Endothelium

definition

The endothelium is a single layer of flat cells that lines all vessels and thus represents an important barrier between the intra- and extravascular space (as the space inside and outside the blood vessels).

construction

The endothelium forms the innermost cell layer of the Intima, the inner layer of the three-layer wall construction one artery.

The cells contain one or more cell nuclei and are relatively flat. They are in Longitudinal direction arranged and thereby ensure a smooth flow of blood through the vessels.

The endothelium exists from individual cells, by dense cell contacts are interlocked. These contacts include Adherence contacts, Tight junctions and Gap junctions.
They separate the intravascular space from the deeper layers of the vessel wall and thus prevent the Contact between blood cells and the extracellular matrix (i.e. the liquid outside the vessels).
At the same time they also control the Passage of plasma components. You thus have an influence on the Endothelial permeability. Which dissolved substances through which cell contacts can pass is, among other things, by a top layer of sugar chains influenced. This apical surface is also known as Glycocalyx designated. In addition, the Glykokalix bind different fabrics and thereby that Affect the inside of the cell. At the opposite side, the basal side of the cell, are the endothelial cells via local contacts with the subendothelial layer interlocked.

function

The endothelium has some functions, the depending on the size and position of the vessel can be different.

For one thing, it has the function to create a barrier. The Tight junctions as strong cell contact between the endothelial cells prevent the passive passage of components that are dissolved in the blood. So they form one dense diffusion barrier to protect unwanted concentrations of substances in the subendothelial layer.

The apical surface with sugar residues settled prevents that Attachment of blood cells. Only through one corresponding activation of selectins and other molecules can bind substances to it. So the endothelium also bears for blood clotting at.
in the intact condition it prevents that Formation of a blood clot and after one Vascular injury it promotes clotting.

Furthermore, through the endothelium, the Regulated vessel size become. The endothelial cells are with internal muscle cells the middle layer, the Media, through so-called myoendothelial contacts connected. About this contact, which is mostly about Gap junctions is made, practice one vasodilatory influence on the muscles.

The endothelium can also locally Release nitric oxide (NO). The release of nitric oxide can be caused by the Shear forces triggered by the Friction of blood flowing by come about with high blood pressure. Another option is through the stimulus vasodilator agentsthat bind to surface receptors on the endothelium. This is a vasodilator substance. If necessary, it can also be a vasoconstricting substance pour out. This is that Protein endothelin.

Classification

The endothelium can be in different basic types to be grouped. The different construction is based according to the function of the organ. The structure has a strong influence on the Permeability of the endothelium (Endothelial permeability) for the im blood and fabrics.

Closed (continuous) endothelium

The closed endothelium comes on most common in front. Among other things, especially at Capillaries and other segments of the vascular system as well as in Muscle, connective and nerve tissue and in the lung and skin.

Characteristic for this type of endothelium are indentations directed towards the lumen (Caveolae). They serve that Transport of substances through the cell (transcellular) by being able to develop into a channel in the short term as part of transcytosis. In addition, the indentations serve the Surface enlargement. Then is on the membrane of the endothelium enough space for numerous channels, pumps, transporters and receptorswhich are extremely important for the process inside cells.

A special shape of the continuous endothelium represents the Endothelium of the brain capillaries They are compared to the rest of the cells 100 times lower permeability. Tight cell contacts hold the paracellular path well closed. This property is of particular importance to the Blood-brain barrier. This allows only selected fabrics through the membrane.

Fenestrated endothelium

Fenestrated endothelium is coming especially in endocrine glands, Intestinal lining, in Capillaries of the kidney and in certain regions of the central nervous system.

Basically it is similar in structure the closed endothelium. In contrast, the fenestrated endothelium has as the name suggests Window in the membrane. It refers to sieve plate-like collectionswhich, depending on the organ, is located under these windows Diaphragm. Here are also sugar residues with negative charge. Because of the negative charge you can equally charged proteins do not step through the windows. Against it can Water and water soluble molecules diffuse quickly through it.

Discontinuous endothelium

This type of endothelium comes about only in a few organs before, for example in the liver or in Bone marrow. In the liver have the endothelial cells large pores without a diaphragmthat pull through the cells. They make that possible Most plasma proteins pass through.
However, it is still unclear whether these pores permanently anchored in the membrane are or only temporary.

A special shape of the discontinuous endothelium comes into the Spleen cells in front. Exist there real gaps between cellsthrough which the blood cells can then recirculate into the vascular system.

Malfunctions

Different risk factors again arterial hypertension, increased Cholesterol levels and especially the Nicotine consumption seriously change the Function of the intact endothelium. One then speaks of one Endothelial dysfunction.

For example oxidative stress the Nitric oxide mechanism change and it arise highly toxic metabolitesthat can damage the endothelium.

Endothelial damage ultimately forms the basis for the Development of arteriosclerosis. It refers to pathological wall changes especially of elastic and larger arteries. On the wall injuries form atheromatous plaques, a build-up of invaded lipids, cholesterol, and mast cells called foam cells. This leads to Narrowing of the vessel lumen (Stenosis) and Reduced blood flow (Ischemia) of the following tissue. There is then the risk that this will result in a necrosis developed (Infarct), which can have life-threatening consequences.