Endothelium

Structure

The endothelium is a fabric of the epithelial type which is different from the others epithelium S because it derives from the embryonic Mésoderme (majority of the others drifting of the Ectoderme or the Endoderme). It is a cell punt and polarized; the apical face is in contact with the light of the vessel, the basal face as for it being fixed on a basal Lame consisted of Collagène.

According to the size of the vessel, the morphology of the cell is very different. In the large vessels, such as the Artery S and the Vein S, it has a prismatic form like the majority of the epithelial cells. In the capillary S smallest, it forms a small tube inside whose the Sang circulates.

The cells are connected between them and with the basal Lame by Desmosomes and hémidesmosomes. It is thus about a fabric which can resist strong mechanical requests (and near the Cœur they are high), but permeable with the dissolved substances of small sizes and water. The cells such as the Hématie S cannot cross it but the Leucocyte S, very deformable, can it.

The basal Lame is a collagenic structure, synthesized by the endothelium and a certain mechanical resistance brings to him.

Function

The most obvious function of the endothelium is to contain the Sang inside the blood-vessels, while authorizing the exchange of the nutritive substances with the interior medium. In this function, the cells endothéliales and the basal Lame cooperate while acting like a molecular filter.

Another function is the control of the blood Coagulation. The epithelial cells inhibit this coagulation whereas the basal blade support the aggregation of the Thrombocyte S. a rupture of the endothelium is thus clogged at once and limit the hemorrhages.

The endothelium also takes part in the control of the Vasomotricité. In answer to various Hormone S, the endothelium can generate a substance the EDHF (or NO) causing a relieving of the vascular muscular layer and a Vasodilatation.

Specialization of the endothelium

According to the bodies, the endothelium can specialize in order to fulfill a specific function of the body.

In the kidney, the basal Lame is fenestrated. I.e., that it comprises holes which give him a molecular function of filter. These holes make it possible all the molecules to pass except to the Protéine S too much large and play a fundamental role in the operation of the Rein.

In the Nervous system, on the contrary, the cells endothéliales are connected by tight junctions preventing the Molécule S from passing between the cells. The endothelium is here responsible for the specific functions of the hemato-encephalic Barrière.

mechanical and biochemical endothelia and constraint

Mechanical constraints:

The pulsatile flow of blood generates 3 types of hemodynamic forces: hydrostatic pressure generated by the liquid, the cyclic stretching or cyclic stretching and shear stresses or shear stress.

• The cyclic stretching corresponds to a distension of the wall induced by a gradient of pressure transmurale. It is function thickness of the vessel, its composition and the degree of contraction of the smooth muscular cells. The principal effect of the cyclic stretching is the induction of an oxydative stress on the level of the cells endothéliales.

• The shear stress corresponds to the frictional forces exerted by blood flow against the vascular wall. It is determined by the blood flow, viscosity of the blood and by the diameter of the vessel. The median value of this constraint, in a right portion of vessel, lies between 1 and 2 Pa, according to the type of vessels.

Answers of the cells endothéliales:

In vitro, in answer to these stimuli, two phases of answers of the cells endothéliales are to be distinguished:

- an early answer which starts in the seconds which follow the beginning of the constraints related to flow. This answer is related to the mechanical force imposed by flow. One speaks about sensing mechanical.

- a late answer corresponding to the adaptation of the cells to a constant flow.

Thus, of the early metabolic variations and the later morphological variations are induced in order to allow the cell to adapt to its environment:

- early metabolic variation: it corresponds to the modification of the transcriptome and the protéome of genes having in their promoter a called specific sequence Shear Stress Response Element (SSRE). Certain genes implied in the vasoactivity, the cellular adhesion, the coagulation and of the growth factors thus see their modified expression.

- late morphological variation: it corresponds to the reorientation of the cells in the direction of flow. A reorganization of the cytosquelette, and more particularly of the filaments of actine, in the direction of flow is observed.

Biochemical constraints:

Activation endothéliale, generated by the biochemical constraints, is mainly due to humoraux mediators such as the cytokines like the tumor necrosis Factor  (TNF ), the hormones or the growth factors. These substances are brought by blood or are produced locally by the cells endothéliales themselves or cells of the vascular compartment. The cells endothéliales are thus sensitive to the oxydative stress caused by the increased presence of oxygenated derivatives, such as the ions superoxydes, the hydrogen peroxides and the radicals hydroxyls, overflowing the antioxydant system (superoxydismutase). This stress leads to acquisition by the cell of an pro-inflammatory phenotype and to a modification of the interactions between the endothelium, the leucocytes and the plates.

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