The epididymis (; plural: epididymides or ) is a tube that connects a

testicle A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testoste ...

to a

vas deferens The vas deferens or ductus deferens is part of the male reproductive system of many vertebrates. The ducts transport sperm from the epididymis to the ejaculatory ducts in anticipation of ejaculation. The vas deferens is a partially coiled tube ...

in the

male reproductive system The male reproductive system consists of a number of sex organs that play a role in the process of human reproduction. These organs are located on the outside of the body and within the pelvis. The main male sex organs are the penis and the te ...

. It is a single, narrow, tightly-coiled tube in adult humans, in length. It serves as an interconnection between the multiple

efferent ducts The efferent ducts (or efferent ductules or ductuli efferentes or ductus efferentes or vasa efferentia) connect the rete testis with the initial section of the epididymis.Hess 2018 There are two basic designs for efferent ductule structure: * ...

at the rear of a testicle (proximally), and the vas deferens (distally).

Anatomy

The epididymis is situated posterior and somewhat lateral to the testis. The epididymis is invested completely by the

tunica vaginalis The tunica vaginalis is the pouch of serous membrane that covers the testes. It is derived from the vaginal process of the peritoneum, which in the fetus precedes the descent of the testes from the abdomen into the scrotum. After its descent, ...

(which is continuous with the tunica vaginalis covering the testis). The epididymis can be divided into three main regions: * The head ( la, caput). The head of the epididymis receives

spermatozoa A spermatozoon (; also spelled spermatozoön; ; ) is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote. (A zygote is a single cell, with a complete set of chromos ...

via the

of the mediastinium of the

testis A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testostero ...

at the superior pole of the testis. The head is characterized

histological Histology, also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures vis ...

ly by a thick epithelium with long

stereocilia Stereocilia (or stereovilli or villi) are non-motile apical cell modifications. They are distinct from cilia and microvilli, but are closely related to microvilli. They form single "finger-like" projections that may be branched, with normal cell ...

(described below) and a little smooth muscle. It is involved in absorbing fluid to make the sperm more concentrated. The concentration of the sperm here is dilute. * The body ( la, corpus, links=no). This has an intermediate epithelium and smooth muscle thickness. * The tail ( la, cauda, links=no). 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 A pseudostratified epithelium is a type of epithelium that, though comprising only a single layer of cells, has its cell nuclei positioned in a manner suggestive of stratified epithelia. As it rarely occurs as squamous or cuboidal epithelia ...

. 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. They also secrete carnitine, sialic acid,

glycoproteins Glycoproteins are proteins which contain oligosaccharide chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosy ...

, 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. * 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 The epithelial sodium channel (ENaC), (also known as amiloride-sensitive sodium channel) is a membrane-bound ion channel that is selectively permeable to sodium ions (). It is assembled as a heterotrimer composed of three homologous subunits α ...

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 Seminiferous tubules are located within the testes, and are the specific location of meiosis, and the subsequent creation of male gametes, namely spermatozoa. Structure The epithelium of the tubule consists of a type of sustentacular cells known ...

, 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 do not reach full motility until they reach the

vagina In mammals, the vagina is the elastic, muscular part of the female genital tract. In humans, it extends from the vestibule to the cervix. The outer vaginal opening is normally partly covered by a thin layer of mucosal tissue called the hymen ...

, where the alkaline pH is neutralized by acidic vaginal fluids.

Development

In the embryo, the epididymis develops from tissue that once formed the

mesonephros The mesonephros ( el, middle kidney) is one of three excretory organs that develop in vertebrates. It serves as the main excretory organ of aquatic vertebrates and as a temporary kidney in reptiles, birds, and mammals. The mesonephros is included ...

, a primitive

kidney The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blo ...

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 The term paradidymis (organ of Giraldés) is applied to a small collection of convoluted tubules, situated in front of the lower part of the spermatic cord, above the head of the epididymis. These tubes are lined with columnar ciliated epitheli ...

'', a small body caudal to the efferent ductules. A

Gartner's duct Gartner's duct, also known as Gartner's canal or the ductus longitudinalis epoophori, is a potential embryological remnant in human female development of the mesonephric duct in the development of the urinary and reproductive organs. It was disco ...

is a homologous remnant in the female.

Function

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. 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 female reproductive system is made up of the internal and external sex organs that function in the reproduction of new offspring. In humans, the female reproductive system is immature at birth and develops to maturity at puberty to be abl ...

. 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 Ejaculation is the discharge of semen (the ''ejaculate''; normally containing sperm) from the male reproductory tract as a result of an orgasm. It is the final stage and natural objective of male sexual stimulation, and an essential compone ...

