Definition
The epithelium is one of the four basic tissues of the body and is also called the covering tissue. Almost all body surfaces are covered by epithelium. These include both external surfaces, such as the skin, and the internal surfaces of hollow organs, such as the bladder.
The epithelium is an extensive group of cells, in which the cells are very close together. The epithelial cells each border on two different spaces and are thus polar cells with an apical (facing outwards or into the body cavity) and a basal (bordering the other tissue) side. The epithelium is separated from other tissues by the basement membrane.
Laterally, the cells are in contact with other cells via various cell connections. The tasks of the epithelium are very different. For example, the epithelium of the skin has the task of protecting the underlying tissue from external damage, such as mechanical effects or sunlight, and of preventing the penetration of bacteria.
Internal epithelia that line hollow organs serve primarily to seal them off from 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 tissue layers, as it does not contain any blood vessels itself.
By means of diffusion, nutrients and oxygen can reach the epithelia through the basement membrane. There are different types of epithelia, which can be classified differently. They can be single-layered or multi-layered, consist of flat or high cells, contain glands (e.g. skin glands) and may have a keratinization (as in the skin). In addition, the apically located cells may have protuberances, so-called microvilli, which are conducive to the exchange of nutrients by increasing their surface area.
Endothelium
The endothelium is a special form of epithelium that lines the inner wall of blood and lymph vessels. It is a single-layered squamous epithelium that rests on a basement membrane. Endothelium is found in all vessels of the cardiovascular system and enables the exchange of various substances between blood and tissue.
Through the production of nitric oxide (NO) it is also involved in the regulation of blood pressure and can have an inhibitory or activating effect on coagulation. A further task of the endothelium is the regulation of inflammatory processes. By activating the endothelium, white blood cells can attach themselves to it, which can then migrate into the underlying inflamed tissue.
There are different types of endothelium, which occur in different parts of the body and differ in their structure and permeability. The continuous endothelium is relatively impermeable and allows only a very specific exchange of certain substances between blood and tissue. This type occurs in the brain, for example, as the so-called blood-brain barrier.
The fenestrated endothelium has “windows” which are closed in most cases (except in the kidney) by diaphragms. The permeability is thus somewhat restricted. Fenestrated endothelium is found, for example, in the renal glomeruli (renal corpuscles) and in the intestine.
The most permeable endothelium is the discontinuous endothelium, which has relatively large gaps. The basement membrane is also partially ruptured or does not exist in this tissue type. This occurs mainly in the liver.
