Chapter 22: Male Reproductive System

The male reproductive system comprises the testes, genital excurrent ducts, accessory sex glands including the seminal vesicles, prostate, and bulbourethral glands, and the external genitalia consisting of the penis and scrotum. The principal functions of the testes, which are housed in the scrotum, are the production of sperm (spermatogenesis) and the synthesis of steroid hormones (androgens) like testosterone. Genetic sex, determined by the presence of the Y chromosome, triggers a cascade of differentiation, initiated by the SRY gene, which leads to the formation of the testis. Key hormones driving male embryonic development include testosterone, which facilitates the growth of the genital ducts, and Müllerian-inhibiting factor (MIF), secreted by Sertoli cells to suppress the development of female reproductive structures. For spermatogenesis to occur successfully, the testes must be maintained at a temperature approximately 2 degrees C–3 degrees C lower than normal body temperature, a condition managed by the pampiniform venous plexus, cremaster muscle, and dartos muscle. Structurally, the testes are covered by the thick tunica albuginea and divided into approximately 250 lobules containing highly convoluted seminiferous tubules. Within the interstitial tissue surrounding these tubules, Leydig cells produce testosterone and are characterized by abundant smooth endoplasmic reticulum and sometimes the rod-shaped crystals of Reinke. The seminiferous tubules contain the specialized seminiferous epithelium, consisting of supporting Sertoli cells and differentiating spermatogenic cells. Sertoli cells create the crucial blood-testis barrier, formed by tight junctional complexes, which isolates the antigenic haploid germ cells from the immune system and maintains a favorable microenvironment rich in androgen-binding protein (ABP). Spermatogenesis is a roughly 74-day process in humans divided into the spermatogonial phase (mitosis of stem cells), the spermatocyte phase (meiosis, which reduces the chromosome number to haploid), and the spermatid phase (spermiogenesis), where the haploid spermatids undergo extensive remodeling. This remodeling includes DNA condensation using protamines, the formation of the acrosomal cap containing essential enzymes like acrosin, and the development of the flagellum. Following spermiation, the newly released, nonmotile sperm traverse the intratesticular ducts (straight tubules and rete testis) into the excurrent duct system. In the highly coiled duct of the epididymis, lined by pseudostratified epithelium with characteristic stereocilia, sperm acquire motility and undergo further maturation, including temporary decapacitation. The ductus deferens, the longest part of this system, features a thick, three-layered muscular coat that propels sperm forcefully during ejaculation. Semen, the final fluid, is alkaline and comprised largely of secretions from the accessory sex glands; the seminal vesicles contribute a viscous fluid rich in fructose (the primary nutrient for sperm), while the largest accessory gland, the prostate, contributes clear fluid containing important markers like prostate-specific antigen (PSA) and prostatic acid phosphatase (PAP). The prostate gland's transitional zone is the frequent site of benign prostatic hypertrophy (BPH), while carcinoma often originates in the peripheral zone. Lastly, the erectile tissues of the penis, the corpora cavernosa and corpus spongiosum, engorge with blood during erection, a vascular event initiated by parasympathetic stimulation and the subsequent release of nitric oxide (NO), which relaxes smooth muscle.