. Immobilin is predominantly secreted into the proximal caput epididymis 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 peristaltic action of muscle layers in the wall of the

, and are mixed with the diluting fluids of the

prostate The prostate is both an accessory gland of the male reproductive system and a muscle-driven mechanical switch between urination and ejaculation. It is found only in some mammals. It differs between species anatomically, chemically, and phys ...

seminal vesicle The seminal vesicles (also called vesicular glands, or seminal glands) are a pair of two convoluted tubular glands that lie behind the urinary bladder of some male mammals. They secrete fluid that partly composes the semen. The vesicles are 5� ...

s, and other accessory glands prior to ejaculation (forming

semen Semen, also known as seminal fluid, is an organic bodily fluid created to contain spermatozoa. It is secreted by the gonads (sexual glands) and other sexual organs of male or hermaphroditic animals and can fertilize the female ovum. Sem ...

). 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

and these men both subsequently impregnated their partners within the next couple of years. This has also been proven in a similar case in Western Europe in the early 1990s.

Antioxidant defenses

During their transit through the epididymis, the

undergo a series of transformations in preparation for their ultimate task of fertilizing the

oocyte An oocyte (, ), oöcyte, or ovocyte is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The femal ...

. 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 damage Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species and a biological system's ability to readily detoxify the reactive intermediates or to repair the resulting damage. Disturbances in the normal r ...

O'Flaherty C. Orchestrating the antioxidant defenses in the epididymis. Andrology. 2019 Sep;7(5):662-668. doi: 10.1111/andr.12630. Epub 2019 May 1. PMID: 31044545. The antioxidant proteins produced include catalase,

glutathione peroxidase Glutathione peroxidase (GPx) () is the general name of an enzyme family with peroxidase activity whose main biological role is to protect the organism from oxidative damage. The biochemical function of glutathione peroxidase is to reduce lipid h ...

glutathione-S-transferase Glutathione ''S''-transferases (GSTs), previously known as ligandins, are a family of eukaryotic and prokaryotic phase II metabolic isozymes best known for their ability to catalyze the conjugation of the reduced form of glutathione (GSH) to x ...

peroxiredoxin Peroxiredoxins (Prxs, ; HGNC root symbol ''PRDX'') are a ubiquitous family of antioxidant enzymes that also control cytokine-induced peroxide levels and thereby mediate signal transduction in mammalian cells. The family members in humans are P ...

s, superoxide dismutases,

thioredoxin reductase Thioredoxin reductases (TR, TrxR) () are enzymes that reduce thioredoxin (Trx). Two classes of thioredoxin reductase have been identified: one class in bacteria and some eukaryotes and one in animals. In bacteria TrxR also catalyzes the reduction ...

and

thioredoxin Thioredoxin is a class of small redox proteins known to be present in all organisms. It plays a role in many important biological processes, including redox signaling. In humans, thioredoxins are encoded by ''TXN'' and '' TXN2'' genes. Loss-of-fu ...

s. 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 Epididymitis is a medical condition characterized by inflammation of the epididymis, a curved structure at the back of the testicle. Onset of pain is typically over a day or two. The pain may improve with raising the testicle. Other symptoms may i ...

. It is much more common than testicular inflammation, termed

orchitis Orchitis is inflammation of the testes. It can also involve swelling, pains and frequent infection, particularly of the epididymis, as in epididymitis. The term is from the Ancient Greek ὄρχις meaning "testicle"; same root as ''orchid''. S ...

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 Post-vasectomy pain syndrome (PVPS) is a chronic and sometimes debilitating genital pain condition that may develop immediately or several years after vasectomy. Because this condition is a syndrome, there is no single treatment method, therefore e ...

and for refractory cases of epididymitis. Epididymectomy is also performed for sterilization on some

male Male (symbol: ♂) is the sex of an organism that produces the gamete (sex cell) known as sperm, which fuses with the larger female gamete, or ovum, in the process of fertilization. A male organism cannot reproduce sexually without access to ...

animals Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...

of livestock species so they can be used to detect estrus in females ready for artificially insemination.

Non-human animals

The epididymis is present in all male reptiles, birds, and mammals. 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.

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 A micrograph or photomicrograph is a photograph or digital image taken through a microscope or similar device to show a magnified image of an object. This is opposed to a macrograph or photomacrograph, an image which is also taken on a m ...

of epididymis -

H&E stain Hematoxylin and eosin stain ( or haematoxylin and eosin stain or hematoxylin-eosin stain; often abbreviated as H&E stain or HE stain) is one of the principal tissue stains used in histology. It is the most widely used stain in medical diagnos ...

Image:Epididymis.JPG, Micrograph File:Slide4UMR.JPG, Deep dissection of epididymis

Notes

External links

* {{Authority control Scrotum