Hormonal regulation of preovulatory follicle maturation in the rat

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Abstract
Estrogen-priming of the hypophysectomized immature female rat promotes preantral follicle development in the absence of endogenous gonadotropins and such an animal is useful for study of the intraovarian glycoprotein-steroidal hormone interactions which underlie morphological and functional ovarian follicle development. The functional characteristics (gonadotropin binding and steroidogenesis) of in vitro granulosa cells harvested at different stages of follicular maturity following treatment with exogenous hormones in vivo is reported. Preovulatory follicle maturation, induced by FSH [follicle-stimulating hormone, follitropin] was studied up until antrum formation as was the acquisition by granulosa cells of the ability to respond directly to LH [luteinizing hormone, lutropin] or hCG [human chorionic gonadotropin]. Before the increases in available granulosa cell membrane, LH/hCG receptors associated with follicular antra formation, effects of hCG or other hormones with interstitial cell-stimulating activity are mediated via interactions with cells outside the lamina basalis. In vivo studies with estrogen-primed hypophysectomized immature rats indicate that androgens [testosterone] secreted by LH/hCG-stimulated thecal and/or interstitial cells may act directly on the preantral follicle to promote atresia. In vitro studies showed a stimulatory effect of androgen on FSH-responsive progesterone secretion by granulosa cells isolated from preantral follicles. These effects, if shown to operate within the ovary during the normal cycle, need not be mutually exclusive because FSH stimulation of granulosa cells in vivo may be a major determinant of follicular responses to androgen. The increase in follicle size and antrum formation accompanying FSH treatment in vivo are associated with: increases in the steroidogenic potential of isolated granulosa cells; the induction of granulosa cell LH/hCG receptors and of steroidogenic responsivity to hCG; and stimulation of granulosa cell aromatase activity. FSH has a critical role in the organization of preovulatory follicular morphology and function.