Figure 2.

The HPG axis. The hypothalamus regulates the biosynthesis and secretion of pituitary hormones LH and FSH through GnRH. LH induces Leydig cells to secrete testosterone, which reduces GnRH and LH production through negative feedback. FSH acts on Sertoli cells to induce the secretion of inhibin, which in turn inhibits the production of FSH. Sertoli cells, under the combined action of testosterone and FSH, stimulate the proliferation and maturation of germ cells. Testosterone is also essential for promoting muscle and bone development. Furthermore, testosterone is metabolized by aromatase (CYP19A1) into 17β-estradiol (E2) which exerts inhibition on the hypothalamus. Normal spermatogenesis depends on the coordinated action of all these hormones.