ABSTRACT
After stimulation by GnRH, the gonadotropes also secrete FSH into the circulation. This glycoprotein hormone binds to specific receptors on Sertoli cells of the testes, stimulating production of androgen-binding protein (ABP). FSH is necessary for the initiation of spermatogenesis. However, full maturation of spermatozoa also appears to require testosterone. Indeed, the major action of FSH on spermatogenesis may be stimulation of ABP production, which allows a high intratubular concentration of testosterone to be maintained. In addition to ABP, there is substantial evidence that other compounds are synthesized and secreted by the gonads. For example, testicular Sertoli cells (like ovarian granulosa cells) secrete peptide and protein products that act in endocrine, paracrine, and even autocrine fashion to modulate the processes of gametogenesis. Inhibin and activin are members of the same superfamily of growthregulating factors as discussed in Ch. 4.The inhibins (A and B) are glycoproteins that circulate in plasma and inhibit GnRH-stimulated FSH release by the pituitary. It is not known whether they also exert a significant negative feedback at a hypothalamic locus. Activin, another gonadal glycoprotein, has the opposite action, stimulating FSH release. At the gonadal level, inhibin increases whereas activin decreases testosterone secretion; thus, FSH can influence Leydig cell function indirectly by modulating production of inhibin and activin. Other paracrine interactions may also be important in maintaining a proper testicular environment for spermatogenesis. Testosterone from Leydig cells further stimulates differentiation and proliferation of peritubular myoid cells. The latter secrete a protein known as PModS that stimulates Sertoli cell function. Each of these pathways may vary in functional activity and significance at different points in the cycle of spermatogenesis. Part A is an overall diagram of pituitary and testicular control of Leydig cell, peritubular myoid cell, and Sertoli cell secretion, as well as spermatogenesis.
