Operation of the follicle-stimulating hormone (FSH)-inhibin B feedback loop in the control of primate spermatogenesis.

This paper reviews our current understanding of the function and operation of the follicle-stimulating hormone (FSH)-inhibin feedback loop in the male rhesus monkey (Macaca mulatta). Inhibin B is the major testicular inhibin in the monkey, and the pattern of secretion of this hormone during postnatal development is temporally coupled to that of gonadotropin. Inhibin B secretion by the Sertoli cell is stimulated by FSH and inhibited by luteinizing hormone (LH), the latter presumably acting via Leydig cell production of testosterone (T). The dynamics of the FSH-inhibin B feedback loop in the adult monkey is revealed following unilateral orchidectomy (UO). Interestingly, a sustained, 50% deficit in inhibin B secretion occurs after UO and this persistent error signal, in turn, results in elevated concentrations of FSH in the circulation. The elevated secretion of FSH appears to be the principal drive for the increased sperm output by the remaining testis. Available data for the functioning of the FSH-inhibin B feedback loop in the human male are placed in perspective, and a model for the negative feedback regulation of sperm number in primates is proposed.