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

Abstract

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.

Endocrine characteristics of late pregnant hyperketonaemic ewes and their reproductive performance following the induction of ovarian cyclicity out of the breeding season

“…In other respects, a hyperketonaemia-related impairment of follicular development is supposed to be a more likely reason for the poor ovarian response. The time needed for development of primordial follicular structures into preovulatory follicles is longer in sheep than in cattle, and it takes approximately 180 days (Cahill and Mauleon, 1980;Monniaux et al, 1997a and1997b). So in the current study the dominant follicles expected to yield fully competent oocytes following the induction of cyclicity on days 92-94 after lambing started to develop before delivery.…”

Ovarian features contributing to the variability of PMSG-induced ovulation rate in sheep

“…Follicles recruited by PMSG are much smaller than those recruited at luteolysis of an unstimulated cycle to undergo preovulatory enlargement ( > 2 mm: Driancourt & Cahill, 1984;Tsonis et al, 1984). It is unclear, at present, whether such small 0-8-2-0 mm follicles, which nor¬ mally need 10-15 days to mature (Cahill & Mauléon, 1980), manage to catch up through the limited increase in the mitotic index of the granulosa cell induced by PMSG (sheep: Tumbull et al, 1977;cattle: Monniaux et al, 1983) and ovulate with an adequate number of granulosa cells. About half of these 0-8-2 mm follicles were atretic, an observation in good agreement with previous reports on atresia in the sheep ovary (Brand & de Jong, 1973;Turnbull et al, 1977;Cahill et al, 1979).…”

Seasonal effects on ovarian follicular development in pony mares

“…Thus, as in ewes (Turnbull et al, 1978 ;Cahill and Mauleon, 1980), the only part of folliculogenesis lacking during winter is preovulatory development. Furthermore, anestrous follicles seem to be sensitive to endogenous or exogenous hormones since Oxender, Noden and Hafs (1977) have noticed some spikes of estradiol-17(3, and ovulation has been successfully induced with pituitary extracts in a fortnight in seasonally anovulatory mares (Douglas, Nuti and Ginther, 1974 ;Lapin and Ginther, 1977).…”