The Reproductive System of the Male Is Regulated by the HypothaIamusrWhich Is Hormonally Linked to the Anterior Pituitary and Testes by Luteinizing Hormone and Follicle-Stimulating Hormone
The reproductive system of male mammals is regulated by intricate feedback mechanisms involving the hypothalamus» anterior pituitary» and testes (Figure 40-3). The hypothalamus synthesizes and secretes the decapeptide gonadotropinreleasing hormone (GnRH).
Secreted in a pulsatile manner, GnRH acts directly on gonadotropic cells in the anterior pituitary. On stimulation by GnRH, these gonadotropes synthesize and secrete the gonadotropins ∕b∕∕ic∕e-sh>nw∕nting hormone (FSH) and luteinizing hormone (LH). Both FSH and LH are heterodimeric glycoproteins made up of two non- Covalently linked polypeptides. The alpha (α) subunit protein is common to both FSH and LH» whereas the beta (β) subunit is specific for each. Individual gonadotropes have the ability to synthesize and secrete FSH, LH, or both. The release of FSH and LH depends on the pulsatile pattern of GnRH secretion. Irregular, low-amplitude GnRH pulses result in FSH release, whereas high-frequency GnRH pulses induce the release of LH.Within the testis, LH binds to membrane receptors on the Leydig cells and stimulates them to convert cholesterol to testosterone. Synthesized androgens diffuse into blood and lymph, where they are bound to androgen-binding protein (ABP) produced by the Sertoli cells. High local concentrations of androgens within the testis are considered essential for normal spermatogenesis to occur. ABP enhances the accumulation of testosterone and dihydrotestosterone in high concentrations within the seminiferous tubules and the interstitium of the testis. Within the testis, the target cells for testosterone are the peritubular myoid cells and the Sertoli cells, which envelop and support the developing sperm cells. ABP also facilitates the transport of androgens from the testis to the epididymis, where these hormones influence epididymal transit and the further maturation of spermatozoa.
Studies have demonstrated that FSH specifically targets receptors on the Sertoli cells within the seminiferous tubules.
FSH and testosterone stimulate a variety of Sertoli cell functions, including the synthesis and secretion of ABP, inhibin, activin, estrogen, and several products (e.g., transferrin) that are involved in the transfer of nutrients to germ cells; meiosis; spermatocyte maturation; spermiation; and Leydig cell function. SertoIi and Leydig cells appear to interact in a paracrine fashion. Steroid production of Leydig cells can be stimulated by a product released by Sertoli cells, the secretion of which is enhanced by FSH. A potential candidate for such a substance is inhibin, which is produced by Sertoli cells in response to FSH and stimulates steroidogenesis in Leydig cells. Inhibint along with testosterone, is involved in the complex feedback regulation of pituitary function. Gonadal steroids are known to suppress FSH release, but inhibin appears to be the most potent inhibitor of FSH secretion from the pituitary. Testosterone, dihydrotestosterone, and estrogen regulate LH synthesis and secretion through negative feedback exerted at the level of hypothalamus or the anterior pituitary gland. Because FSH and LH are necessary for high testicular concentrations Ofsubstances responsible for normal spermatogenesis, exogenous administration of testosterone or inhibin to enhance fertility would be contraindicated since they would impede the secretion of those factors responsible for maintaining an optimal Spermatogenic environment.