Seminal vesicles

The seminal vesicles are a pair of tubular glands in the male reproductive system. They are located behind the bladder, superior to the prostate gland, and lateral to the vas deferens. The seminal vesicles are positioned in the pelvic cavity, just above the base of the bladder, and adjacent to the ampulla of the vas deferens. Each seminal vesicle is approximately 5-10 cm long and is coiled into a compact structure. The ducts of the seminal vesicles join with the vas deferens to form the ejaculatory ducts, which then pass through the prostate and open into the prostatic urethra.

Structure and Anatomy

The seminal vesicles are an integral part of the male reproductive system, contributing to the production of seminal fluid. Below is a detailed description of their anatomy, including location, structure, blood supply, and surrounding structures.

Location

The seminal vesicles are paired glands located in the pelvic cavity. They are positioned:

• Posterior to the Bladder: The seminal vesicles are located behind the urinary bladder, near its base.
• Superior to the Prostate: They sit just above the prostate gland, which is inferior to them.
• Lateral to the Vas Deferens: Each seminal vesicle lies adjacent to the ampulla of the vas deferens, which joins with the seminal vesicle duct to form the ejaculatory duct.
• Anterior to the Rectum: The seminal vesicles are located in front of the rectum, making them palpable through the rectum during a digital rectal examination.

Structure

The seminal vesicles are tubular, coiled glands that are approximately 5 to 10 cm in length when uncoiled, though they appear much shorter in their compact form due to their highly folded and coiled structure.

Tubular Shape:Each seminal vesicle is a long, coiled tube, resembling a honeycomb-like structure internally, that helps increase its surface area for secretion. This convoluted appearance gives the seminal vesicle the ability to store and secrete large volumes of fluid.

Lumen and Mucosa:The lumen of the seminal vesicle is lined with pseudostratified columnar epithelium, which produces seminal fluid. The mucosa is arranged in complex folds that increase the gland’s secretory surface area, facilitating the production and storage of seminal fluid.

Layers

The wall of the seminal vesicle consists of several layers:

• Mucosal Layer: The inner lining, or mucosa, is responsible for producing the gland’s secretions. It is folded and contains columnar epithelial cells and goblet cells.
• Smooth Muscle Layer: Surrounding the mucosa is a layer of smooth muscle. This muscle contracts during ejaculation, helping to expel the fluid stored in the seminal vesicle into the ejaculatory duct.
• Fibrous Capsule: The outermost layer is a fibrous connective tissue capsule, which provides structural support and protection to the gland.

Ducts

Each seminal vesicle has a duct that joins with the ampulla of the vas deferens to form the ejaculatory duct. The ejaculatory ducts then pass through the prostate gland and empty into the prostatic urethra.

• Duct of the Seminal Vesicle: This duct is short and merges with the terminal part of the vas deferens to form the ejaculatory duct.
• Ejaculatory Ducts: The seminal fluid produced by the seminal vesicles is combined with sperm from the vas deferens within the ejaculatory ducts. These ducts then open into the prostatic urethra at the seminal colliculus.

Blood Supply

The seminal vesicles receive their blood supply from branches of the internal iliac artery, specifically the inferior vesical artery and the middle rectal artery.

Arterial Supply

• Inferior Vesical Artery: This artery primarily supplies the seminal vesicles and the prostate. It arises from the internal iliac artery.
• Middle Rectal Artery: This artery also supplies blood to the seminal vesicles, along with portions of the rectum.

Venous Drainage

The venous drainage of the seminal vesicles mirrors their arterial supply, with blood flowing into the prostatic venous plexus and the internal iliac veins.

Nerve Supply

The nerve supply to the seminal vesicles is derived from the inferior hypogastric plexus, which contains both sympathetic and parasympathetic fibers.

• Sympathetic Innervation:The sympathetic nerves from the pelvic plexus are responsible for stimulating the contraction of the smooth muscle in the seminal vesicle walls during ejaculation. This helps propel the seminal fluid into the ejaculatory ducts.
• Parasympathetic Innervation:Parasympathetic fibers play a role in regulating the secretory functions of the seminal vesicles, enhancing their production of seminal fluid.

Lymphatic Drainage

Lymphatic drainage from the seminal vesicles is directed toward the internal iliac lymph nodes and the external iliac lymph nodes.

This drainage system is important for the spread of infection or disease, such as cancer, from the seminal vesicles to other parts of the body.

