View source for Epididymis

{{Short description|Tube that connects a testicle to a vas deferens}}
{{Infobox anatomy
| Name        = Epididymis
| Latin       = epididymis
| Greek       = ἐπιδιδυμίς 
| Image       = Epididymis-KDS.jpg
| Caption     = Adult human testicle with epididymis highlighted: A. Head of epididymis, B. Body of epididymis, C. Tail of epididymis, and D. Vas deferens
| Width       = 194
| Image2      = Gray1148.png
| Caption2    = The right [[testicle]], exposed by laying open the [[tunica vaginalis]].
| Precursor   = [[Mesonephric duct]]s
| System      = 
| Artery      = 
| Vein        = [[Pampiniform plexus]]
| Nerve       = 
| Lymph       = 
}}
The '''epididymis''' ({{IPAc-en|ɛ|p|ᵻ|ˈ|d|ɪ|d|ᵻ|m|ɪ|s}}; {{plural form}}: '''epididymides''' {{IPAc-en|ɛ|p|ᵻ|d|ᵻ|ˈ|d|ɪ|m|ə|d|iː|z}} or {{IPAc-en|ɛ|p|ᵻ|ˈ|d|ɪ|d|ə|m|ɪ|d|iː|z}}) is an elongated tubular [[genital organ]] attached to the posterior side of each one of the two male [[Gonad|reproductive glands]], the [[testicle]]s. It is a single, narrow, tightly coiled tube in adult humans, {{convert|6|to|7|cm|in}} in length; uncoiled the tube would be approximately 6 m (20 feet) long.<ref name=":1">{{Cite book
|title=Glenn's urological surgery
|last1=Kim
|first1=Howard H.
|last2=Goldstein
|first2=Marc
|editor1-last=Graham
|editor1-first=Sam D.
|editor2-last=Keane
|editor2-first=Thomas E.
|editor3-last=Glenn
|editor3-first=James F.
|edition=7th
|year=2010
|page=356
|chapter=Chapter 53: Anatomy of the epididymis, vas deferens, and seminal vesicle
|publisher=Lippincott Williams & Wilkins
|place=Philadelphia
|chapter-url=https://books.google.com/books?id=GahMzaKgMKAC&pg=PA356
|isbn=978-0-7817-9141-0
}}</ref> 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|sperm cells]].

==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).<ref name=":2">{{Cite book |title=Gray's anatomy : the anatomical basis of clinical practice |date=2021 |others=Susan Standring |isbn=978-0-7020-7707-4 |edition=Forty-second |location=[New York] |oclc=1201341621}}</ref>{{Rp|page=1296}}

The epididymis can be divided into three main regions:
* The head ({{langx|la|caput}}). The head of the epididymis receives [[spermatozoa]] via the [[efferent ducts]] of the [[mediastinum testis|mediastinum]] of the [[testis]]<ref name=":0" /> at the superior pole of the testis.<ref name=":2" />{{Rp|page=1296}} The head is characterized [[histological]]ly by a thick [[epithelium]] with long [[stereocilia]] (described below) and a little [[smooth muscle]].<ref name=":0">{{Cite book|title=Color Atlas of Veterinary Histology|last1=Bacha|first1=William|last2=Bacha|first2=Linda|publisher=Wiley-Blackwell|year=2012|isbn=978-0470958513|pages=226}}</ref> It is involved in absorbing fluid to make the [[sperm]] more concentrated. The concentration of the sperm here is dilute.
* The body ({{langx|la|corpus|links=no}}). This has an intermediate epithelium and smooth muscle thickness.<ref name=":0" />
* The tail ({{langx|la|cauda|links=no}}). This has the thinnest epithelium of the three regions and the greatest quantity of smooth muscle.<ref name=":0" /> The tail is distally continuous with (the convoluted portion of) the [[Ductus Deferens|ductus deferens (s. vas deferens)]].<ref name=":2" />{{Rp|page=1296}}

== 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 (anatomy)|lumen]].<ref name="pmid30659401">{{cite journal| author=Sharma S, Hanukoglu I| title=Mapping the sites of localization of epithelial sodium channel (ENaC) and CFTR in segments of the mammalian epididymis. | journal=Journal of Molecular Histology | year= 2019 | volume= 50 | issue= 2 | pages= 141–154 | pmid=30659401 | doi=10.1007/s10735-019-09813-3 | s2cid=58026884 }}</ref> The stereocilia are much shorter in the cauda (tail) segment.<ref name="pmid30659401" /> 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.
* '''Intraepithelial macrophages'''<ref>{{cite journal |vauthors=Da Silva N, Cortez-Retamozo V, Reinecker HC |title=A dense network of dendritic cells populates the murine epididymis |journal=Reproduction |volume=141 |issue=5 |pages=653–63 |date=May 2011 |pmid=21310816 |pmc=3657760 |doi=10.1530/REP-10-0493|display-authors=etal}}</ref><ref>{{cite journal |vauthors=Shum WW, Smith TB, Cortez-Retamozo V |title=Epithelial basal cells are distinct from dendritic cells and macrophages in the mouse epididymis |journal=Biology of Reproduction |volume=90 |issue=5 |pages=90 |date=May 2014 |pmid=24648397 |doi=10.1095/biolreprod.113.116681|display-authors=etal |pmc=4076373}}</ref>

=== Stereocilia ===
The stereocilia of the epididymis are long cytoplasmic projections that have an actin filament backbone.<ref name="pmid30659401" /> These filaments have been visualized at high resolution using fluorescent phalloidin that binds to actin filaments.<ref name="pmid30659401" />
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<sup>+</sup> ions into the cell is localized on stereocilia.<ref name="pmid30659401" />

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|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 ducts|efferent ductules]].

