Inhibin--fact or artifact.

Peripheral concentrations of follicle-stimulating hormone (FSH) in male as well as in female animals appear to be partly regulated by inhibin, a protein which is secreted by the gonads. The molecular structure of this substance is still unknown, and the mechanism(s) of its action on the pituitary or hypothalamic level is not clear. Much of the confusion about inhibin stems from the fact that no generally accepted definition of inhibin exists and that fundamentally different biological assay systems have been used by different groups. Therefore this short review starts with a discussion of the definition of inhibin and the assay principles. From the available information on the site of origin of the hormone it appears likely that inhibin is produced in the Sertoli cells of the testis and the granulosa cells of the ovary. The available data on the chemical nature of inhibin suggest that different principles, acting on different sites of the hypothalamic--pituitary axis, might be present in preparations with inhibin-like activity. Finally, with respect to the biological significance of inhibin, it seems that inhibin could play a more important role in the feedback regulation of FSH in the adult female than in the adult male animal.