Chapter 42: Structure and Function of the Male GU System

Embryonic development is governed by the presence of the SRY gene on the Y chromosome, which prompts the differentiation of gonads into testes. Testicular hormones, including anti-müllerian hormone (AMH) and testosterone, subsequently guide the development of internal ducts (from wolffian ducts) and external genitalia (via dihydrotestosterone or DHT). Anatomical structures include the testes, located in the scrotum and maintained at a temperature 2°C to 3°C below body temperature by mechanisms like the pampiniform plexus, and the genital duct system (epididymis and vas deferens) which transports and matures sperm. Spermatogenesis, the continuous process of sperm generation in the seminiferous tubules, begins at puberty and is tightly regulated by the hypothalamic–pituitary axis. The hypothalamus secretes gonadotropin-releasing hormone (GnRH), stimulating the anterior pituitary to release luteinizing hormone (LH), which triggers testosterone production in Leydig cells, and follicle-stimulating hormone (FSH), which initiates sperm maturation in Sertoli cells. Testosterone is essential for the growth of primary and secondary male sex characteristics and promotes anabolic effects. Neural control of sexual function involves the parasympathetic nervous system mediating erection via the release of nitric oxide, while emission and ejaculation are sympathetic nervous system reflexes. Finally, the chapter addresses the reduced function of the gonads, known as hypogonadism (categorized by the source of dysfunction as primary, secondary, or tertiary), and the gradual changes accompanying aging, sometimes termed andropause, which involve decreased testosterone levels and structural changes like prostate enlargement.