The Anatomy of the Epididymis

The epididymis is a long, coiled tube that transports sperm from the testes to the vas deferens. The epididymis consists of three parts, the caput (head), corpus (body), and cauda (tail.) During their journey through the various parts of the epididymis, sperm mature and gain the ability to swim. Because of its role in sperm transport and maturation, functional and structural issues with the epididymis can be associated with infertility or reduced fertility.

Anatomy

The epididymis is a coiled tube in three sections. It sits inside the scrotum, attached to the testes. The sperm move from the testes, through the epididymis, and into the vas deferens prior to being mixed with various secretions at the time of ejaculation. During their time moving through the epididymis, sperm mature and gain additional function. In particular, they gain the ability to propel themselves forward. Sperm must pass through at least the middle of the corpus to mature enough to fertilize an egg on their own. However, intracytoplasmic sperm injection (ICSI) can be used to fertilize an egg with less mature sperm.

The structure of the epididymis changes along the path from the testes to the vas deferens. The walls of the tube are thickest at the head and thickness decreases along the length of the tube to the tail. As the walls get thinner, more and more water is reabsorbed into the body, and sperm concentration increases. If uncoiled, the epididymis could be as long as 20 feet, and sperm take two to four days to travel from one end of the tube to the other. Although this may seem like a long time, it is relatively short when compared to how long sperm take to move through the epididymis of other species.

The epididymis is divided into three segments.

• The caput, or head, is where sperm enter the epididymis from the testes. At this stage, sperm are not motile. They are also very dilute. Concentration of sperm starts in the caput.
• The corpus, or body, is where sperm begin to gain motility.
• The cauda, or tail, is where sperm are stored. Sperm can be stored in the cauda for up to a few days. The cauda epididymis can store enough sperm for two to three normal semen samples.This is substantially less sperm than is stored in the cauda of many other species.

Anatomical Variations

There are a number of congenital conditions that can lead to abnormalities of the epididymis. One such condition is undescended testes, or cryptorchidism, a condition where the testes are trapped in the abdomen and do not descend into the scrotum. A significant number individuals with cryptorchidism also have abnormalities of the epididymis. Changes in the epididymis are also associated with conditions that affect kidney development, as parts of the epididymis are derived from the same tissue as the kidneys.

Regardless of the cause, in general, there are four types of congenital anomalies seen in the epididymis.

1. Problems with how the epididymis attaches to the testes. These include issues where the epididymis isn't attached to the testes as well as issues where only some sections of the epididymis attached to the testes and not others.
2. Epididymal cysts, which can also develop later in life. In many individuals, these cysts are asymptomatic and do not cause problems. In others, cysts can obstruct the flow of sperm and potentially cause issues with fertility.
3. Epidydimal agenesis occurs when part of the epididymis doesn't develop properly. The missing part of the epididymis is usually the corpus or the cauda, since the caput originates from a different type of tissue. Some people will only experience agenesis on one side of their body, and not the other.
4. Epididymal duplication occurs when there is a secondary epididymis branching off of the main epididymis. Many individuals with such duplication do not have any symptoms.

Function

The primary functions of the epididymis are sperm transport and sperm maturation. The epididymis serves this function across many mammalian species. As sperm travel through the epididymis they are exposed to a number of signals from the cells of the epididymis that drive their maturation. Hundreds, or even thousands, of different genes have been identified as being expressed in the different sections of the epididymis, and many have been shown to be involved in sperm maturation in humans and other species.

Associated Conditions

There are several acquired health conditions that can affect the epididymis. Epididymitis, or inflammation of the epididymis, is probably the most common. In sexually active, adult men, this is often the result of a sexually transmitted infection, such as chlamydia or gonorrhea. In children and teenagers, epididymitis is more likely to be caused by urinary tract infections and/or undiagnosed anatomical abnormalities. Elderly men may also experience epididymitis related to urinary tract infections or due to conditions that cause chronic inflammation.

Certain types of infertility may also be related to the health of the epididymis. In particular, obstructive infertility occurs when there is a blockage that prevents the passage of sperm. This blockage may be partial, in which case some sperm may get through, or complete. Not all obstructive infertility is the result of blockages in the epididymis, but many cases are. Obstructions in the epididymis can be the result of scarring from infection, congenital abnormalities, or injuries.

Other problems with the epididymis—such as attachment issues and agenesis—can also affect fertility. Sometimes, but not always, these fertility concerns can be addressed with surgery to restore a pathway along which sperm can mature. At other times, sperm can be harvested from before the blockage and used for assisted reproduction.

It is also important to note that vasectomy can affect the health of the epididymis. Vasectomy is a surgical sterilization process that disrupts and blocks the vas deferens so that sperm can not get into the ejaculate. In some, but not all, individuals, the process may be reversible. Whether or not the procedure is reversible appears to be, at least in part, because of the ways in which vasectomy affects the health of the epididymis. A buildup of sperm in the epididymis after vasectomy is thought to potentially lead to damage, and there may be damage from inflammation and other immune responses, such as granuloma formation, as well.

Tests

In general, tests of the epididymis are not performed unless an individual experiences pain in the scrotum or symptoms of infertility. If needed, ultrasound can be used to examine the epididymis and to detect any abnormalities. It can also sometimes be used to distinguish be epididymitis and other health concerns such as testicular torsion.

Biopsy of the epididymis may be used to identify abnormal growths. Needle aspiration can also be used to collect sperm for diagnostic procedures or assisted reproduction. Magnetic resonance imaging (MRI) can be used to image the epididymis and testes to identify congenital and other abnormalities. Semen analysis can also be used to identify issues with sperm quality, which may or may not be linked to the function of the epididymis.