Porth's Essentials of Pathophysiology, 4e

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Disorders of the Male Genitourinary System

C h a p t e r 3 9

The bulbourethral glands are peanut-sized glands located on either side of the membranous urethra (see Fig. 39-1). A duct of each gland joins the initial portion of the penile urethra. The glands, which structurally resemble mucus-secreting glands, produce a clear mucus- like secretion. 2 Sexual stimulation causes release of the secretion, which constitutes the major portion of the pre- seminal fluid and serves to lubricate the penile urethra. Hypothalamic/Pituitary Control of Testicular Function The hypothalamus and the anterior pituitary gland play an essential role in promoting spermatogenesis in the testes and maintaining the endocrine function of the tes- tes by means of the gonadotropic hormones (i.e., ante- rior pituitary hormones that promote the function and growth of the testes in the male). 1,3 The synthesis and release of the gonadotropic hormones from the pituitary gland are regulated by gonadotropin-releasing hormone (GnRH), which is synthesized by the hypothalamus and secreted into the hypothalamohypophysial portal circu- lation (Fig. 39-5). Two gonadotropic hormones are secreted by the pitu- itary gland: FSH and luteinizing hormone (LH). In the male, LH also is called interstitial cell–stimulating hor- mone. The production of testosterone by the interstitial cells of Leydig is regulated by LH. Follicle-stimulating

hormone binds selectively to Sertoli cells surrounding the seminiferous tubules, where it functions in the initiation of spermatogenesis. Under the influence of FSH, Sertoli cells produce androgen-binding protein, plasminogen activator, and inhibin. Androgen-binding protein binds testosterone and serves as a carrier and storage site for testosterone in Sertoli cells. Although FSH is necessary for the initiation of spermatogenesis, full maturation of the spermatozoa requires testosterone. Androgen-binding protein also serves as a carrier of testosterone from the testes to the epididymis. Plasminogen activator, which converts plasminogen to plasmin, functions in the final detachment of mature spermatozoa from Sertoli cells. Circulating levels of the gonadotropic hormones are regulated in a negative feedback manner by testoster- one. 1,3 High levels of testosterone suppress LH secretion through a direct action on the pituitary and an inhibi- tory effect on the hypothalamus. Follicle-stimulating hormone is thought to be inhibited by a substance called inhibin, produced by Sertoli cells. Although two forms of inhibin have been identified, only inhibin B has been found in males. 3 Inhibin suppresses FSH release from the pituitary gland without effecting LH release. The pitu- itary gonadotropic hormones and Sertoli cells in the tes- tes form a classic negative feedback loop in which FSH stimulates inhibin secretion and increased inhibin levels suppress FSH release from the hypothalamus. Unlike the cyclic hormonal pattern in the woman, in the man, FSH, LH, and testosterone secretion and spermatogenesis occur at relatively unchanging rates during adulthood. Sexual Function The penis is themale external genital organ throughwhich the urethra passes. It functions both as a sexual organ and as an organ of urine elimination. Anatomically, the external penis consists of a shaft that ends in a tip called the glans (see Fig. 39-1). The loose skin of the shaft folds to cover the glans, forming what is called the prepuce , or foreskin . It is the foreskin that is removed during cir- cumcision. The glans of the penis contains many sen- sory nerves, making this the most sensitive portion of the penile shaft. The shaft of the penis is composed of three masses of erectile tissue held together by fibrous strands and covered with a thin layer of skin (Fig. 39-6B). The two dorsal masses of tissue are called the corpora cav- ernosa and the third ventral mass, in which the spongy part of the urethra is enclosed, is called the corpus spon- giosum. 2 The corpora cavernosa and corpus spongiosum are cavernous sinuses that normally are relatively empty but become engorged with blood during penile erection. The physiology of the male sex act involves a com- plex interaction between autonomic nervous system– mediated spinal cord reflexes, higher neural centers, and the vascular system 1,2 (Fig. 39-6A). It involves erec- tion, emission, ejaculation, and detumescence. Erection involves increased flow of blood into the corpora cav- ernosa and penile rigidity, emission is the contraction of the vas deferens and ampulla with expulsion of sperm into the internal urethra, and ejaculation is the

Hypothalamus

GnRH

Anterior pituitary

Feedback inhibition

LH FSH

Testis

Inhibin

Sertoli cells

Interstitial cells of Leydig

Testosterone

Initiation of spermatogenesis

Maturation of spermatozoa

Target tissues

FIGURE 39-5. Hypothalamic-pituitary feedback control of spermatogenesis and testosterone levels in the male. The dashed line represents negative feedback. FSH, follicle- stimulating hormone; GnRH, gonadotropin-releasing hormone; LH, luteinizing hormone.