Classification
The endothelium can be divided into different basic types. The different types depend on the function of the organ. The structure has a strong influence on the permeability of the endothelium (endothelial permeability) for the substances found in blood and tissue.
The closed endothelium is most common. Among others, especially in capillaries and other segments of the vascular system as well as in muscle, connective and nervous tissue and in the lung and skin. Characteristic for this type of endothelium are invaginations (caveolae) directed towards the lumen.
They are used to transport substances through the cell (transcellularly) by being able to develop into a channel for a short time during the process of transcytosis. In addition, the invaginations serve to increase the surface area. At the membrane of the endothelium there is then sufficient space for numerous channels, pumps, transporters and receptors, which are enormously important for the process inside the cell.
A special form of the continuous endothelium is the endothelium of the brain capillaries. They are 100 times less permeable than the remaining cells. Dense cell contacts keep the paracellular pathway well closed.
This property is of particular importance for the blood–brain barrier. This means that only selected substances can pass through the membrane. Fenestrated endothelium is found especially in endocrine glands, intestinal mucosa, in capillaries of the kidney and in certain regions of the central nervous system.
In principle, it is similar in structure to the fenestrated endothelium.In contrast, the fenestrated endothelium, as the name suggests, has windows in the membrane. These are screen plate-like accumulations, and depending on the organ, a diaphragm is located under these windows. There are also sugar residues with negative charge.
Because of the negative charge, proteins with the same charge cannot pass through the windows. Water and water-soluble molecules, on the other hand, can diffuse quickly through them. This type of endothelium is only found in a few organs, for example in the liver or bone marrow.
In the liver, the endothelial cells have large pores without a diaphragm that pass through the cells. They allow the passage of most plasma proteins. However, it is still unclear whether these pores are permanently anchored in the membrane or only temporarily. A special form of the discontinuous endothelium is found in the spleen cells. There there are real gaps between cells, through which the blood cells can then recirculate into the vascular system.
