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Seminal vesicles
The seminal vesicles are accessory glands of the male reproductive system. They are a pair of contorted or twisted tubes, which are located between the bladder and the rectum. At puberty, the seminal vesicles form sacs and contribute up to 65-75% of the seminal fluid. Unlike what their name suggests, the seminal vesicles are not involved in the storage of spermatozoa. This article will talk about the anatomical structure, histology, embryology and function of the seminal vesicles. Following this will be an overview of the clinical pathology related to these vesicles.
Anatomy
Characteristics
Each seminal vesicle is relatively pyramidal in shape and is approximately 5 cm long. The diameter of each seminal vesicle tube is 3-4cm and they are 10 cm in length when uncoiled. The superior aspects of the seminal vesicles are covered with peritoneum. The base of each vesicle is directed superiorly as well as in a posterolateral direction. At the lower end, each vesicle narrows to form a straight duct, which joins with the ductus deferens. The ductus deferens transports sperm and when it joins with the duct of the seminal vesicle it forms the ejaculatory duct. The duct then opens into the prostatic urethra at an opening known as the verumontanum.
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Location
The anterior aspect of each seminal vesicle is in contact with the posterior wall of the bladder. The posterior aspect of each vesicle, however, is separated from the rectum by the rectoprostatic or Denonvillier’s fascia. Inferior to the seminal vesicles is the prostate with the ureters located anterior to them. Medial to the seminal vesicles are the ampulla of ductus deferens, whereas the veins of the prostatic venous plexus lie lateral to them.
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Arterial supply
The arteries that supply the seminal vesicles are branches of the inferior vesical and middle rectal arteries.
Innervation
The parasympathetic nerve supply of the seminal vesicles is derived from the inferior hypogastric plexus. The sympathetic supply is from the superior lumbar and hypogastric nerves.
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Histology
Under a low magnification level, the lumen shows a honeycombed appearance due to the irregular lumen and diverticula. The wall of the seminal vesicles consists of three main layers. The outer layer is comprised of connective tissue, whereas the middle layer is comprised of smooth muscle. An inner mucosal layer is also present.
The middle smooth muscle layer is arranged into two layers: an outer longitudinal layer and an inner circular layer. Both of these layers are supplied by the sympathetic nervous system.
The inner layer is comprised of pseudostratified columnar epithelium and also contains secretory cells. These secretory cells have lipid droplets in their cytoplasm giving a foamy appearance. The epithelial cells have some variation in their nuclear size and shape and can often contain brown lipofuscin granules. Although the seminal vesicles do not store spermatozoa, spermatozoa are often seen under the microscope, and are thought to enter the vesicles by reflux through the ampulla.
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Embryology
The seminal vesicles develop from the mesonephric or Wolffian ducts. These ducts are derived from the mesoderm, the middle layer of one of the three primary germ layers in the embryo. The other two layers are the ectoderm and the endoderm. The seminal vesicles are formed from budding off the distal mesonephric ducts at around 10-12 weeks gestation.
Function
The seminal vesicles form a functional unit with the ampulla of the ductus deferens and the ejaculatory ducts. This functional unit develops slowly after the onset of puberty. The seminal vesicles contribute up to 85% of the seminal fluid, with the prostate gland secreting the majority of the rest. The seminal vesicles secrete a yellow, alkaline and viscous fluid that contains fructose, fibrinogen, prostaglandins and vitamin C, as well as other specific proteins.
During ejaculation, the sympathetic nervous system innervates the muscular wall of the seminal vesicles, causing it to contract. This results in secretion of the seminal fluid into the ejaculatory duct and then into the urethra.
Clinical notes
Seminal vesiculitis
Inflammation of the seminal vesicles is referred to as seminal vesiculitis and it usually occurs secondary to prostatitis or inflammation of the prostate, but may occur independently. Seminal vesiculitis can be caused by:
- bacterial infection
- spreading of inflammation from the testis or epididymis
- obstruction of blood to the perinal area
- from excessive heat
There are two types of seminal vesculitis: acute and chronic. Acute seminal vesculitis causes pain in the lower abdomen, whereas chronic seminal vesculitis can cause discomfort superior to the pubis region and in the perineal area. Seminal vesiculitis can also cause haematospermia or blood in the sperm, as well as painful urination and ejaculation. Treatment includes antibiotics and abstinence from sexual intercourse.
Seminal vesicle cysts
Seminal vesicle cysts are fluid-filled sacs and can be either congenital or acquired. Congenital seminal vesicle cysts are present since birth. These cysts rarely occur and are thought to be due to obstruction at the junction where the seminal vesicle joins with the ejaculatory duct. They usually are unilateral and become symptomatic early in adult life. Acquired cysts most often occur secondary to surgery or prostatic infection and are usually bilateral. Symptoms occurring due to a cyst may be absent, but may include:
- dysuria or painful urination
- haematuria or blood in the urine
- abdominal pain
- pain with ejaculation
Physical examination of the seminal vesicles in clinical practice is difficult and they are usually not palpable. The area superior to the prostate is usually soft with no tenderness present. A seminal vesicle cyst can be felt in this area as a compressible mass.
Laboratory examination of the seminal fluid can be carried out with a semen sample. Fructose levels in the semen are a measure of seminal vesicle function and low or absent levels of fructose may indicate an obstruction within the seminal vesicle.
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