FIG 1. Components of the normal pituitary-testis axis. GnRH acts on the pituitary to stimulate the release of the gonadotropins. In this figure the testis is represented by the large oval. LH acts on the Leydig cells to stimulate testosterone (T) synthesis and secretion while FSH acts on Sertoli cells. T stimulates spermatogenesis through actions on peritubular and Sertoli cells and inhibits LH release through a classical feed-back pathway. Inhibin can negatively regulate FSH release from the pituitary while the development of germ cells to sperm is a result of the biological activities of the Sertoli cells. Sertoli cells and germ cells communicate through a number of paracrine factors.