Hypothalamo-Pituitary Unit, Testis, and Male Accessory Organs

The function of the hypothalamo-pituitary-testicular-accessory organ axis is to ensure normal androgenization (including embryonic, infantile, pubertal, and adult sexual maturation), male sexual behavior, and reproductively competent sperm output. Thus the system is important for both the health of individuals and preservation of humankind. All elements in the system acting together as an integrative regulatory network are critical for male reproductive health. Dysfunction may result in ambiguous genitalia, sex reversal, pubertal delay, eunuchism, impaired spermatogenesis, and reduced systemic androgen exposure. Furthermore, partial deficiency of reproductive hormones may contribute to some of the features of male aging, impair recovery from protracted critical illness, and induce visceral adiposity with insulin resistance. This chapter reviews the hypothalamo-pituitary-testicular axis, its components, and their joint regulation. Where practicable, emphasis is on newer discoveries in humans. Hormonal methods to regulate male fertility and to limit age-associated frailty constitute examples of how knowledge of male reproductive physiology is being translated into clinical practice.