The epididymis is an essential component of the male reproductive tract. It is a site of sperm storage and maturation until the time of their discharge to the ductus deferens or vas deferens. In this article we aim to cover the embryology, anatomy and functions of the epididymis. At the end we will conclude with some clinical points relevant to this structure.
The major factor in sex determination during development is the presence of Y chromosome. The testis determining factor (TDF) is encoded by the gene present on the sex-determining region on the Y chromosome.
The development of reproductive organs is mainly from the intermediate mesenchyme. Mesonephric (Wolffian duct) and paramesonephric (Mullerian duct) ducts develop alongside each other. In males, sexual differentiation occurs due to regression of Mullerian ducts and development of mesonephric ducts into main reproductive organs like epididymis, vas deferens and ejaculatory ducts. The production of anti-mullerian hormone by the sertoli cells, causes regression of the Mullerian ducts.
The complex coiled elongated structure of the epididymis develops gradually during embryonic and early postnatal life. Interaction of Wolffian duct epithelium with surrounding mesenchymal cells, and cellular proliferation along with directed cell rearrangement cause the elongation of the structure. This process largely depends on the presence of androgens and mesenchymal factors. The proximal region (from initial segment to corpus ) undergoes coiling by birth. The coiling of the rest of the structure and epithelial differentiation occurs postnatally. Lumicrine factors released by the testicles play very important role during postnatal development
Macroscopic & Microscopic Anatomy
The epididymis is a comma shaped elongated, fine tubular structure that is compressed and raveled tightly. The degree of compression results in the epididymis appears almost solid. The epididymis is found on the posterior surface of the testes, and sits along the entire length of the posterior testes.
Head, Body & Tail
It is divided into a head (caput or globus major), body (corpus) and tail (cauda or globus minor), with the caput reaching over the top of the testes. The head is in contact with the upper end of the testis by efferent ductules, while the tail is connected with its lower end by reflection of the tunica vaginalis.
Serous membrane covers the lateral surface, head and tail of the epididymis. Most of the body is also completely invested by this membrane, except along the posterior border. A small pouch known as sinus of the epididymis lies between the body and the testis, which is also known as digital fossa.
The head is the section that receives the seminal fluid from the testes, and has a thin myometrium structure. The sperm in this region is still relatively dilute. The tail has a thicker myometrial wall, and absorbs more fluid, resulting in a higher concentration of the resultant fluid. Vas deferens continues from the tail, which ascends to the deep inguinal ring within the spermatic cord.
At the upper extremity of the epididymis lies the developmental remnant of mesonephros called the appendix epididymis.
The efferent ductules from the mediastinal testis become enlarged and convoluted, forming the lobules of the epididymis while entering into the head of the epididymis. Each lobular duct opens into a coiled duct of epididymis, forming the body and tail of the structure. These coils are held together by fibrous connective tissue. The epithelium here is pseudostratified columnar epithelium with two main cell types, principal and basal cells. Other cells like apical, narrow, halo and clear cells are also present but less common.
Principal cells are found throughout the length of the duct but show structural differences in each region. The microvilli on these cells serve to resorb fluid from the testicular secretion. These cells also secrete glycoproteins which help in the maturation process of spermatozoa.
Basal cells also appear on the entire epididymis. They lie at the base and are considered precursors of the principal cells. Apical cells are mostly found in the region of head with numerous mitochondria in cytoplasm. The clear cells are present in the tail region. They have few microvilli and are considered as active endocytic cells.
Epithelium lies on thin lamina propria and circularly arranged smooth muscle layer. Close to the vas deferens the muscular layer becomes thicker and organised into three layers.
The blood supply to the epididymis is the same as that of the testes, namely the testicular arteries, which enter the scrotum through the spermatic cord. These arteries arise from the abdominal aorta and travel in the retroperitoneal space before crossing over the superior surface of the ureters They pass through the deep inguinal ring, and ultimately enter the inguinal canal along with the other components of the spermatic cord. These arteries also supply the vas deferens in part, but that also has a separate blood supply.
Venous drainage of the epididymis and testes is via a network of veins known as the pampiniform plexus. Thee veins also pass in the spermatic cord, with the right network terminating in the inferior vena cava, and the left terminating in the left renal vein.
Lymphatic drainage very much follows the testicular arteries and veins, and terminates at the para-aortic lymph nodes, near the region that the testes developed and descended during foetal life.
The sperms that enter the caput are relatively non-motile. They require two to three months of time and transport from the caput to the cauda, in order to develop the ability to swim and fertilize the egg. This process is facilitated by fluid that is at the correct pH, oxygen levels, energy substrate and temperature for sperm maturation. It is only once the sperm enter the female reproductive tract that they undergo the capacitation process, which renders the sperm able to bind to the egg and deliver its contents.
Ejaculation in the male is a process where the sperms will move from the tail of the epididymis to the vas deferens. The latter propel the sperms forwards with its peristaltic muscular contractions. They are mixed with the seminal fluid from the accessory glands before they are ejaculated, with the final mixture of these substances being called semen.
It is the connection of the caput of the epididymis to the upper pole of the testes that enables the seminal fluid to enter the structure, be transported up through the vas deferens and ultimately be ejaculated. It is therefore is an essential component in male fertility. As the epididymis is so contorted and compressed, it offers a large degree of storage for sperm.