Epididymis
The epididymis is a narrow and elongated tubular structure, which is made up of the meeting and bunching of the tubules that continue to the seminiferous ducts. It is located in the posterior part of the male gonad (testicle), and its anatomical size is approximately 5 centimeters in length. It is customary to distinguish three sectors: head, body and tail, which is continuous with the vas deferens.
The function of the epididymis is the transport, storage and maturation of spermatozoa, to develop their motility and fertilization capacity.
Embryology
The epididymis develops from the mesonephric tubules and the proximal Wolffian duct.
During the embryonic stage, and before epithelial differentiation, the mesenchymal androgen receptor, together with inhibin beta A, facilitates the elongation and convolution/coiling of the tubule.
Early in epididymal development, coiling occurs in a single plane, but later three-dimensional (3-D) coiling is seen in the initial segment (IS) and in the caput of the epididymis.
At birth, the anterior Wolffian duct transforms completely into all three regions of the epididymis along the anterior/posterior axis.
Structure
The epididymis is a highly specialized organ with a structural unit, which presents characteristics of a tortuous and folded duct, which is part of the male reproductive system.
Anatomy
The epididymis is arranged in the shape of a crescent, on a strip that extends from the cranial pole to the caudal pole of the testicle.
Gross anatomy of the epididymis shows, in man, an approximate length of 3.8-5 centimeters (cm) by 12 millimeters (mm) wide.
For its anatomical description, it is customary to classically divide it into three observable zones, according to their similarity in:
- head (caput in Latin), located in the cranial pole of the testicle,
- body (corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus hab corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus hab corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus corpus in Latin), and
- tail (cauda in Latin), which continues with the deferential duct.
The total length of the epididymis, if we could unroll it into a single isolated tubule, would vary among different mammalian species. The estimated length, of the adult epididymis, reaches: more than 1 meter (m) in the mouse, 3 meters in the rat, 6 meters in the human and 15-18 meters in the stallion horse.
- In the man
A particularity of the human epididymis are the poorly differentiated segments: the caput does not show the classic bulbous appearance, the cauda does not show the dilated tubules and their reserve capacity of sperm is limited.
Histology
Microscopic anatomy (right). Mouse.
The epididymis is formed in the head (caput) by efferent ducts, which start from the rete testis, and in the body and the tail through the epididymal duct.
- Them efferent conductillos
and the initial segment (IS) are a loosely coiled tubule of broad diameter. The epithelial cells of this segment are elongated and have tall stereocilia.
The head segment (caput) is characterized by a narrow lumen diameter. They are lined by pseudostratified epithelium with ciliated cells, and surrounded by connective tissue and smooth muscle fibers.
- The epididynamic conduit
has a lumen diameter that increases distally within the body (corpus) and tail (cauda) of the epididymis.
The body and tail are lined with a columnar pseudostratified epithelium with stereocilia. The height of the cells and cilia decreases towards the tail.
The different types of cells within these segments are responsible for the creation of a specialized microenvironment for their secretory and endocytic functions, which promotes sperm maturation and subsequent storage.
- In the man
In the human epididymis, the connective tissue septa are underdeveloped in longitudinal histological section, are incomplete and do not define compartments.
The efferent ducts in men are not small straight tubules, which connect the rete testis with the epididymal tubule. Some efferents are coiled, some are branched, while others are infinite and do not fuse with the epididymal tubule.
The intraluminal diameter of the epididymal tubule is relatively small throughout the organ.
Basal cells are the hallmark of the epididymis in man, residing at the base of the epithelium throughout the entire organ.
The efferent ducts are characterized by numerous hair cells that are abundantly and evenly distributed.
Cells of the epididymis
The cells that line the epididymal duct form a pseudostratified epithelium and are called:
BM= basal membrane. My=Myoid cell.
- main cells (Principal cell),
- narrow cells (Narrow cell),
- Apical cells,
- clear cells (Clear cell),
- basal cells and
- halo cells (Halo cell).
- In addition, a population of mononuclear phagocytes (dendritic cells and macrophages) reside at the base of the epithelium along with migratory halo cells.
In each of the four main regions: initial segment, caput, corpus and cauda, this set of cells defines the structural integrity of the tubule and also the composition of the contents of the lumen.
- Main cell
It is the most numerous cell type in the epithelium of the epididymis and exists throughout the entire length of the duct. They represent 65-80% of the epididymal epithelium depending on the region. have a immobile cellular modification called stereovilli or stereocilia, which are tuft-like apical extensions 5-10 micrometers (μm) in length.
- Apical cell
They are located mainly in the initial segment (IS) of the epididymal epithelium and also have endocytic activity.
- Close cell
also exist exclusively within the initial segment, and are narrower than adjacent principal cells. These cells have been shown to secrete H+ ions into the lumen of the epididymis and are responsible for endocytosis.
- Clear cell
They lie exclusively within the head, body, and tail regions of the epididymis and do not lie within the initial segment (IS). They have high endocytic activity.
- Basal cell
They are found throughout the tubule and adhere to the basement membrane. They are an integral part of the structure of the tubules.
