epididyme, either of a pair of elongated crescent-shaped structures attached to each of the two male reproductive organs, the testes (see testis). Sperm cells produced in the testes are transported to the epididymes, where they mature and are stored. Each epididymis has three regions, called, respectively, the head, body, and tail. The head is the uppermost and largest part of the epididymis; it lies on the top surface of the testis. The body is attached to the anal side of the testis and extends the length of the gland. The smallest region is the tail, which begins at the point of separation of the epididymis from the testis. Sperm cells mature primarily in the head and body of the epididymis and are stored in the tail.
The epididymis receives sperm from the tubules in the mediastinum testis, the region in the testis in which all its sperm-producing tubules converge and empty. Leading from the mediastinum to the head of the epididymis are 15–20 small, tightly coiled ducts called the ductuli efferentes. The cells lining the ductuli have pigment granules, secretory granules, and cilia (hairlike structures). In the head region of the epididymis, all the ductuli efferentes connect to one large vessel, the ductus epididymidis. This duct is also extremely coiled, being about 4 to 5 m (13 to 16 feet) long when stretched out. The ductus epididymidis extends through both the body and the tail region of the epididymis. In the tail region it becomes thicker, less coiled, and larger in diameter. As it emerges from the end of the epididymis, it straightens out to form the ductus deferens.
During ejaculation, sperm are propelled through the ductuli efferentes and ductus epididymidis in two ways. First, the muscle tissue, by contracting, narrows the ducts, propelling the sperm. Second, the cilia located in the ductuli efferentes can propel sperm by their continual swaying motions. As sperm pass through the various ducts, they acquire small amounts of fluids that help to keep them alive. These secretions include high concentrations of potassium, sodium, and a substance known as glycerylphosphorylcholine, which is an energy source for sperm.