Influences of season, cycle and breed on follicular growth rates in sheep.

Follicular growth was studied in 16 ewes of different breeds (Romanov, mean ovulation rate 3.0, and Ile-de-France, mean ovulation rate 1.6), stage of cycle (Day 0 or 7) and season (December and June). The follicular growth rates, determined by measuring the mitotic index before and 2 h after colchicine treatment, varied greatly between animals studied and did not vary significantly between breeds, time of cycle or season. From 3 layers of granulosa cells until antrum formation the mitotic index increased slowly but then the follicles grew rapidly reaching maximum growth rate at a follicular diameter of 0.85 mm; thereafter the mitotic index decreased almost to zero in preovulatory follicles. The mean time for a follicle to pass from 3 layers of granulosa cells (200 cells) to preovulatory size (3 x 10(6) cells) was estimated to be about 6 months. The total number of normal follicles with > 3 layers of granulosa cells in Ile-de-France ewes was similar in the anoestrous (3 ovaries studied) and breeding (3 ovaries) seasons, but there were more antral follicles in the latter. Highly significant correlations existed in each ewe between the number of follicles and the mean mitotic index per class, suggesting the existence of an intraovarian mechanism regulating folliculogenesis.