Epididymis
Encyclopedia
The epididymis is part of the male
Male
Male refers to the biological sex of an organism, or part of an organism, which produces small mobile gametes, called spermatozoa. Each spermatozoon can fuse with a larger female gamete or ovum, in the process of fertilization...

 reproductive system
Reproductive system
The reproductive system or genital system is a system of organs within an organism which work together for the purpose of reproduction. Many non-living substances such as fluids, hormones, and pheromones are also important accessories to the reproductive system. Unlike most organ systems, the sexes...

 and is present in all male amniote
Amniote
The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...

s. It is a narrow, tightly-coiled tube connecting the efferent ducts
Efferent ducts
The efferent ducts connect the rete testis with the initial section of the epididymis.There are two basic designs for efferent ductule structure:...

 from the rear of each testicle
Testicle
The testicle is the male gonad in animals. Like the ovaries to which they are homologous, testes are components of both the reproductive system and the endocrine system...

 to its vas deferens
Vas deferens
The vas deferens , also called ductus deferens, , is part of the male anatomy of many vertebrates; they transport sperm from the epididymis in anticipation of ejaculation....

. A similar, but probably non-homologous
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...

, structure is found in cartilaginous fishes.

Regions

The epididymis can be divided into three main regions
  • The head (Caput). The head of the epididymis receives spermatozoa via the efferent ducts
    Efferent ducts
    The efferent ducts connect the rete testis with the initial section of the epididymis.There are two basic designs for efferent ductule structure:...

     of the mediastinum
    Mediastinum
    The mediastinum is a non-delineated group of structures in the thorax, surrounded by loose connective tissue. It is the central compartment of the thoracic cavity...

     of the testis. It is characterized histologically by a thin myoepithelium
    Myoepithelium
    Myoepithelial cells are cells usually found in glandular epithelium as a thin layer above the basement membrane but generally beneath the lumenal cells. These may be positive for alpha smooth muscle actin and can contract and expel the secretions of exocrine glands...

    . The concentration of the sperm here is dilute.
  • The body (Corpus)
  • The tail (Cauda). This has a thicker myoepithelium than the head region, as it is involved in absorbing fluid to make the sperm more concentrated.


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.

Histology

The epididymis is covered by a two layered pseudostratified epithelium
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...

. The epithelium
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...

 is separated by a basement membrane
Basement membrane
The basement membrane is a thin sheet of fibers that underlies the epithelium, which lines the cavities and surfaces of organs including skin, or the endothelium, which lines the interior surface of blood vessels.- Composition :...

 from the connective tissue
Connective tissue
"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...

 wall which has smooth muscle
Smooth muscle
Smooth muscle is an involuntary non-striated muscle. It is divided into two sub-groups; the single-unit and multiunit smooth muscle. Within single-unit smooth muscle tissues, the autonomic nervous system innervates a single cell within a sheet or bundle and the action potential is propagated by...

 cells. The major cell types in the epithelium are:
  • Principal cells: columnar cells that, with the basal cells, form the majority of the epithelium. These cells cells extend from the lumen to the basal lamina, They also have non-motile stereocilia
    Stereocilia
    In the inner ear, stereocilia are the mechanosensing organelles of hair cells, which respond to fluid motion in numerous types of animals for various functions, including hearing and balance. They are about 10–50 micrometers in length and share some similar features of microvilli...

    , which are long and branching in the head region and shorter in the tail region. They also secrete carnitine
    Carnitine
    Carnitine is a quaternary ammonium compound biosynthesized from the amino acids lysine and methionine. In living cells, it is required for the transport of fatty acids from the cytosol into the mitochondria during the breakdown of lipids for the generation of metabolic energy. It is widely...

    , sialic acid
    Sialic acid
    Sialic acid is a generic term for the N- or O-substituted derivatives of neuraminic acid, a monosaccharide with a nine-carbon backbone. It is also the name for the most common member of this group, N-acetylneuraminic 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.

