Each testis has its own epididymis, a comma-shaped structure you can usually feel as a soft ridge along the back and top of the testis. It has three parts: the head (caput), which receives sperm from the testis; the body (corpus); and the tail (cauda), which joins the vas deferens.
Sperm leaving the testis are not yet ready for the job. It is during their passage through the epididymis that they acquire forward motility and the ability to fertilise.1 Much of what we know about this comes from animal studies, and human research is more limited than people might expect.1
Human sperm pass through in a matter of days, and the reserve of stored sperm outside the testes is small.2 Most of the time it takes to make a sperm is spent in the testis during spermatogenesis, not in the epididymis.
What does the epididymis do?
It matures, transports and stores sperm. By the time sperm reach the tail, they are ready to be carried up the vas deferens at ejaculation.
In an autopsy study of healthy men, just over half of the sperm held in the epididymis (52%) were in the tail, with the rest split between the head and body.2 The human epididymis appears less developed as a storage reservoir than in the laboratory animals most research is based on.1
- Maturation: sperm gain the ability to swim forwards and fertilise1
- Transport: muscle in the wall moves sperm steadily along the tube
- Storage: the tail holds sperm ready for the next ejaculation2
- Environment: the lining shapes the fluid that surrounds sperm as they mature
How long do sperm spend in the epididymis?
Only a few days in humans. One classic estimate put transit through the head, body and tail at roughly 0.7, 0.7 and 1.8 days respectively.
Those figures come from Amann and Howards, who also concluded that maturation in the head and body takes less than two days, and that men carry relatively small reserves of sperm outside the testes.2
A later autopsy study found that transit was faster in men who produced more sperm each day, and that age made no difference.3 For the full timeline from new sperm cell to ejaculate, see how long it takes sperm to regenerate.
What problems can affect the epididymis?
The common ones are epididymitis (inflammation, usually from infection), cysts and blockages. Blockages matter most for fertility, because they stop sperm reaching the ejaculate.
Epididymitis causes pain, swelling and tenderness at the back of the testis, sometimes with fever or urinary symptoms. It needs prompt medical assessment, partly because it can be confused with testicular torsion, which is an emergency, and partly because untreated infection can scar the tube.
Epididymal cysts are smooth, fluid-filled lumps that are usually harmless, though any new lump deserves an examination. Scarring or blockage along the ducts can cause obstructive azoospermia: in one clinical series, 37% of men with azoospermia had ductal obstruction rather than a sperm production problem.4
When should you see a doctor?
See a doctor promptly for new pain, swelling or a lump in the scrotum. Sudden, severe testicular pain needs urgent emergency care, because torsion has to be ruled out quickly.
If you are having fertility tests, the doctor will feel the epididymis as part of the examination. A swollen or firm epididymis alongside a very low or absent sperm count can point towards a blockage, which may be repairable or may allow sperm to be retrieved directly for IVF.
Common questions
Can you feel your epididymis?
Usually, yes. It feels like a soft, slightly bumpy ridge along the back of each testis. Knowing what it normally feels like helps you tell it apart from a new lump during self-checks.
Does a vasectomy affect the epididymis?
Sperm are still made after a vasectomy but can no longer travel past the cut vas deferens, so the epididymis becomes the end of the line. Researchers use vasectomy and its reversal as a model for how blockage affects the human epididymis.1
Is epididymitis a sexually transmitted infection?
It can be caused by sexually transmitted infections such as chlamydia or gonorrhoea, but urinary infections and other causes are also possible. Either way, it needs testing and treatment, so see a GP or sexual health clinic.
Read more on this
How long does it take sperm to regenerate?
Lifestyle factors that affect male fertility: a practical guide
Sources
- Sullivan R, Mieusset R. The human epididymis: its function in sperm maturation. Hum Reprod Update. 2016;22(5):574-87. PubMed
- Amann RP, Howards SS. Daily spermatozoal production and epididymal spermatozoal reserves of the human male. J Urol. 1980;124(2):211-5. PubMed
- Johnson L, Varner DD. Effect of daily spermatozoan production but not age on transit time of spermatozoa through the human epididymis. Biol Reprod. 1988;39(4):812-7. PubMed
- Jarow JP, Espeland MA, Lipshultz LI. Evaluation of the azoospermic patient. J Urol. 1989;142(1):62-5. PubMed
Not medical advice. Written by Eric Stephens from the research cited above; not reviewed by a doctor. If something worries you, see a GP.