Activins, inhibins, and follistatins: further thoughts on a growing family of regulators.

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.