Surrounding Structures

The seminal vesicles are located in close proximity to several important structures in the pelvic cavity:

• Bladder: The seminal vesicles lie just behind the bladder, and their fluid contributions are stored temporarily in the prostatic urethra until ejaculation.
• Prostate: Located just below the seminal vesicles, the prostate gland plays a complementary role in semen production by contributing additional fluid and enzymes. The ejaculatory ducts formed by the union of the seminal vesicle ducts and vas deferens pass through the prostate.
• Rectum: The seminal vesicles are anterior to the rectum. This anatomical position allows them to be palpated during a digital rectal exam, which can help assess their size and health in clinical settings.
• Vas Deferens: Each seminal vesicle is located near the terminal portion of the vas deferens, and the two structures unite to form the ejaculatory ducts. The vas deferens carries sperm from the testes, which then mix with seminal vesicle secretions during ejaculation.

Microscopic Anatomy

At the microscopic level, the seminal vesicles consist of:

• Epithelial Lining:The inner mucosal lining of the seminal vesicles is composed of pseudostratified columnar epithelium. These epithelial cells secrete the fluid that forms a large portion of semen.
• Secretory Cells:The cells in the epithelium are specialized to produce a fluid rich in fructose, prostaglandins, and other nutrients that nourish sperm and facilitate their motility.
• Smooth Muscle Layer:Beneath the mucosa, the smooth muscle layer contracts during ejaculation, helping to expel the stored fluid into the ejaculatory ducts. The contraction of the smooth muscle is controlled by the sympathetic nervous system.

Embryological Development

The seminal vesicles develop from the mesonephric (Wolffian) ducts during the embryonic period. These ducts are responsible for the development of other structures in the male reproductive system, including the vas deferens and epididymis.

Androgen Dependence: The development of the seminal vesicles is regulated by androgens (male sex hormones). Testosterone stimulates the growth and maturation of these glands during fetal development and puberty.

Variations in Anatomy

The size and shape of the seminal vesicles can vary slightly between individuals. While the average length is about 5-10 cm, some men may have slightly larger or smaller seminal vesicles. Any significant anatomical variations, such as cysts or malformations, can affect their function and lead to reproductive issues.

Function

The seminal vesicles play a critical role in the male reproductive system, primarily by producing and secreting a significant portion of the fluid that makes up semen. This fluid provides nourishment for sperm, helps with their motility, and creates a favorable environment for fertilization. Below is a detailed description of the key functions of the seminal vesicles.

Production of Seminal Fluid

One of the primary functions of the seminal vesicles is to produce seminal fluid, which makes up about 60-70% of the total volume of semen.

Secretion of Fructose

• The seminal vesicles secrete a fluid rich in fructose, a simple sugar that provides an energy source for sperm. Fructose is critical for sperm motility, as it fuels the movement of sperm as they travel through the female reproductive tract in search of an egg to fertilize.
• Fructose concentration in seminal fluid is an important marker of seminal vesicle function and overall semen quality.

Production of Proteins and Enzymes

• The seminal vesicles produce several important proteins and enzymes that play a role in semen coagulation and liquefaction. After ejaculation, semen initially coagulates, forming a gel-like consistency. The proteins secreted by the seminal vesicles help form this initial clot, which keeps the semen in place in the vagina after deposition.
• Later, enzymes such as proteolytic enzymes and fibrinolysin, which are also secreted by the prostate gland, help break down this clot, allowing sperm to swim freely.

Secretion of Prostaglandins

• Prostaglandins are lipid compounds that are also secreted by the seminal vesicles. They have several important functions, including enhancing sperm motility and aiding the fertilization process.
• Prostaglandins help by stimulating contractions in the female reproductive tract, which assists in the movement of sperm toward the egg. These contractions create a wave-like motion in the uterine and fallopian tube muscles, facilitating sperm transport.

Contribution to Semen Volume

The seminal vesicles are responsible for producing the majority of the fluid volume in semen, approximately 60-70%. This fluid acts as a medium in which sperm can travel and survive.

Dilution of Sperm

• The fluid produced by the seminal vesicles helps dilute sperm from the vas deferens. This dilution is necessary to ensure that sperm do not become too concentrated, which could reduce their motility.
• By adding a large volume of fluid to the semen, the seminal vesicles create a favorable environment for sperm, allowing them to remain viable and motile for longer periods.

Enhancement of Sperm Motility

The seminal vesicles produce substances that enhance sperm motility, including prostaglandins and fructose. These compounds provide sperm with the energy needed to move and assist with their ability to swim effectively through the female reproductive tract.

pH Regulation and Protection of Sperm

The fluid produced by the seminal vesicles is slightly alkaline, which helps protect sperm from the acidic environment of the vagina and other parts of the female reproductive system.

