The epididymis is a coiled tube behind each testis. This reproductive organ is where the sperm are stored after being produced in the seminal vesicles. It's also where they undergo their final stage of maturation. The epididymis is linked directly to the ejaculatory duct by the vas deferens.
Sperm are initially stored in the head of the epididymis, which rests at the top of the testis. The body of the epididymis runs down the back of the testicle, and is estimated to be nearly 20 feet long if uncoiled. Sperm move through the epididymis over a period of approximately 10–15 days, during which time they mature and gain the ability to move on their own.
Finally, sperm reach the tail (cauda) of the epididymis. Here, sperm are stored before they leave the body; the cauda epididymis holds around 50–80% of the sperm present in the epididymis. They wait there until ejaculatory contractions move them into the vas deferens, which takes them to the pelvic cavity. Finally, sperm are transported to the ejaculatory duct, combined with semen, and ejected through the urethra during an orgasm.
If there is no ejaculation for several weeks — or if a vasectomy prevents sperm from leaving the epididymis — sperm is eventually reabsorbed by the epididymis.
When the testicles produce sperm, they’re still immature and not ready to fertilize an egg. The epididymis plays an essential role in the maturation process, and, therefore, in fertility.
First, the epididymis absorbs fluids, increasing sperm concentration, translating to a higher sperm concentration in the semen. As sperm move through the epididymis, they gain motility and fertilization ability. The epididymis is also important for protecting sperm; this organ releases antioxidants to counter reactive oxygen species that can harm sperm. It also guards sperm from any dangerous substances in the blood.
Overall, successful maturation (involving complete transit through the epididymis) and protection of sperm is critical for male fertility and reproduction. Research has found that immature sperm result in low fertilization rates as well as defects in the embryos that achieve fertilization.