Inhibin, a feedback inhibitor of pituitary FSH secretion, and its homodimer, activin, have been the subject of a growing body of literature in the last 5 years. These factors play a role not only in endocrine feedback in the reproductive system but also in paracrine and autocrine regulation of both reproductive and nonreproductive organs, including the liver, kidney, and brain. Additionally, the messages coding for both subunits and their receptors are exquisitely regulated, both spatially and temporally, during embryonic development. The cloning of a family of activin receptors; the development of specific immunoassays for inhibins A and B, and activins A and B; the description of alpha subunit, beta subunit, and receptor loss of function transgenic mouse models; and the cloning of two new beta subunit homologs have increased our understanding of the possible roles this complex family of proteins plays in development and endocrine function. This review largely confines itself to the roles of inhibins and activins in the male and female reproductive system, and is intended as an update to a 1992 review published in this journal.

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Activin is a protein originally isolated from follicular fluid as a factor stimulating FSH release from the pituitary. The present experiments support the hypothesis that activins may also regulate follicle development by autocrine/paracrine mechanisms. Granulosa-oocyte complexes were isolated by collagenase/dispase dispersion of ovaries from 14- or 21-day-old rats and cultured in serum-free medium. Within 24 h, the cells had spread to form a monolayer. Hormones and growth factors were added at this time. Cell number and thymidine incorporation were measured after an additional 72 h. In the presence of insulin and transferrin, activin-A increased both granulosa cell number and thymidine incorporation more than 2-fold. This effect could be inhibited by follistatin, an activin-binding protein. In addition, activin-A, in the presence of FSH, induced reorganization of follicular structures from monolayer culture of cells from 14-day-old rats and caused cells from primary follicles to develop into large follicle-like structures. These structures contained oocytes, a cumulus layer, an antrum, and a multilayered follicular wall with a diameter of more than 1 mm. Electron microscopy revealed that the cells in the follicle-like structure were connected by gap junctions. Oocytes showed a mature morphology and had closely associated cumulus layers. Dissociation of the follicular wall in these follicle-like structures was induced by the addition of LH, resembling the induction of ovulation in vivo. The findings are important for understanding follicular development and atresia.

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The effects of growth factors were investigated on the proliferation of a normal placental cytotrophoblast cell line (NPC). Epidermal growth factor (EGF), transforming growth factor alpha (TGF alpha) and insulin-like growth factor-I (IGF-I) stimulated NPC cell proliferation. In contrast, TGFbeta1 was found to be a negative regulator, inhibiting EGF-induced cell proliferation. When EGF/TGF alpha receptor was analysed by radio-ligand binding, two binding sites of different affinities were revealed in the proliferating NPC cells but only the low affinity binding site was detected in the non-proliferating cytotrophoblast cells in primary cultures. The results suggest that EGF stimulates cytotrophoblast proliferation through high affinity binding sites.