Alkaline Environment

• The alkalinity of the seminal fluid neutralizes the acidic conditions that sperm encounter after ejaculation. The vaginal environment is naturally acidic (pH 3.5-4.5), which is hostile to sperm. The seminal vesicles’ secretions raise the pH level of the semen to a more neutral or slightly alkaline range, providing a buffer that protects sperm.
• This protection increases the chances of sperm surviving long enough to reach and fertilize the egg.

Protection from Urethral Acidity

In addition to the vaginal environment, sperm must pass through the male urethra, which can contain traces of urine that is also slightly acidic. The seminal fluid from the seminal vesicles helps to flush and neutralize the urethra before sperm pass through, providing additional protection from acidic conditions.

Facilitation of Semen Coagulation and Liquefaction

The seminal vesicles contribute to the process of semen coagulation and liquefaction, both of which are essential for sperm function and fertilization.

Semen Coagulation

• After ejaculation, the proteins and enzymes secreted by the seminal vesicles, along with those from the prostate, help coagulate semen. This coagulation forms a gel-like mass that keeps the sperm near the cervix in the female reproductive tract, preventing them from being lost immediately after ejaculation.
• The coagulation helps the sperm stay localized in the area where fertilization is more likely to occur.

Semen Liquefaction

• After the initial coagulation, liquefaction must occur to allow sperm to swim freely. Enzymes such as proteolytic enzymes and fibrinolysin, contributed by the prostate gland, break down the coagulated mass, restoring semen to a more fluid state.
• This process allows sperm to regain mobility and begin their journey toward the egg for potential fertilization.

Sperm Storage and Delayed Release

Although the seminal vesicles do not directly store sperm, they do store seminal fluid that is released during ejaculation.

Storage of Seminal Fluid

• The seminal vesicles store the seminal fluid they produce until ejaculation occurs. During ejaculation, the smooth muscle surrounding the seminal vesicles contracts to expel the fluid into the ejaculatory ducts, where it mixes with sperm from the vas deferens.
• The fluid is not released continuously but rather held in reserve until needed during ejaculation.

Delayed Release During Ejaculation

• The seminal vesicle secretions are released during the second phase of ejaculation, after the sperm has been expelled from the vas deferens. This delayed release ensures that the sperm and seminal fluid are mixed efficiently to form semen.
• The timing of this release is regulated by the sympathetic nervous system, which controls the contraction of smooth muscle in the seminal vesicles and vas deferens.

Contribution to Fertility

The seminal vesicles play an essential role in male fertility by providing the right environment for sperm to survive and function effectively.

• Nutrient Supply for Sperm: The fructose secreted by the seminal vesicles provides a vital energy source for sperm. This energy is necessary for the sperm to swim effectively and reach the egg. Without the nutrients supplied by the seminal vesicles, sperm would not have the energy needed for motility, which would reduce the chances of fertilization.
• Facilitating Fertilization: The prostaglandins secreted by the seminal vesicles help facilitate fertilization by stimulating contractions in the female reproductive tract, aiding sperm movement. These contractions help propel the sperm toward the egg, increasing the likelihood of fertilization.
• Maintaining Sperm Viability: The alkaline secretions from the seminal vesicles protect sperm from the acidic environments they encounter during their journey. This ensures that sperm maintain their viability as they pass through the male and female reproductive tracts, allowing them to fertilize the egg under optimal conditions.

Clinical Significance

The seminal vesicles play a crucial role in male reproductive health, and any dysfunction or abnormality in these glands can have significant clinical consequences:

• Seminal Vesiculitis: Inflammation of the seminal vesicles, known as seminal vesiculitis, can result from bacterial infections, often associated with prostatitis or urinary tract infections. Symptoms may include pelvic pain, painful ejaculation, blood in semen (hematospermia), and urinary discomfort.
• Cysts and Obstruction: Seminal vesicle cysts or obstruction of the ejaculatory ducts can interfere with the passage of seminal fluid, leading to low semen volume or fertility issues (oligospermia or azoospermia). These conditions can be diagnosed via imaging such as transrectal ultrasound (TRUS).
• Male Infertility: Abnormalities in seminal vesicle function, such as insufficient production of seminal fluid or low fructose levels, can negatively impact sperm motility and survival, contributing to male infertility.
• Prostate and Seminal Vesicle Involvement in Cancer: In cases of prostate cancer, the cancer may invade the seminal vesicles, which can affect the prognosis and treatment options. Seminal vesicle invasion often indicates a more advanced stage of the disease.