Role of intrafollicular regulators and FSH in growth and development of large antral follicles in sheep

Abstract

Manipulation of circulating concentrations of hormones and ovarian follicle status was carried out on Day 11-12 of the oestrous cycle in sheep. All follicles visible on the ovary were ablated by cautery and ewes were treated with oestradiol or ovine follicular fluid (oFF) to suppress FSH or with PMSG to increase circulating gonadotrophic activity. One group underwent unilateral ovariectomy which greatly increased endogenous FSH and was the only treatment which significantly affected LH pulse frequency. The size distribution of antral follicles, the extent of atresia and the mitotic index of granulosa cells of follicles on Day 15 showed that (a) treatment with oFF inhibited the growth of follicles beyond 2 mm diameter by suppressing the mitotic index of the granulosa cells and (b) the concentration of FSH in peripheral plasma was related to the ability of small antral follicles to grow during the late luteal-early follicular phase of the cycle. Subsequently, it was demonstrated that oFF inhibits, in a dose-dependent manner, folliculogenesis sustained by PMSG in ewes on Days 12-15. Inhibition of folliculogenesis was represented by a decrease in those follicles > 4 mm, an increase in the relative proportion of follicles < 2 mm, and minimal change in the average number of follicles visible on the ovarian surface, and a decrease in the mitotic index of granulosa cells of follicles < 2 mm. There was no change in the extent of atresia. This supports the concept of a mitotic inhibitor present in charcoal-treated oFF, which can act at the ovarian level to prevent follicles passing beyond 2 mm diameter, even in the presence of excess exogenous gonadotrophin. Such a factor could be involved in selection of the follicle(s) destined to ovulate.