Porth's Essentials of Pathophysiology, 4e

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Genitourinary and Reproductive Function

U N I T 1 1

Semen Production The accessory genital glands include the paired seminal vesicles and bulbourethral glands and the single prostate gland (see Fig. 39-1). 1,2 These glands secrete fluids that form the bulk of the ejaculatory fluid or semen. The seminal vesicles, which lie on the posterior wall of the bladder, consist of highly tortuous tubes that secrete fluid for the semen. Each of the paired seminal vesicles is lined with secretory epithelium containing an abundance of fructose, prostaglandins, and several other proteins. The fructose secreted by the seminal vesicles provides the energy for sperm motility. The prostaglandins are thought to assist in fertilization by making the cervical mucus more receptive to sperm and by causing reverse peristaltic contractions in the uterus and fallopian tubes to move the sperm toward the ovaries. A short excretory duct from the each seminal vesicle combines with the ampulla of the duc- tus deferens to form the ejaculatory duct, which enters the posterior part of the prostate and continues through until it ends in the prostatic portion of the urethra. Contraction of the smooth muscle coat of the seminal vesicles during ejaculation discharges their secretion into the ejaculatory ducts and helps to flush sperm out of the urethra. The prostate, which is the largest of the accessory glands, is located in the pelvis, inferior to the bladder, where it surrounds the prostatic portion of the urethra. The prostate gland secretes a thin, milky, alkaline fluid containing citric acid, calcium, acid phosphate, a clot- ting enzyme, and a profibrinolysin. 2 During ejaculation, the capsule of the prostate contracts, and the added fluid increases the bulk of the semen. Both vaginal secretions and the fluid from the vas deferens are strongly acidic. Because sperm mobilization occurs at a pH of 6.0 to 6.5, the alkaline nature of the prostatic secretions is essential for successful fertilization of the ovum.

The process of spermatogenesis begins with the sper- matogonial phase, during which the spermatogonia undergo mitotic division to form more spermatogonia as well as primary spermatocytes (Fig. 39-3B). During the next phase of spermatogenesis, the primary sper- matocyte undergoes meiotic division during which its 46 chromosomes (23 pairs) are divided so that 23 chromo- somes go to one secondary spermatocyte and the other 23 to another secondary spermatocyte. During the final stage of spermatogenesis, the secondary spermatocytes divide to form spermatids, which develop into mature spermatozoa or sperm cells. After their development in the seminiferous tubules, the spermatozoa move into a highly convoluted network of ducts called the rete testis (see Fig. 39-2). The spermato- zoa are then transported through the efferent ductules and into a single duct called the epididymis , which is the final site for sperm maturation. 1,2 The epididymis also serves as a reservoir for sperm. Sperm can be stored in the epi- didymis for several months. If held longer they are eventu- ally phagocytized by the epithelial cells of the epididymis. When the male is sexually stimulated and ejaculates, the smooth muscle in the wall of the epididymis contracts vig- orously, moving sperm into the next segment of the ductal system, the ductus deferens , also called the vas deferens (Fig. 39-4). The ductus deferens ascends along the poste- rior border of the testes and then enters the abdomen in the spermatic cord, which serves as a conduit for all of the structures passing to and from the testes. After leaving the spermatic cord, the ductus deferens descends in the pelvis to the level of the bladder, where its distal end enlarges to form the ampulla . Surgical disconnection of the vas deferens in the scrotal area (i.e., vasectomy) serves as an effective method of male contraception. Because sperm are stored in the ampulla, men can remain fertile for 4 to 5 weeks after performance of a vasectomy.

Urinary bladder opening

Seminal vesicle

Surface of urinary bladder

Ampulla

Ejaculatory duct Prostate gland

Prostatic urethra Membranous urethra

Spongy urethra

Penis

Ductus deferens

Epididymis

Urethra

Testis

FIGURE 39-4. The excretory ducts of the male reproductive system and the path that sperm follows as it leaves the testis and travels to the urethra.

Seminiferous tubules