The [[epoophoron]] is a [[homology (biology)|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.<ref>{{Cite journal|last=Cornwall|first=Gail A.|date=2009|title=New insights into epididymal biology and function|journal=Human Reproduction Update|volume=15|issue=2|pages=213–227|doi=10.1093/humupd/dmn055|issn=1355-4786|pmc=2639084|pmid=19136456}}</ref> During their transit in the epididymis, sperm undergo maturation processes necessary for them to acquire motility and fertility.<ref>{{cite journal |author=Jones RC |title=To store or mature spermatozoa? The primary role of the epididymis |journal=International Journal of Andrology |volume=22 |issue=2 |pages=57–67 |date=April 1999 |pmid=10194636 |doi=10.1046/j.1365-2605.1999.00151.x|doi-access=free }}</ref> 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.<ref>{{cite journal|title=Characteristics of the Epididymal Luminal Environment Responsible for Sperm Maturation and Storage|journal=Front Endocrinol (Lausanne)|year=2018|doi=10.3389/fendo.2018.00059|pmid=29541061|pmc=5835514|last1=Zhou|first1=Wei|last2=De Iuliis|first2=Geoffry N.|last3=Dun|first3=Matthew D.|last4=Nixon|first4=Brett|volume=9|page=59| doi-access=free }}</ref>

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|peristaltic]] action of [[muscle]] layers in the wall of the [[vas deferens]], and are mixed with the diluting fluids of the [[prostate]], [[seminal vesicle]]s,  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.<ref name = "silber">{{cite journal | doi=10.1016/S0015-0282(16)59733-7 | title=Pregnancy caused by sperm from vasa efferentia | year=1988 | last1=Silber | first1=Sherman J. | journal=Fertility and Sterility | volume=49 | issue=2 | pages=373–375 | pmid=3338593 | doi-access=free }}</ref><ref name = "wolf">{{cite journal | pmid=8062964 | year=1994 | last1=Weiske | first1=W. H. | title=Pregnancy caused by sperm from vasa efferentia | journal=Fertility and Sterility | volume=62 | issue=3 | pages=642–643 | doi=10.1016/S0015-0282(16)56959-3 | doi-access=free }}</ref> 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.<ref name = "silber"/> This has also been proven in a similar case in Western Europe in the early 1990s.<ref name = "wolf"/>

===Antioxidant defenses===

During their transit through the epididymis, the [[spermatozoon|spermatozoa]] 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|oxidative damage]].<ref name = "O’Flaherty2019">{{cite journal|pmid=31044545 |date=2019 |last1=O'Flaherty |first1=C. |title=Orchestrating the antioxidant defenses in the epididymis |journal=Andrology |volume=7 |issue=5 |pages=662–668 |doi=10.1111/andr.12630 |s2cid=143424672 |doi-access=free }}</ref> The antioxidant proteins produced include [[catalase]], [[glutathione peroxidase]]s, [[glutathione-S-transferase]]s, [[peroxiredoxin]]s, [[superoxide dismutase]]s, [[thioredoxin reductase]] and [[thioredoxin]]s.<ref name = "O’Flaherty2019"/> 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 [[DNA damage (naturally occurring)|oxidative damage to the sperm DNA]].<ref name = "O’Flaherty2019"/>

==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 (medicine)|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]].<ref name=":1" /> The caput epididymidis is fused to the testis in [[Eutheria|eutherian]] mammals, but not in [[Marsupial|marsupials]].<ref>{{Cite book |last1=Vogelnest |first1=Larry |url=https://books.google.com/books?id=396VDwAAQBAJ&pg=PT143 |title=Current Therapy in Medicine of Australian Mammals |last2=Portas |first2=Timothy |date=2019-05-01 |publisher=Csiro Publishing |isbn=978-1-4863-0753-1 |language=en}}</ref>

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.<ref name="VB">{{cite book |author=Romer, Alfred Sherwood |title=The Vertebrate Body |author2=Parsons, Thomas S. |publisher=Holt-Saunders International |year=1977 |isbn=0-03-910284-X |location=Philadelphia, PA |pages=394–395}}</ref>

==Gallery==
<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
</gallery>[https://commons.m.wikimedia.org/wiki/File:Epididymal_head,_with_measurement,_longitudinal_view.png]

==See also==
{{Anatomy-terms}}
* [[Epididymis evolution from reptiles to mammals]]
* {{Annotated link|Blue balls|Epididymal hypertension}}
* [[List of distinct cell types in the adult human body]]

==Notes==
{{Reflist}}

==External links==
{{wiktionary}}
* {{BUHistology|16903loa}}

{{Male reproductive system}}
{{Authority control}}

[[Category:Human male reproductive system]]
[[Category:Mammal male reproductive system]]
[[Category:Sex organs]]
[[Category:Scrotum]]