Adult human testicle with epididymis: A. Head of epididymis, B. Body of epididymis, C. Tail of epididymis, and D. Vas deferens
The epididymis (//; plural: epididymides // or //) is part of the male reproductive system and is present in all male amniotes. It is a single, narrow, tightly-coiled tube (in adult humans, six to seven meters in length) connecting the efferent ducts from the rear of each testicle to its vas deferens.
The epididymis can be divided into three main regions:
- The head (Latin: Caput). The head of the epididymis receives spermatozoa via the efferent ducts of the mediastinium of the testis. It is characterized histologically by a thin myoepithelium. The concentration of the sperm here is dilute.
- The body (Latin: Corpus)
- The tail (Latin: Cauda). This has a thicker myoepithelium than the head region, as it is involved in absorbing fluid to make the sperm more concentrated.
In reptiles, there is an additional canal between the testis and the head of the epididymis and which receives the various efferent ducts. This is, however, absent in all birds and mammals.
The epididymis is covered by a two layered pseudostratified epithelium. The epithelium is separated by a basement membrane from the connective tissue wall which has smooth muscle cells. The major cell types in the epithelium are:
- Main cells: columnar cells that, with the basal cells, form the majority of the epithelium. These cells extend from the lumen to the basal lamina, They also have non-motile stereocilia, which are long and branching in the head region and shorter in the tail region. They also secrete carnitine, sialic acid, glycoproteins, and glycerylphosphorylcholine into the lumen.
- Basal cells: shorter, pyramid-shaped cells which contact the basal lamina but taper off before their apical surfaces reach the lumen. These are thought to be undifferentiated precursors of principal cells.
- Apical cells: predominantly found in the head region
- Clear cells: predominant in the tail region
- Intraepithelial lymphocytes: distributed throughout the tissue.
The stereocilia of the epididymis are structures which aid in absorption. They are long cytoplasmic projections but have no motility.
Unlike the stereocilia of the ear, which plays a role in hearing, the stereocilia in the epididymis are more like the long microvilli of other epithlium that serve an absorptive function. These extensions increase the surface area of the cell allowing for greater absorption and secretion.
The stereocilia have no internal microtubule structure and unlike true cilia, are non-motile. The internal actin network increases the surface area just like microvilli. Because sperm are initially nonmotile as they leave the seminiferous tubules, large volumes of fluid are secreted to propulse the spermatozoa, along with the cilia of the pathway to the epididymus. The core function of the stereocilia is to resorb this large volume of fluid (90% of fluid volume), as the spermatozoa start to become motile here. This absorption creates a fluid flow that moves the immobile spermatozoa from the seminiferous tubule to the epididymis. They do not reach full motility (hypermotility) until they reach the vagina where the alkaline pH is neutralized by acidic vaginal fluids.
In the embryo, the epididymis develops from tissue that once formed the mesonephros, a primitive kidney found in many aquatic vertebrates. Persistence of the cranial end of the mesonephric duct will leave behind a remnant called the appendix of the epididymis. In addition, some mesonephric tubules can persist as the paradidymis, a small body caudal to the efferent ductules.
Role in storage of sperm and ejaculant
Spermatozoa formed in the testis enter the caput epididymis, progress to the corpus, and finally reach the cauda region, where they are stored. Sperm entering the caput epididymis are incomplete—they lack the ability to swim forward (motility) and to fertilize an egg. It stores the sperm for 2–3 months. During their transit in the epididymis, sperm undergo maturation processes necessary for them to acquire these functions. Final maturation is completed in the female reproductive tract (capacitation).
During ejaculation, sperm flow from the lower portion of the epididymis (which functions as a storage reservoir). They have not been activated by products from the prostate gland, and they are unable to swim, but are transported via the peristaltic action of muscle layers within the vas deferens, and are mixed with the diluting fluids of the seminal vesicles and other accessory glands prior to ejaculation (forming semen).
The epithelial cells of the epididymis possess numerous apical modifications that are often referred to as stereocilia, as under the light microscope they look like cilia. However, as electron microscopy has revealed them to be structurally and functionally more similar to microvilli, some now refer to them as stereovilli.
Epididymotomy is the placing of an incision into the epididymis and is sometimes considered as a treatment option for acute suppurating epididymitis. Epididymectomy is the surgical removal of the epididymis sometimes performed for post-vasectomy pain syndrome and for refractory cases of epididymitis.
This article uses anatomical terminology; for an overview, see anatomical terminology.
- Kim, Howard H.; Goldstein, Marc (2010). "Chapter 53: Anatomy of the epididymis, vas deferens, and seminal vesicle". In Graham, Sam D.; Keane, Thomas E.; Glenn, James F. Glenn's urological surgery (7th ed.). Philadelphia: Lippincott Williams & Wilkins. p. 356. ISBN 978-0-7817-9141-0.
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- How sperm are re-absorbed into the body. http://www.vasectomy-information.com/moreinfo/reabsorb.htm (accessed 2/16/06)
- Efferent Ducts and Epididymis. http://www3.umdnj.edu/histsweb/lab16/lab16efferentandepididmys.html (accessed 2/16/06).
- Jones R (1999). "To store or mature spermatozoa? The primary role of the epididymis". Int J Androl 22 (2): 57–67. doi:10.1046/j.1365-2605.1999.00151.x. PMID 10194636.
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|Look up epididymis in Wiktionary, the free dictionary.|
- BU Histology Learning System: 16903loa
- inguinalregion at The Anatomy Lesson by Wesley Norman (Georgetown University) (testes)