epithelium

definition

The epithelium is one of the four basic types of tissue in the body and is also known as the covering tissue. Almost all body surfaces are covered by epithelium. This includes both external surfaces, such as the skin, and the internal surfaces of hollow organs, such as the bladder.

The epithelium is a flat group of cells in which the cells are very close to one another.

The epithelial cells each border two different spaces and are therefore polar cells with an apical (facing outwards or into the body cavity) and a basal (adjacent to the other tissue) side.

To other tissues the epithelium is about that Basement membrane delimited. At the side, the cells are in contact with other cells via various cell connections.

The tasks of the epithelium are very different.
The Epithelium of the skin has the task of protecting the underlying tissue from external damage, such as mechanical influences or solar radiation, and preventing the penetration of bacteria.

Inner epithelia, which line the hollow organs, serve primarily to seal against the outside (for example the epithelium of the bladder) and to exchange substances.

Certain epithelia also take over the production of various substances, such as Secretions, hormones, or enzymes.

The epithelium is supplied with nutrients by deeper layers of tissue, as it does not contain any blood vessels itself. Can by means of diffusion Nutrients and oxygen through the Basement membrane get to the epithelia.

There are different types of epithelia that can be classified differently. You can single-layer or multi-layer be made up of flat or tall cells, Glands (e.g. Skin glands) and possibly a Keratinization have (as with the skin).

In addition, the apically located cells can have protrusions, so-called Microvilli, which, by increasing the surface area, is conducive to the Nutrient exchange are.

Endothelium

The Endothelium is a special form of epithelium that forms the inner wall of Blood and lymph vessels lines. It is a monolayer squamous epithelium, which rests on a basement membrane.

Endothelium is found in all vessels of the cardiovascular system and enables various substances to be exchanged between blood and tissue.

By producing Nitric oxide (NO) is it also involved in the regulation of the Blood pressure and can be inhibitory or activating on the Coagulation act.

Another task of the endothelium is the regulation of Inflammatory processes. By activating the endothelium, White blood cells accumulate there, which can then migrate into the underlying inflamed tissue.

There are different types of endothelium that occur in different parts of the body and differ in their structure and permeability.

The continuous endothel is relatively impermeable and allows only a very targeted exchange of certain substances between blood and tissue. This kind comes for example in the brain as so-called Blood-brain barrier in front.

The fenestrated endothelium has "windows" that in most cases (except in the kidney) are closed by diaphragms. This limits the permeability somewhat.
Fenestrated endothelium comes in, for example Renal glomeruli (Kidney corpuscles) and im Intestines in front.

The most permeable endothelium is that discontinuous endothelium, which has relatively large gaps. Also the Basement membrane is partially broken in this type of tissue or does not exist. This mainly occurs in the liver in front.

Epithelium of the eye

in the eye come Epithelia in different areas. Outwardly the eye becomes through the Cornea (Cornea), which in turn consists of six layers.

The outermost layer of the cornea is the epithelial layer. This is about 50 µm thick and consists of five to six cell layers. Inwardly, the epithelial cells are above the basal cells in the Basement membrane anchored.

The outer cells attached to the Tear film adjoin, wear many Microvilli. These are the smallest protuberances on the cells that are used to absorb and release various nutrients.

The innermost layer of the cornea is a monolayer endotheliumwhich is the demarcation to the front Eye chamber represents, which can regulate the permeability of the aqueous humor to the cornea.

Also the Retina of the eye has an epithelium, the so-called Pigment epithelium. This is the demarcation between Retina and Choroid. It is a single-layer epithelium, which many Color pigments (especially melanin) and is therefore colored black.
The pigment epithelium belongs to Blood-retinal barrier and is also very important for the process of vision, as it discards dead photoreceptors.

Epithelium of the stomach

The stomach is inside of the Gastric mucosa lined its innermost layer single-layer, high prismatic epithelium forms. This means that the epithelial cells have an elongated shape.
The individual cells are about special connections, so-called Tight junctions, connected with each other.

Form the epithelium and the adjacent layers Dimples in the stomach (Foveolae gastricae), into which the gastric glands end.
There are many in the epithelium Adjoining cellswho produce mucus, which is very important in protecting the stomach.

Epithelium of the esophagus

The esophagus, or the Esophagus is of multilayered uncornified squamous epithelium lined.