They are responsible for absorbing the cytoplasmic droplets that are released from the sperm during maturation in the lumen of the epididymis.
- Cello.
They exist throughout the epithelium and are the primary immune cells in the epididymis. In addition, a population of mononuclear phagocytes (dendritic cells and macrophages) reside at the base of the epithelium along with the migrating halo cells.
The epididymal epithelium is surrounded by connective tissue. Smooth muscle is thinnest in the head with a single layer of myoid cells and becomes progressively thicker with three layers toward the tail of the epididymis.
Function
The epididymis is an important accessory male sexual organ, in which sperm motility and fertilization capacity develop.
From a functional point of view, the ducts of the epididymis are responsible for both maturation and activation of spermatozoa.
The epididymis has well-described roles, such as sperm concentration, protection, transport, and storage.
- Transit
A transit time along the lumen of the epididymal ducts is crucial to transform spermatozoa from an infertile and immobile state into cells capable of complete fertilization. The composition of the lumen fluid, which bathes the sperm, is considered one of the most complex systems in the body in terms of chemical components and physical interactions with proteins and lipids.
Sperm transit time is fast in man, 2-6 days, compared to 10-13 days in rodents. It is speculated that sperm maturation may occur more rapidly in the male.
Blood-epididymal barrier
Within the tubule, the contents of the lumen and maturing sperm are protected from immune surveillance behind a blood-epididymal barrier (BEB) or blood-epididymal barrier. This barrier consists of three components: an anatomical barrier, a physiological barrier, and an immunological barrier.
- The anatomical barrier is formed by narrow unions, formed in the basal and apical membranes of the main cells, which prevent molecules and cells from entering or coming out of the light. The occludens zonula of the epididym is well developed and its ultrastructure shows an extensive mesh of close union. These complexes of union between the epithelial cells of the thybulus of the epididymis contain many proteins, such as claudias, conexins and catenins.
- The physiological barrier is composed of conveyors and channels. Specific conveyors, located along the basolateral and apical membranes of epithelial cells of the epididym, regulate the movement of molecules within or outside the light of the tubulum. The physiological barrier is a reflection of the permeability of the basolateral and apical membranes to the molecules.
This physiological barrier is necessary to create the appropriate microenvironment within the epithelium and lumen for development (meiosis and differentiation) and the maturation of sperm. - The immune barrier is made up of different immune components within and outside the tubulum, which limit access to the immune system. Its function is to protect sperm from the attack of this system, by kidnapping the antigens of sperm and by restricting the entry of immunoglobulins and immune cells.
These three components determine a restrictive resultant, which decreases or controls the entry of molecules or even cells from the circulating blood or from the interstitium, so that they do not enter this tissue and compartment.
The epididymis contributes to the formation of seminal plasma from the generation of cauda epididymal fluid (ECF, also known as cauda epididymal fluid). This fluid is acidic in nature and contains proteins such as β-N-acetylglucosaminidase and fibronectin, synthesized in the epididymis and involved in sperm maturation. At the time of ejaculation, the spermatozoa immersed in the FCE are expelled into the vas deferens from contractions of the smooth muscle around the tail of the epididymis.
During sperm transit through the epididymis, a wide variety of changes occur in the environment of the lumen of the tubule. These changes include the release and uptake of fluids, ions, antioxidants, and the exosomes known as epididymo-somes.
In cases of azoospermia (when there are no evident spermatozoa in the ejaculate), if the testicles continue to produce spermatozoa, the way to obtain them is to enter the epididymis through a puncture, since in this area there will be a high concentration of spermatozoa.
In animal biology
- Located in the upper and lateral portion of the testicle.
- It presents a triangular section to the court.
- Responsible for the maturation of male gametes.
- Structure:
- Head: variable length according to the species. Its main function is to reabsorb water.
- Efferent behaviors.
- Cells (basal or deep, glandular, with stereociles).
- Body or duct: the sperm ends its ripening.
- Cola: connection to the deferential duct. It is the storage site of mature sperm.
- Head: variable length according to the species. Its main function is to reabsorb water.
- Travel time: regeneration time.
The epididymal ducts also store sperm and expel them into the urethra during ejaculation by peristaltic contractions of its smooth muscle. Sperm can remain stored in the ducts of the epididymis or are reabsorbed.
Diseases
- An infection that directly attacks the epididymum is epidimitis, which causes inflammation of the same epididymum and scrotum, producing a strong testicular pain, also produces mild to moderate fever.
The infection and inflammation of the epididym are involved in 13-15% of cases of human infertility by male factor.
- The cysts of the epididymus are a fairly common pathology. These lesions, usually benign, are mainly located in the head of the epididym. They are solitary or multiple lesions with aqueous content, lymphocytes, sperms and abundant decay cells. The number and size are variable from millimeters (mm) to centimeters (cm). They cause from unspecific discomfort to intense pain associated with increased volume and secondary deformation of the epididym.
- Epididymic tumors in man represent less than 5% of all para-testicular tumors.
80% of epididymic tumors are benign. They are presented between the third and fourth decades of life, usually asymptomatic and slow growth.