In mammals, spermatogenesis begins with diploid stem cells that resemble other somatic cells; it ends with highly specialized motile haploid cells that are remarkably unique in appearance and function. Continuous production of spermatozoa throughout life requires that spermatogonia replenish themselves. Type B spermatogonia undergo mitosis to give rise to diploid primary spermatocytes. The spermatocytes then cross the blood-testis barrier formed by the Sertoli tight junctions to the adluminal compartment. Spermiogenesis refers to the acquisition by the germ cell of several organelles and accessory structures such as the acrosome and the flagellum. Testosterone and follicle-stimulating hormone (FSH) are the two major regulatory hormones of spermatogenesis. FSH binding to its receptor activates adenylate cyclase, and the resultant rise in cAMP triggers binding of cAMP response element modulator (CREM) to ACT (activator of CREM). The complex then acts as a molecular master-switch for a number of genes involved in spermatogenesis.