Gross Anatomy Of The Male Reproductive Tract
The testes are a pair of oval, slightly flattened bodies measuring about 4 cm in length and 2.5 cm in diameter. Together with the epididymides, they lie in the scrotum, an extra-abdominal sac just below the penis. The walls of the cavity in which the testes and epididymis reside are known as the tunica vaginalis. The tunica vaginalis forms from intra- abdominal peritoneum that migrates into the primitive scrotum during development of the male internal genitalia. After migration of the testis into the scrotum, the channel down which the testis has moved (processus vaginalis) is obliterated.
The epididymis is a comma-shaped structure that clasps the posterolateral margin of the testis. It is formed from the duct of the epididymis, an irregularly twisted tube. The epididymal duct is about 600 cm long. It begins at the top of the testis as the head of the epididymis. After an extraordinarily tortuous course it ends as the tail of the epididymis, then becomes the vas deferens (Fig. 7.1).
The testicular arteries supply blood to the testes and epididymides. These arteries arise from the aorta just below the renal arteries. The testicular arteries end in a dense vascular plexus, the pampiniform plexus, which courses just under the tunica vaginalis surrounding the testes. The plexus drains into the testicular veins. The pampiniform plexus dissipates heat out of the scrotum by vasodilatation and thereby has an important role in temperature regulation of the testes. Like the ovarian veins, the right testicular vein empties into the inferior vena cava, and the left testicular vein into the left renal vein. Lymphatic drainage of the testes is to the para-aortic nodes.
All the blood and lymph vessels to the testis and epididymis are bundled in a structure known as the spermatic cord. This structure also contains the vas deferens and any remnants of the processus vaginalis. The spermatic cord enters the scrotum from the abdomen through the inguinal canal.
The testis is the site of spermatogenesis and sex steroid production in the male. The epididymis is the site of final sperm maturation. The scrotum is basically a specialized dermal pouch that protects the testis and epididymis from physical injury and aids in heat regulation of the testes. Spermatozoa are very heat sensitive. Because the testes and epididymides are outside the body cavity, intratesticular temperature is typically lower than in the abdomen. Disorders of urogenital development that result in testicular retention within the abdominal cavity or inguinal canal can have dramatic effects on future fertility and increase the risk for testicular tumors (relative risk 3–8). Most of these are seminomas (Chapter 40).
Vas (ductus) deferens and seminal vesicles
The vas deferens is a direct continuation of the epididymis. It is a 45-cm-long structure that begins at the lower end of the epididymis and ascends along the posterior aspect of the testis in loose coils. After leaving the back of the testis, the vas deferens traverses the spermatic cord into the abdomen. The vas deferens may be felt as a firm hard cord on the posterior aspect of the spermatic cord as it traverses the scrotum toward the superficial inguinal ring. After crossing into the abdomen, the vas deferens curves medially across the external iliac artery toward the pelvis. From there, it crosses the obturator nerve and vessels and the vesicular vessels. The vas deferens then crosses over the ureter to meet the duct of the seminal vesicle. Together, the vas deferens and the duct of the seminal vesicle form the ejaculatory duct that opens into the prostatic portion of the urethra. The ejaculatory duct is short (2.5 cm) and lies very close to its companion contralateral duct as they pass forward through the prostate.
The seminal vesicles are a pair of hollow, sacculated structures located at the base of the bladder in front of the rectum. Each vesicle is about 5 cm long and more intimately connected to the bladder than to the rectum. During embryonic development, the seminal vesicles form as diverticula of the vas deferens. The structures share common blood and lymphatic supplies.
Blood supply to the vas deferens and seminal vesicles is mainly from the inferior vesicular artery. The artery accompanies the vas deferens into the scrotum where it anastomoses with the testicular artery. Lymphatic drainage is to the internal and external iliac nodes. The vas deferens functions in sperm transport. The seminal vesicles produce approximately 50–60% of the volume of the seminal fluid.
Important seminal vesicle-derived semen components include fructose and prostaglandins.
The prostate is a partly glandular, partly muscular organ that surrounds the beginning of the male urethra, firmly affixed by a connective tissue sheath just behind the symphysis pubis. The organ is about 2.5 × 3.5 × 4.5 cm. The median lobe of the prostate, histologically referred to as the transition zone, is wedge-shaped, directly surrounds the urethra and separates it from the ejaculatory ducts. When hypertrophied, the median lobe may obstruct the flow of urine. Median lobe hypertrophy occurs commonly in elderly men.
The anterior prostate is composed mostly of fibromuscular tissue. The glandular tissue of the prostate is situated at the sides of the urethra and immediately posterior to it. This glandular tissue is sub- divided into a central and peripheral zone based on embryology (Chapter 6) and histology (Chapter 8). The peripheral zone is much larger than the central zone and composed of about 50 incompletely defined lobules. Each lobule contains minute ducts that empty directly into the urethra just above the ejaculatory ducts.
The blood supply to the prostate gland is variable, but most commonly arises from the common origin of the internal pudendal and inferior gluteal arteries off the internal iliac (hypogastric) arteries. The veins draining the prostate are wide and thin-walled, forming a plexus that communicates with the plexus draining the bladder. Both drain into the internal iliac veins. The prostatic plexus also communicates with the vertebral venous plexuses; therefore, a tumor in the prostate may give rise to secondary growth in the vertebral column. Lymphatic drainage of the prostate follows that of the seminal vesicles and bladder neck into the iliac chain of nodes.
All the muscular tissues in the vas deferens, prostate, prostatic urethra and seminal vesicles are involved in ejaculation. Prostate secretions contribute ∼15% of the volume of the seminal fluid. Important prostate-derived components include acid phosphatases, zinc, citrate and proteases that aid in semen liquefaction. Liquefaction enables sperm to escape the very viscous initial ejaculate.
The penis is composed chiefly of cavernous (erectile) tissue and is traversed by the urethra. The posterior surface of the flaccid penis is nearest the urethra and the opposite, more extensive surface is dorsal (Fig. 7.2). Most of the erectile tissue of the penis is arranged in three longitudinal columns: the paired corpora cavernosa and the single median corpus spongiosum. The tip of the penis is called the glans. The glans of the penis also contains erectile tissue and is continuous with the corpus spongiosum. The glans is covered with a retractable folded layer of thin skin, called the prepuce or foreskin. Although it is not typically indicated medically, the operation of circumcision removes the foreskin and is still widely practiced in some societies.
The internal pudendal arteries supply blood to the penis, entering the organ on its dorsal surface and penetrating deeply into the erectile tissue of the corpora cavernosa. Veins draining the penis enter the prostatic plexus either directly or through the dorsal vein of the penis. Erection of the penis occurs when the extensive cavernous spaces of the corpora cavernosa and corpus spongiosum fill with blood. Engorgement of the penis inhibits venous return and allows maintenance of erection.
Innervation of the penis is critical for its erection. Penile nerve supply is derived from the pudendal nerve (2nd, 3rd, 4th sacral nerves) and from the pelvic autonomic plexuses. The lymphatic drainage of the penis is into the medial group of superficial inguinal lymph nodes. The function of the penis is penetration. Penetration of the vagina of the female allows deposition of semen near the uterine cervix.