Inhibin, activin, and follistatin in reproductive medicine

Objective: To review the available information regarding the polypeptide factors inhibin, activin, and follistatin in reproductive physiology. Design: The protein structure, tissue expression, regulation, and effects of these factors are outlined, with an emphasis on the reproductive tissues in both females and males. Although some information is only available in animal model systems, human data has been selected whenever possible. Conclusions: Inhibin and activin are closely related peptides with opposing actions, whereas follistatin is a structurally unrelated peptide that may act indirectly through modulation of inhibin-activin effects. These three peptides are secreted in highest levels by the adult gonads; however, they are also present in a wide variety of reproductive and nonreproductive tissues where they are believed to exert local, tissue-specific effects. Within the reproductive system, these peptides play a role in the regulation of gonadotropin biosynthesis and secretion, ovarian and placental steroidogenesis, and oocyte and spermatogonial maturation.