Epididymis: Definition, Structure and Function

Diagram of the anatomy of the epididymis.

What Is the Epididymis?

The epididymis is a long, thin, tightly-coiled tube connecting each testicle to a vas deferens in the male reproductive system. It is used to transport sperm from the testes and for collecting and storing sperm.

The epididymis is found in all male mammals, birds and reptiles.

Anatomy and Function

The epididymis is identified as a curved formation located on the posterior, or rear, of each testicle. While sharing some visible similarities to the testes, the epididymis is smaller, and the tubes are bigger and not as tightly packed.

The epididymis is composed of three parts:

  • Head (caput epididymidis) – Sperm produced in the testis enter the head of the epididymis, located near the top of each testis, where sperm are stored until maturation. Sperm entering the head are not fully formed, and are not able to display motility (swim forward) or fertilize an egg.
  • Body (corpus epididymidis) – The body of the epididymis is a long, twisting tube where maturation of the sperm takes place. During this stage the sperm develop motility and fertility, a process taking anywhere from 2-6 days on average. However, sperm do not develop full motility until they reach the vagina, where the alkaline pH becomes neutralized by acidic vaginal fluids.
  • Tail (cauda epididymidis) – During ejaculation, sperm travel from the tail (which acts as a storage chamber) into the vas deferens. From here they are propelled by the contractions in the walls of the vas deferens, and are combined with semen (fluids from the prostate, seminal vesicles, and other accessory glands) just before ejaculation.

Additionally, a thin sheet of serous membrane tissue known as the tunica vaginalis surrounds and separates the epididymis from the testicle.

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