The epididymis (; : epididymides or ) is an elongated tubular genital organ attached to the posterior side of each one of the two male Gonad, the . It is a single, narrow, tightly coiled tube in adult humans, in length; uncoiled the tube would be approximately 6 m (20 feet) long.
Structure
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 mediastinium of the testis
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The body (). This has an intermediate epithelium and smooth muscle thickness.
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The tail (). This has the thinnest epithelium of the three regions and the greatest quantity of smooth muscle.
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:
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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.
-
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.
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Apical cells: predominantly found in the head region
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Clear cells: predominant in the tail region
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Intraepithelial lymphocytes: distributed throughout the tissue.
-
Intraepithelial macrophages
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 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 Sperm 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 ducts.
The epoophoron is a homologous remnant in the female.
Function
Role in storage of sperm and ejaculant
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 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.
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 the creating of the viscoelastic luminal environment that serves to mechanically immobilize spermatozoa until ejaculation. Immobilin is predominantly secreted into the proximal caput epididymidis prior to 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 peristalsis action of muscle layers in the wall of the vas deferens, and are mixed with the diluting fluids of the prostate, , 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.
Antioxidant defenses
During their transit through the epididymis, the
spermatozoon undergo a series of transformations in preparation for their ultimate task of fertilizing the
oocyte. In order to protect the spermatozoa during their transit through the epididymis, the epididymal
epithelium produces a variety of
antioxidant proteins that help protect the spermatozoa from
oxidative stress.
The antioxidant proteins produced include
catalase, glutathione peroxidases, glutathione-S-transferases,
, superoxide dismutases, thioredoxin reductase and
.
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 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 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.
The caput epididymidis is fused to the testis in
Eutheria mammals, but not in
Marsupial.
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.Romer, Alfred Sherwood (1977). 003910284X, Holt-Saunders International. 003910284X
==Gallery==
of epididymis - H&E stain]]
See also
-
Epididymis evolution from reptiles to mammals
-
List of distinct cell types in the adult human body
Notes
External links
