Seminal Vesicle Anatomy

The seminal vesicles are paired organs of the male genitourinary system. Specifically, they are genital organs. The male genital organs include the penis, testes, excretory genital ducts, ductus deferens, seminal vesicles, prostate, and bulbourethral glands. Of these organs, the seminal vesicles, prostate, and bulbourethral glands are considered accessory glandular structures.

These organs work together to produce and excrete semen ("seed" in Latin), which is composed of mature spermatozoa residing in a thickened, mucous fluid. The spermatozoa are produced by the testes, and they travel to the urethra by means of the ductus deferens. Along this route, the genital ducts and accessory glandular structures produce mucous secretions that combine with the spermatozoa to create semen. Smooth muscular contractions then propel the semen through the urethra and into the female reproductive tract by means of the penis.

The following image depicts the seminal vesicles, ejaculatory ducts, and the prostate.

Anterior aspect of the seminal vesicles, terminal parts of the deferent ducts, and the prostate. (Source: Gray's Anatomy.)

Inspection

Manual examination of the seminal vesicles can be attempted via palpation with the index finger in the rectal vault and the fingerpad directed anteriorly. The seminal vesicles lie immediately superior to the prostate gland at the base of the bladder and flare out laterally as one palpates in an inferior to superior direction. Although the prostate gland can be readily felt, the seminal vesicles are not normally palpable. However, in the instance of chronic infection (such as tuberculosis) or advanced cancer of the prostate, the glands may feel indurated. ^[1] Cystic masses of the seminal vesicles may also be felt on palpation and are typically embryologic remnants of the paramesonephric ducts. Primary carcinoma of the seminal vesicles is very rare.

The seminal vesicles are bilateral, lobulated glands (see the following image). They are soft and approximately 5-7 cm long. The vesicles are blind pouches and are rounded on their most superior aspects and taper to their inferior aspects, where they constrict to ultimately form short ducts. They descend inferomedially while lying on the fundic portion of the posterior surface of the urinary bladder. The seminal vesicles are immediately inferior and lateral to the ampullary portions of the ductus deferentes. ^[2] This bilateral arrangement most closely resembles the letter "V."

Anterior aspect of the seminal vesicles, terminal parts of the deferent ducts, and the prostate. (Source: Gray's Anatomy.)

The ureters pass in between the superior, rounded aspects of the seminal vesicles and the superior portions of the ampullae of the ductus deferentes. The short ducts of the seminal vesicles join the lateral aspects of the ductus deferentes at an acute angle, creating the ejaculatory ducts at the base of the prostate gland.

The seminal vesicles lie below the inferior-most aspect of the peritoneum in the pelvic cavity. They are covered by endopelvic fascia, which is an abundant extraperitoneal connective tissue. ^{[2, 1, 3]}

Arterial supply and innervation

The seminal vesicles receive their arterial supply in a way that is similar to that of the prostate gland. The primary arterial supply is from the inferior vesical, internal pudendal, and middle rectal arteries. These 3 primary sources all arise from the anterior division of the internal iliac artery.

The innervation to the seminal vesicles is from the sympathetic nervous system. These fibers originate from the inferior hypogastric plexus.

Embryology

Developmentally, when the testes begin to form during the seventh week of life, the mesonephric (Wolffian) duct differentiates into the male genital system. The mesonephric duct ultimately forms the epididymis, ductus deferens, seminal vesicles, and ejaculatory ducts by way of the mesonephric ductal system. Initially, the epididymis and ductus deferens form from the mesonephric duct. Then, a local dilation of the ductus deferens forms before the ductus deferens joins the urethra. This local dilatation evaginates, ultimately becoming the seminal vesicles.

The seminal vesicles are each composed of a long, tortuous tube that is 15 cm in length. ^[4] If the seminal vesicle is sectioned, the tube is visible, having been cut in multiple different orientations. This gives a honeycombed appearance at low magnification. ^[5] Its mucosa is lined with pseudostratified columnar epithelium, which is secretory in nature. The highly granular cells produce a yellowish, alkaline fluid that contains fructose, various proteins, and vitamin C. These cells are highly influenced by testosterone levels, which dictate the size and activity levels of the cells. The fructose portion of this fluid provides nourishment for the highly active spermatozoa. This fluid, produced by both seminal vesicles, accounts for over 50% of the total volume of seminal fluid. The remainder is contributed by the prostate gland, with a small contribution from the bulbourethral glands.

The secretory cells of the seminal vesicles are underlain by the lamina propria, which is composed of elastic fibers surrounded by an inner circular and outer longitudinal layer of smooth muscle. This smooth muscle is sympathetically innervated; during ejaculation, the fluid produced by the seminal vesicles is forced through the ejaculatory ducts and into the urethra.

Natural variants

Minor variations of the anatomy of the seminal vesicles are common, but developmental abnormalities are not common. When observed, the abnormalities are typically cysts of both the prostate and seminal vesicles. The cysts are small, midline structures posterior to the prostate. These cysts represent embryologic remnants of the paramesonephric (Mullerian) ducts. If problematic, the pathophysiology relates to local obstruction of the neck of the bladder. ^[1]

Pathophysiologic variants

Blocked duct

Blockages of the posttesticular portion of the reproductive tract may occur. These blockages may result in infertility, they may be congenital or acquired, and they may be mechanical or functional in nature.

One type of congenital obstruction of the seminal vesicles occurs in cystic fibrosis, an autosomal recessive genetic disease. Many, if not most, men with cystic fibrosis have malformed or absent seminal vesicles along with malformed or missing ductus deferentes and ejaculatory ducts. These pathologies give rise to reproductive tract obstruction.

Functionally, the seminal vesicles may become obstructed following damage to the anterior portion of the inferior hypogastric plexus of nerves, which represent postganglionic outflow to the genital organs. Such damage to the innervation of the seminal vesicles may occur in cases of trauma and gross tumor growth in the pelvis. Given the difficulty in identifying and dissecting the inferior hypogastric plexus of nerves during surgical procedures such as retroperitoneal lymph node dissection, iatrogenic injuries may also occur. Finally, medications that impair the contractility of the seminal vesicles may also lead to functional obstruction. ^{[1, 6]}

Tuberculosis

Mycobacterium tuberculosis begins as a pulmonary infection. Genitourinary organs are then reached via a hematogenous route, with the kidneys and prostate gland the most commonly secondarily affected. Urine containing the microorganism may subsequently invade the seminal vesicles. Fibrosis and necrosis of the seminal vesicles are common sequelae. Palpation of the seminal vesicles may reveal enlargement and induration. Pathology is typically bilateral, affecting both glands. Scarring, calcification, and morphologic alteration can affect the functioning of the glands. ^[1]