Intraovarian control of progesterone biosynthesis by granulosa cells and corpus luteum

Abstract

Some of the recent developments on the intraovarian control of follicular development and luteinization have been discussed. Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) regulate the growth and maturation of the follicle. However, the locus of control, by which the responsiveness of follicles to LH and FSH is regulated at different stages of follicular development, resides within the ovary itself. Granulosa cells obtained from small follicles have more FSH receptors and can accumulate more cyclic AMP and progesterone in response to FSH compared to cells obtained from large mature follicles. The situation is reversed in the case of LH responsiveness; larger follicles are more responsive than small ones. The steroidogenic potential of follicular cell types to secrete oestrogen and progesterone is altered with the stage of the follicle. In vivo studies in the rhesus monkey demonstrated that thecal cells of the preovulatory follicle are the principal source of oestrogen and progesterone of the ovarian venous blood. In in vitro cultures, thecal cells alone could secrete more oestrogen than granulosa cells, and mixing of the two cell types augmented oestrogen synthesis. Addition of exogenous testosterone (0.15μg/ml) to granulosa cell cultures resulted in increased secretion of oestrogens. Granulosa cells in culture produced more progesterone than thecal cells. These findings suggest that the thecal layer is the main source of ovarian vein levels of oestrogen and progesterone and the two cell types may be responsible for the high levels of these steroids in the follicular fluid. A luteinization inhibitor, present in the follicular fluid of small and medium-sized follicles may regulate the responsiveness of follicles to gonadotrophins and prevent luteinization of granulosa cells prior to ovulation. Evidence for the presence of another inhibitor, a LH receptor binding inhibitor (LHRBI) in frozen aqueous extracts of porcine corpus luteum and its ability to inhibit progesterone secretion by cultured granulosa cells of mature porcine follicles has also been presented.