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
Motility
Motility is a biological term which refers to the ability to move spontaneously and actively, consuming energy in the process. Most animals are motile but the term applies to single-celled and simple multicellular organisms, as well as to some mechanisms of fluid flow in multicellular organs, in...

) and to fertilize an egg. During their transit in the epididymis, sperm undergo maturation processes necessary for them to acquire these functions. Final maturation is completed in the female
Female
Female is the sex of an organism, or a part of an organism, which produces non-mobile ova .- Defining characteristics :The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon, is produced by the male...

 reproductive tract (capacitation
Capacitation
Capacitation is the penultimate step in the maturation of mammalian spermatozoa and is required to render them competent to fertilize an oocyte.This step is a biochemical event; the sperm move normally and look mature prior to capacitation....

).

During ejaculation
Ejaculation
Ejaculation is the ejecting of semen from the male reproductory tract, and is usually accompanied by orgasm. It is usually the final stage and natural objective of male sexual stimulation, and an essential component of natural conception. In rare cases ejaculation occurs because of prostatic disease...

, sperm flow from the lower portion of the epididymis (which functions as a storage reservoir). They have not been activated by products from the prostate gland, and they are unable to swim, but are transported via the peristaltic
Peristalsis
Peristalsis is a radially symmetrical contraction and relaxation of muscles which propagates in a wave down the muscular tube, in an anterograde fashion. In humans, peristalsis is found in the contraction of smooth muscles to propel contents through the digestive tract. Earthworms use a similar...

 action of muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...

 layers within the vas deferens
Vas deferens
The vas deferens , also called ductus deferens, , is part of the male anatomy of many vertebrates; they transport sperm from the epididymis in anticipation of ejaculation....

, and are mixed with the diluting fluids of the seminal vesicle
Seminal vesicle
The seminal vesicles or vesicular glands are a pair of simple tubular glands posteroinferior to the urinary bladder of male mammals...

s and other accessory glands prior to ejaculation (forming semen
Semen
Semen is an organic fluid, also known as seminal fluid, that may contain spermatozoa. It is secreted by the gonads and other sexual organs of male or hermaphroditic animals and can fertilize female ova...

).

The epithelial cells of the epididymis possesses numerous apical modifications that are often referred to as stereocilia, as under the light microscope they look like cilia. However, as electron microscopy has revealed them to be structurally and functionally more similar to microvilli, some now refer to them as stereovilli.

Pathology

An inflammation of the epididymis is called epididymitis
Epididymitis
Epididymitis is a medical condition in which there is inflammation of the epididymis . This condition comprises gradual onset of testicular pain that can vary from mild to severe, and the scrotum may become red, warm and swollen...

. It is much more common than orchitis.

Embryology and vestigial structures

A Gartner's duct
Gartner's duct
Gartner's duct is a potential embryological remnant in human female development of the mesonephric ducts in the development of the urinary and reproductive organs...

 is a homologous
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...

 remnant in the female.

In the embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

, the epididymis develops from tissue that once formed the mesonephros
Mesonephros
The mesonephros 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 in the Wolffian body after Caspar Friedrich Wolff who described it in 1759...

, a primitive kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...

 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
Appendix of the epididymis
On the head of the epididymis is a small stalked appendage . It is named the appendix of the epididymis , and is usually regarded as a detached efferent duct....

. In addition, some mesonephric tubules can persist as the paradidymis, a small body caudal to the efferent ductules
Efferent ducts
The efferent ducts connect the rete testis with the initial section of the epididymis.There are two basic designs for efferent ductule structure:...

.

Epididymectomy

Epididymectomy is the surgical removal of the epididymis sometimes performed for Post-Vasectomy Pain Syndrome
Post-Vasectomy Pain Syndrome
Post-vasectomy pain syndrome 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 efforts focus on mitigating/relieving the individual...

.
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