The epididymis (/ɛpɪˈdɪdɪmɪs/; pl.: epididymides/ɛpɪdɪˈdɪmədiːz/or/ɛpɪˈdɪdəmɪdiːz/) is an elongated tubular structure attached to the posterior side of each one of the two male reproductive glands, the testicles. It is a single, narrow, tightly coiled tube in adult humans, 6 to 7 centimetres (2.4 to 2.8 in) in length; uncoiled the tube would be approximately 6 m (20 feet) long.[1] It connects the testicle to the vas deferens in the male reproductive system. The epididymis serves as an interconnection between the multiple efferent ducts at the rear of a testicle (proximally), and the vas deferens (distally). Its primary function is the storage, maturation and transport of sperm cells.
The human epididymis is situated posterior and somewhat lateral to the testis. The epididymis is invested completely by the tunica vaginalis (which is continuous with the tunica vaginalis covering the testis).[2]: 1296
The epididymis can be divided into three main regions:
The body (Latin: corpus). This has an intermediate epithelium and smooth muscle thickness.[3]
The tail (Latin: cauda). This has the thinnest epithelium of the three regions and the greatest quantity of smooth muscle.[3] The tail is distally continuous with (the convoluted portion of) the ductus deferens (s. vas deferens).[2]: 1296
Principal cells: columnar cells that, with the basal cells, form the majority of the epithelium. In the caput (head) region these cells have long stereocilia that are tuft-like extensions that project into the lumen.[4] The stereocilia are much shorter in the cauda (tail) segment.[4] 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 long cytoplasmic projections that have an actin filament backbone.[4] These filaments have been visualized at high resolution using fluorescent phalloidin that binds to actin filaments.[4]
The stereocilia in the epididymis are non-motile. These membrane extensions increase the surface area of the cell, allowing for greater absorption and secretion. It has been shown that epithelial sodium channel ENaC that allows the flow of Na+ ions into the cell is localized on stereocilia.[4]
Because sperm are initially non-motile as they leave the seminiferous tubules, large volumes of fluid are secreted to propel them to the epididymis. The core function of the stereocilia is to resorb 90% of this fluid as the spermatozoa start to become motile. This absorption creates a fluid current that moves the immobile sperm from the seminiferous tubules to the epididymis. Spermatozoa only reach full motility when inside a 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.
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. Epididymal transit takes 2 to 6 days in humans and 10–13 in rodents.[7] During their transit in the epididymis, sperm undergo maturation processes necessary for them to acquire motility and fertility.[8] Final maturation (capacitation) is completed in the female reproductive tract.
The epididymis secretes immobilin, a large glycoprotein that is responsible for the creating of the viscoelastic luminal environment that serves to mechanically immobilize spermatozoa until ejaculation. Immobilin is predominantly secreted into the proximal caput epididymis prior to the acquisition of the potential for sperm motility.[9]
During emission, sperm flow from the cauda epididymis (which functions as a storage reservoir) into the vas deferens where they are propelled by the peristaltic action of muscle layers in the wall of the vas deferens, and are mixed with the diluting fluids of the prostate, seminal vesicles, and other accessory glands prior to ejaculation (forming semen).
Contrary to popular belief, sperm are capable of causing a pregnancy even without ever travelling through the epididymis.[10][11] This has been proven in two cases in the United States in the 1980s where a couple of men's vasa deferentia were directly surgically attached to their efferent ducts and these men both subsequently impregnated their partners within the next couple of years.[10] This has also been proven in a similar case in Western Europe in the early 1990s.[11]
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.
Epididymectomy is also performed for sterilization on some maleanimals of livestock species so they can be used to detect estrus in females ready for artificial insemination.
The epididymis is present in all male reptiles, birds, and mammals.[1] The caput epididymis is fused to the testis in eutherian mammals, but not in marsupials.[13]
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.[14]
Epididymal hypertension – Condition that arises during male sexual arousal when seminal fluid is not ejaculatedPages displaying short descriptions of redirect targets
^ abcdBacha, William; Bacha, Linda (2012). Color Atlas of Veterinary Histology. Wiley-Blackwell. p. 226. ISBN978-0470958513.
^ abcdeSharma S, Hanukoglu I (2019). "Mapping the sites of localization of epithelial sodium channel (ENaC) and CFTR in segments of the mammalian epididymis". Journal of Molecular Histology. 50 (2): 141–154. doi:10.1007/s10735-019-09813-3. PMID30659401. S2CID58026884.
^Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 394–395. ISBN0-03-910284-X.
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