Chapter 36: Male Reproductive System Physiology

Male Reproductive System Physiology begins by elucidating the hypothalamic-pituitary-gonadal axis, where the hypothalamus secretes gonadotropin-releasing hormone (GnRH) in a pulsatile manner to stimulate the anterior pituitary's release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). The text details how LH binds to G-protein-coupled receptors on Leydig cells in the testicular interstitium to drive steroidogenesis, specifically the conversion of cholesterol into testosterone via the cAMP signaling pathway, while FSH acts on Sertoli cells within the seminiferous tubules to support spermatogenesis and produce factors like inhibin, activin, and androgen-binding protein. Significant attention is given to the process of spermatogenesis, which spans approximately 64 days and involves three distinct phases: the mitotic proliferation of spermatogonia, meiotic division to form haploid spermatids, and spermiogenesis, where spermatids undergo morphological restructuring to become motile spermatozoa with a specialized acrosome and flagellum. The summary explains the crucial role of the blood-testis barrier formed by Sertoli cell tight junctions in creating an adluminal compartment that protects developing germ cells from the immune system. Furthermore, the chapter covers the transport and maturation of sperm through the ductal system, including the epididymis and vas deferens, and the contribution of accessory glands like the seminal vesicles and prostate to seminal fluid composition. The neural control of the sexual response is also examined, distinguishing between the parasympathetic mediation of erection via nitric oxide and cGMP, and the sympathetic control of emission and ejaculation. Additionally, the description explores the metabolic fate of androgens, including the conversion of testosterone to the more potent dihydrotestosterone (DHT) by the enzyme 5-alpha-reductase for external genital differentiation, and its aromatization to estradiol. Finally, the chapter addresses pathophysiology, categorizing male reproductive disorders into hypogonadotropic and hypergonadotropic hypogonadism, with specific clinical examples such as Kallmann syndrome, Klinefelter syndrome, and androgen insensitivity, alongside discussions on male infertility and the impact of environmental factors like oxidative stress on sperm quality.