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 85% 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.
Each seminal vesicle is relatively pyramidal in shape and is approximately 5cm long. The diameter of each seminal vesicle tube is 3-4cm and they are 10cm 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 of each vesicle, it narrows to form a straight duct, which joins with the vas deferens. The vas 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.
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 the vas deferens whereas the veins of the prostatic venous plexus lie lateral to them. The arteries that supply the seminal vesicles are branches of the inferior vesical and middle rectal arteries. The parasympathetic nerve supply of the seminal vesicles is derived from the inferior hypogastric plexus and the sympathetic supply from the superior lumbar and hypogastric nerves.
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.
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.
The seminal vesicles form a functional unit with the ampulla of the vas 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.