of development of a primordial follicle to ovulation, is thought to be about 6 months. The initiation of follicular growth as follicles enter the growth phase is influenced by gonadotrophins and, in the sheep, by factors such as breed, season and nutrition. Preantral follicles are characterized by a slow growth rate and no atresia. The number of preantral follicles is influenced by gonadotrophins, age, nutrition, season and unilateral ovariectomy. Antral follicles grow rapidly and produce steroids in response to gonadotrophins. The number of antral follicles varies according to factors such as breed of sheep, season, cycle, unilateral ovariectomy and gonadotrophins. Ovulation rate can be influenced in the short term by factors such as PMSG stimulation, short-term nutrition and unilateral ovariectomy that probably act by changing the number of follicles undergoing atresia. Factors such as breed, age and season probably act by changing the number of follicles entering the growth phase.