The role of FSH in the selection of large ovarian follicles in the rat

Abstract

To determine if the preovulatory surge of FSH plays a role in the selection of a cohort of follicles destined to ovulate during the subsequent estrous cycle, exogenous FSH was administered on the day before proestrus and the ovaries were examined 24 hr later using histological and morphometric techniques. Administration of exogenous FSH 1 day ahead of the endogenous surge of gonadotropins caused the appearance of a cohort of small pre-Graafian follicles (390-500 μm in diameter) 1 day ahead of the time of their usual appearance. Elimination of the FSH/LH surge on proestrus, by means of a barbiturate blockade of the surge of gonadotropins, prevented the influx of small pre-Graafian follicles normally seen on estrus. However, replacement of FSH in barbiturate blocked animals restored the appearance of these small pre-Graafian follicles. These results suggest that the FSH surge of 1 cycle may rescue follicles in the 200-400 μm size range, thereby selecting those follicles which will ovulate during the next cycle.