Epididymis

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). The epididymis can be divided into three main regions: • The head (). The head of the epididymis receives spermatozoa via the efferent ducts of the mediastinum of the testis It is involved in absorbing fluid to make the sperm more concentrated. The concentration of the sperm here is dilute. • The body (). This has an intermediate epithelium and smooth muscle thickness. • The tail (). This has the thinnest epithelium of the three regions and the greatest quantity of smooth muscle. The tail is distally continuous with (the convoluted portion of) the ductus deferens (s. vas deferens). == Histology ==

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: • 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. The stereocilia are much shorter in the cauda (tail) segment. Stereocilia The stereocilia of the epididymis are long cytoplasmic projections that have an actin filament backbone. These filaments have been visualized at high resolution using fluorescent phalloidin that binds to actin filaments. 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 the epithelial sodium channel ENaC that allows the flow of Na+ ions into the cell is localized on stereocilia. 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. == Development ==

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. The epoophoron is a homologous remnant in the female. ==Function==

Role in storage of sperm and ejaculate Spermatozoa formed in the testis enter the caput epididymidis, progress to the corpus, and finally reach the cauda region, where they are stored. Sperm entering the caput epididymidis are incomplete—they cannot swim forward (motility) and to fertilize an egg. Epididymal transit takes 2 to 6 days in humans and 10 to 13 days in rodents. During their transit in the epididymis, sperm undergo maturation processes necessary for them to acquire motility and fertility. Final maturation (capacitation) is completed in the female reproductive tract. The epididymis secretes immobilin, a large glycoprotein that is responsible for creating the viscoelastic luminal environment that serves to mechanically immobilize spermatozoa until ejaculation. Immobilin is predominantly secreted into the proximal caput epididymidis before the acquisition of the potential for sperm motility. 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 before ejaculation (forming semen). Contrary to popular belief, sperm are capable of causing a pregnancy even without ever travelling through the epididymis. 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. The antioxidant proteins produced include catalase, glutathione peroxidases, glutathione-S-transferases, peroxiredoxins, superoxide dismutases, thioredoxin reductase and thioredoxins. Deficiencies in the availability of these antioxidant proteins reduces sperm quality by affecting a variety of the proteins necessary for the motility needed to fertilize oocytes. Reduced antioxidant activity also causes increased oxidative damage to the sperm DNA. ==Clinical significance==

Inflammation An inflammation of the epididymis is called epididymitis. It is much more common than testicular inflammation, termed orchitis. Surgical removal Epididymotomy is the placement of an incision into the epididymis and is sometimes considered 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 male animals of livestock species so they can be used to detect estrus in females ready for artificial insemination. == Other animals ==

The epididymis is present in male reptiles, birds, mammals, and cartilaginous fish. In reptiles, there is an additional canal between the testis and the head of the epididymis, which receives the various efferent ducts. This is, however, absent in all birds and mammals. ==Gallery==

Image:Illu repdt male.jpg|Human male reproductive system Image:Illu testis 1b.jpg|Testis Image:Mesorchium.svg|Schematic drawing: cross-section through a testicle Image:Epididymis high mag.jpg|Micrograph of epididymis - H&E stain Image:Epididymis.JPG|Micrograph File:Slide4UMR.JPG|Deep dissection of epididymis ==See also==