The Anatomy of the Seminal Vesicles

The seminal vesicles are a pair of glands along the back of the bladder base in men and are part of the male genital system. Their main function is to produce a fluid that makes up semen, which is released during ejaculation.

Anatomy

The seminal vesicles look like two tightly coiled tubular sacs on either side of the back of the bladder base, or fundus. They are about 2 cm–4 cm (centimeters) long and 1 cm–2 cm in diameter. They usually extend back and out from the base of the prostate gland, a bit like rabbit ears.

Structure

At their lower end, each seminal vesicle forms a duct, which joins the ductus deferens (which carries sperm cells from the testes). Together, both ducts form the ejaculatory duct, which passes through the prostate gland and joins the prostatic urethra (the portion of the urethra in the prostate gland).

On a microscopic level, the inner lining of the seminal vesicles is extensively folded, and is made up of epithelial cells, which secrete the sugars, proteins, and mucus that contribute to seminal fluid. The outside of the vesicles are lined by a coat of smooth muscle, which can contract during ejaculation.

Location

The seminal vesicles are located between the bladder fundus and the rectum.

Anatomical Variations

Occasionally, boys are born with abnormalities of the seminal vesicles. One or both seminal vesicles may be absent (a condition called agenesis). Men with cystic fibrosis sometimes have missing seminal vesicles.

Rarely, both seminal vesicles may be on one side. Congenital (at birth) abnormalities of the seminal vesicles are often associated with abnormalities in other parts of the male genitourinary system that makes up the genital and urinary organs, such as the prostate, testes, kidneys, or ureters.

Cysts of the seminal vesicles can be congenital or acquired. Acquired cysts are often caused by an infection, obstruction of the duct, or other sources of inflammation. Cysts are usually small and on one side, although rarely they may become quite large.

Although the seminal vesicles usually extend up and out from the prostate base, they may sometimes turn downward, along the back of the prostate. If this is the case, a doctor performing a prostate exam may mistake the seminal vesicles for a prostate abnormality. This confusion can easily be cleared up with imaging (such as a prostate ultrasound or magnetic resonance imaging, or MRI). 

Function

The main function of the seminal vesicles is to produce a fluid that is high in fructose, a sugar that provides nutrients for sperm cells, as well as other proteins, enzymes, and mucus. This fluid is a major component of semen, and it accounts for about 50%–80% of semen volume.

The fluid produced by the seminal vesicles is stored in the vesicles themselves. During ejaculation, strong contractions occur in the muscular walls of the vesicles, pushing the seminal fluid into the ejaculatory ducts and urethra, where it leaves the body. 

Associated Conditions

Cysts and other malformations of the seminal vesicles are often associated with other problems in the male genitourinary system. For example, people with seminal vesicle cysts at birth are often missing one of their kidneys. Also, the ureters, which usually empty into the bladder, may instead empty into the seminal vesicles or other structures. Congenital abnormalities like these may require surgery for correction.

Other conditions related to the seminal vesicles include the following:

• Men may be born with absent or underdeveloped (hypoplastic) seminal vesicles on one or both sides. These men often have abnormalities in their seminal fluid and may be infertile. Men with infertility related to seminal vesicle abnormalities may benefit from assisted reproductive technologies that handle both a woman's egg and a man's sperm.
• Seminal vesicle cysts may cause no symptoms. When they do cause symptoms, they are often vague and include perineal pain (pain in the area between the genitals and rectum) during ejaculation or bowel movements, pain with urination, urinary retention, or recurrent epididymitis (a returning inflammation of the epididymis, a duct behind the testes where sperm passes). Cysts are usually small, but can become large and cause symptoms by pushing on other structures in the pelvis. This is rare. Though treatment may not be required for most seminal vesicle cysts, large symptomatic cysts may be treated with surgical drainage or removal.
• Infection of pelvic organs such as the prostate, bladder, ejaculatory ducts, or epididymis may spread to the seminal vesicles. These infections may bring on fever, pain with urination, or pain in the scrotum, perineum, or rectum. Infections can usually be treated with antibiotics. Rarely, complications such as pelvic abscess or narrowing of a duct (stricture) may require surgery.
• Cancers of other pelvic organs may also spread to the seminal vesicles, particularly prostate cancer. Bladder cancer or, less commonly, rectal cancer may also spread to the seminal vesicles. Rarely do cancers from the testes or kidneys metastasize (spread) to the seminal vesicles. Cancer arising directly from the seminal vesicles also is rare. Before starting treatment, doctors need to know whether the cancer has spread to the seminal vesicles, since it will change prognosis (the prospect for recovery) and treatment strategy.
• Patients with a history of seminal vesicle infection, diabetes, or kidney disease may develop calcifications (calcium deposits) of the seminal vesicles. These calcifications usually do not produce symptoms, and no treatment is required. However, they may be detected on imaging studies performed for other reasons.

Tests

People with seminal vesicle cysts or infections often have symptoms that are difficult to attribute to a specific organ. They may undergo a general workup looking for signs of infection, such as a physical exam, blood tests, or urinalysis. Men with infertility may undergo analysis of their seminal fluid.

The seminal vesicles can be seen in imaging studies, such as an ultrasound, a computed tomography (CT) scan, or an MRI of the pelvis. Ultrasound and CT are the least expensive and most widely available types of imaging. They are often first ordered in men with urinary difficulties or pelvic pain. They can be helpful in detecting signs of infection or inflammation in the seminal vesicles or nearby organs. Complications of infection, such as an abscess (a collection of pus surrounded by inflamed tissue) can be detected with ultrasound and CT, and doctors may use the images to help guide surgical treatment. 

MRI produces the most detailed images of the seminal vesicles. This test is very good at detecting and visualizing seminal vesicle cysts, malformations, and other associated abnormalities of the genitourinary system. MRI is also the best imaging study to determine whether cancer of the prostate, rectum, or bladder has spread to the seminal vesicles.