Growth and sexual differentiation in the male result from a complex feedback circuit involving the hypothalamus, pituitary, and gonads. Neurons of the lateral ventricles of the hypothalamus synthesize and secrete the Gonadotropin Releasing Hormone (GnRH). It is a decapeptide that, through the pituitary peduncle, spills into the median eminence and reaches the anterior pituitary, where it binds to its specific receptors resulting in the release of Luteinizing Hormone (LH) and, to a lesser extent, Follicle-stimulating hormone (FSH) (Fig. 1). GnRH is released in a pulsatile manner every 60–90 minutes, and this pulsatility is critical for the normal functioning of the pituitary-gonadal axis.

Fig1. Male hypothalamus-pituitary-gonad regulation. GnRH gonadotropin-releasing hormone, LH luteinizing hormone, FSH follicle- stimulating hormone. (Copyright EDISES 2021. Reproduced with permission)

Gonadotropins (FSH, LH) have two subunits: α and β. The α subunit is common to all glycoprotein hormones (FSH, LH, TSH, and hCG), while the β subunit is specific for the different hormones. LH and FSH levels vary according to the individual’s age: high at birth, decrease during growth, low or undetectable levels throughout the prepubertal period, and increasing in later stages of development. FSH binds specific receptors on Sertoli cells, stimulating the synthesis and release of inhibin and other factors essential for regulating spermatogenesis (Fig. 1).

Inhibin is a heterodimeric protein hormone belonging to the Transforming Growth Factor-β (TGF-β) superfamily. It exists in two molecular forms: inhibin A and inhibin B, derived from the combination of an α subunit and two β sub units (βA and βB). Both isoforms are required in gonadal maturation processes in men and women. Inhibin B plays an essential role in spermatogenesis and controls, via negative feedback, the pituitary production of FSH. This feedback involves the inhibin B, which acts as an activin antagonist. Activin is structurally homologous to inhibin and stimulates FSH synthesis (Fig. 1).

Sertoli cells also produce Müllerian Inhibiting Substance (MIS), which, during male genital apparatus differentiation, will induce regression of Müllerian structures (fallopian tubes, uterus, upper portion of the vagina).

Leydig cells react to the LH-induced stimulus by converting cholesterol, either from LDL or synthesized within the cells, to pregnenolone and then to dehydroepiandrosterone (DHEA), androstenedione or androstenediol, which will eventually be converted to testosterone (Figs. 2 and 3). The testis produces 3–10 mg per day of testosterone. Once produced, testosterone can perform its functions by binding androgen receptors or be converted, at the target tis sues, into a more potent metabolite, dihydrotestosterone (DHT), by the enzyme 5α-reductase, or into 17β-estradiol by the enzyme aromatase (Fig. 3).

Fig2. Anatomical section of the seminiferous tubule. (Copyright EDISES 2021. Reproduced with permission)

Fig3. Synthesis of testosterone, estradiol and dihydrotestosterone in Leydig cells and peripheral tissues. (Copyright EDISES 2021. Reproduced with permission)

Testosterone and estradiol exert a negative control mechanism on the hypothalamic-pituitary axis. Specifically, testosterone inhibits the production of GnRH, FSH, and LH, resulting in decreased production of testosterone by Leydig cells; estradiol blocks the production of LH and FSH (Fig. 2). Estradiol is essential in regulating bone development (inhibitory action), epiphyseal closure, and lipid metabolism; finally, it possesses a psychotropic action.

Testosterone is secreted in a pulsatile manner and circulates in the peripheral blood form bound to plasma proteins (i.e., SHBG and albumin) and, to a lesser extent, in free form. Testosterone is responsible for sexual differentiation, devel oping and maintaining secondary sexual characteristics, regulating erythropoiesis and libido control, and developing muscle mass and bone structure. DHT, produced by the con version of testosterone, androstenedione, and DHEA at target tissues (mainly skin, liver, and urogenital tissues), is responsible for sexual differentiation (induces the development of external genitalia), sebum production, prostate development, and the appearance of hair. In addition to testosterone production, the testis synthesizes progesterone, 17-OH-progesterone, and androstenedione (D4).

Small amounts of testosterone are present several years before puberty and produced in response to the adrenal androgens DHEA and dehydroepiandrosterone sulfate (DHEA-S), which are responsible for pubic and axillary hair (adrenarche) appearance.

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