The epididymis is a coiled tube of around 20 feet (6 m) in length surrounded by connective tissue and blood vessels. It lies inside the scrotum just behind the testis and is the site where sperm is stored for maturation. Anatomically, the epididymal structure consists of a large upper section, or head, a body and a tail below. The head receives newly formed sperm from the testis while the tail carries mature sperm toward the vas deferens, a tube which leads into the pelvis and, via the ejaculatory duct and urethra, to the penis. As part of its storage process, the epididymis absorbs fluid, adding nourishing substances which serve to nurture sperm.
Sperm is manufactured in the testis before passing into the epididymis. Inside the testis are coils of seminiferous tubules. Sperm are created in the walls of these tubules before draining into the head of the epididymis. When sperm leave the testis, they are immature and lack mobility. They acquire both maturity and mobility as they make their way through the epididymis.
While in the epididymis, sperm acquire the ability to move forward. This becomes possible due to the activation of a protein known as CatSper. CatSper is found on the tail of the sperm and functions as a channel, allowing calcium ions to flow in. This leads to contraction of fibrous proteins and the characteristic beating motion of the tail.
Smooth muscle cells surrounding the walls of the epididymal duct contract and help move sperm along. From the tail of the epididymis the vas deferens arises, the tube which carries mature sperm. The seminal vesicle, a structure that produces nourishing fluid for sperm, joins the vas deferens to form the ejaculatory duct. This opens into the urethra, the tube leading from the bladder, which passes through the prostate to enter the penis.
Epididymitis, or inflammation of the epididymis, usually occurs as a result of infection. It can be sudden or long-lasting, with symptoms including pain in the area of the scrotum, redness and swelling. Usually, bacterial infection has spread back from the vas deferens, sometimes in association with a urinary tract infection or sexually transmitted disease. If left untreated, an abscess could be created, damaging the epididymal tissue, or infection could spread to the testis. Generally, epididymitis can be treated with a combination of antibiotics and pain relief.
Sudden severe pain in the scrotum should always be assessed by a doctor. This is because similar symptoms can be caused by a condition called testicular torsion. Here, the spermatic cord twists round, cutting off the blood supply to the testis, and requiring emergency surgery.