This can be divided into four layers.
The basement membrane lies the so-called Stratum basale from which the regeneration of the cells proceeds.
This is followed by that Stratum parabasale, Stratum spinosum (intermedium) and the Stratum superficiale.

The cells of the inwardly directed superficial stratum perish due to the mechanical stress caused by food and are regularly exfoliated.

In addition, the ducts of glands open into the interior of the esophagus and can be found in the epithelium. The mucous membrane then regenerates from basal to luminal (i.e. directed towards the lumen of the esophagus).
The same epithelium is in the Oral cavity to find.

Epithelium of the intestine

The intestines, like the stomach, are of one monolayer columnar epithelium lined. This applies to both the Large intestine, as well as for the Small intestine.

The exact structure of the Mucous membrane However, it differs somewhat in the individual areas of the digestive tract, as each section has to fulfill specific functions.

The epithelial cells of the intestine become Enterocytes called and have so-called Microvilli on the side facing the inside of the intestine (Lumens) is facing. These are small protuberances that serve to increase the surface area and also as brush border are designated.
The large surface offers plenty of space for recording (Absorption) the food components from the intestine.

In addition, the lining of the intestine has many Gland cells that produce hormones that control digestion.

Epithelium of the skin

The skin (epidermis) is outwardly through a multilayered keratinized squamous epithelium delimited. This is used for mechanical protection, preventing the penetration of bacteria and preventing the body from drying out.

It is called squamous epithelium because the top layer of cells consists of flat cells. Since these constantly die off, turn into horn scales and are peeled off, it is called keratinizing.

So the top layer is that Horny layer (Stratum corneum), in whose cells no nuclei can be found because the cells have already died.
Inside to Body interior close that Stratum lucidum, the Stratum granulosum and the Stratum spinosum on.
The latter is characterized by a strong interlocking of the cells with one another.

The regeneration of the cells takes place in the Stratum basale instead of. The basal cells located there sit on the basement membrane, can divide and this creates new cells for the overlying skin layers.

Epithelium of the lungs

The different sections of the lungs are lined by different epithelia.

The upper airways, i.e. the windpipe and the larger bronchi, are of one multi-row ciliated epithelium covered.
Since this affects a large part of the airways, it is also called respiratory epithelium designated.

Multi-row means that all cells of the epithelium are in contact with the basement membrane, but not all of them reach the surface.

Ciliated epithelium it is called because the cells that reach the surface have numerous small protuberances that move during breathing and are mainly used for absorption.

The smaller bronchi are from uniseriate ciliated epithelium lined, which in the further course is no longer cylindrical, but cubic, i.e. flatter.

In the Alveoli (Alveoli) itself you will find a flat epithelium that allows the exchange of gases between breath and blood without any problems. The epithelial cells of the alveoli can be divided into two types.

The Type 1 alveolar epithelial cellswho are responsible for gas exchange and who Type 2 alveolar epithelial cells, the Surfactant to produce. The surfactant is responsible for ensuring that the alveoli do not collapse.

Carcinomas

In epithelia can also Carcinomas, so malignant tumors arise. There are different types that arise from different types of epithelium.

They are of the so-called Adenomas delineate those who are benign Tumors of glands of the epithelium.
Also Papillomas are benign epithelial growths.

A carcinoma can develop from squamous epithelium, then one speaks of one Squamous cell carcinoma. This is a malignant degeneration that can occur in both keratinizing and non-keratinizing squamous epithelium.
As a result, these carcinomas can appear in many parts of the body.

Horny squamous cell carcinoma of the skin can be triggered by radiation from the sun or X-rays.
Non-keratinizing squamous cell carcinoma come especially in the esophagus (please refer Esophageal cancer), of the tongue ( please refer Tongue cancer), the cervix (please refer cervical cancer) or the Urethrae in front.

Squamous cell carcinomas can be treated surgically or with radiation or chemotherapy, depending on their occurrence.

The epithelial type of the Urothelium (Transitional epithelium) can have a carcinoma. The urothelium dresses them laxative urinary tract out.
Accordingly can Urothelial carcinoma in the Renal pelvis, ureter, urinary bladder or urethra occurrence.
In over 90% of the cases, however, the bladder affected. Urothelial carcinomas often metastasize, so they form daughter tumors.