Seminal vesicles

The seminal vesicles (also called vesicular glands or seminal glands) are a pair of convoluted tubular accessory glands that lie behind the urinary bladder of male mammals. They secrete fluid that largely composes the semen. The vesicles are 5–10 cm (2.0–3.9 in) in size, 3–5 cm (1.2–2.0 in) in diameter, and are located between the bladder and the rectum. They have multiple outpouchings, which contain secretory glands, which join together with the vasa deferentia at the ejaculatory ducts. They receive blood from the vesiculodeferential artery, and drain into the vesiculodeferential veins. The glands are lined with column-shaped and cuboidal cells. The vesicles are present in many groups of mammals, but not marsupials, monotremes or carnivores.. Inflammation of the seminal vesicles is called seminal vesiculitis and most often is due to bacterial infection as a result of a sexually transmitted infection or following a surgical procedure. Seminal vesiculitis can cause pain in the lower abdomen, scrotum, penis or peritoneum, painful ejaculation, and blood in the semen. It is usually treated with antibiotics, although may require surgical drainage in complicated cases. Other conditions may affect the vesicles, including congenital abnormalities such as failure or incomplete formation, and, uncommonly, tumours. The seminal vesicles have been described as early as the second century AD by Galen, although the vesicles only received their name much later, as they were initially described using the term from which the word prostate is derived.

Structure The human seminal vesicles are a pair of glands in males that are positioned below the urinary bladder and at the end of the vasa deferentia, where they enter the prostate. Each vesicle is a coiled and folded tube, with occasional outpouchings termed diverticula in its wall. The lower part of the tube ends as a straight tube called the excretory duct, which joins with the vas deferens of that side of the body to form an ejaculatory duct. The ejaculatory ducts pass through the prostate gland before opening separately into the verumontanum of the prostatic urethra. The vesicles are between 5–10 cm (2.0–3.9 in) in size, 3–5 cm (1.2–2.0 in) in diameter, and have a volume of around 13 mL. The vesicles receive blood supply from the vesiculodeferential artery, and also from the inferior vesical artery. The vesiculodeferential artery arises from the umbilical arteries, which branch directly from the internal iliac arteries. Blood is drained into the vesiculodeferential veins and the inferior vesical plexus, which drain into the internal iliac veins. Lymphatic drainage occurs along the venous routes, draining into the internal iliac nodes. The vesicles lie behind the bladder at the end of the vasa deferentia. They lie in the space between the bladder and the rectum; the bladder and prostate lie in front, the tip of the ureter as it enters the bladder above, and Denonvilliers' fascia and the rectum behind.

Development

In the developing embryo, at the hind end lies a cloaca. This, over the fourth to the seventh week, divides into a urogenital sinus and the beginnings of the anal canal, with a wall forming between these two inpouchings called the urorectal septum. Two ducts form next to each other that connect to the urogenital sinus; the mesonephric duct and the paramesonephric duct, which go on to form the reproductive tracts of the male and female respectively. In the male, under the influence of testosterone, the mesonephric ducts proliferate, forming the epididymis, ductus deferens and, via a small outpouching near the developing prostate, the seminal vesicles. Sertoli cells secrete anti-Müllerian hormone, which causes the paramesonephric ducts to regress. The development and maintenance of the seminal vesicles, as well as their secretion and size/weight, are highly dependent on androgens. The seminal vesicles contain 5α-reductase, which metabolizes testosterone into its much more potent metabolite, dihydrotestosterone (DHT). The seminal vesicles have also been found to contain luteinizing hormone receptors, and hence may also be regulated by the ligand of this receptor, luteinizing hormone.

Microanatomy

The inner lining of the seminal vesicles (the epithelium) is made of a lining of interspersed column-shaped and cube-shaped cells. There are varying descriptions of the lining as being pseudostratified and consisting of column-shaped cells only. When viewed under a microscope, the cells are seen to have large bubbles in their interior. This is because their interior, called cytoplasm, contains lipid droplets involved in secretion during ejaculation. The tissue of the seminal vesicles is full of glands, spaced irregularly. As well as glands, the seminal vesicles contain smooth muscle and connective tissue. This fibrous and muscular tissue surrounds the glands, helping to expel their contents. The outer surface of the glands is covered in peritoneum.

Function The seminal vesicles secrete a significant proportion of the fluid that ultimately becomes semen. Fluid is secreted from the ejaculatory ducts of the vesicles into the vas deferens and ejaculated through the urethra during the male sexual response. About 70-85% of the seminal fluid in humans originates from the seminal vesicles. The fluid consists of nutrients including fructose and citric acid, prostaglandins, and fibrinogen. Nutrients help support sperm until fertilisation occurs; prostaglandins may also assist by softening mucus of the cervix, and by causing reverse contractions of parts of the female reproductive tract such as the fallopian tubes, to ensure that sperm are less likely to be expelled.

Clinical significance

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