Endothelium

Endothelium, a layer of cells lining the inner surface of blood and lymphatic vessels and serous cavities. The concept of E. in the narrow sense refers only to the vascular system; in serous membranes, these cells, developing from the so-called coelomic epithelium, are also called mesothelium, covering epithelium (Deckzellen). The covering cells of the meninges, formerly considered derivatives of mesenchyme and designated as E., are now attributed to the neuroectoderm and tend to be called epithelium. E. consists of a single layer of cells; sometimes with the help of silvering, their outlines can be determined, which most often have an elongated, spindle-shaped form, less often polygonal or irregular. In many cases, the boundaries between individual epithelial cells are not detected at all (capillaries of the liver, kidneys, vascular coat of the eye, etc.). Endothelial cells of vessels develop from mesenchymal syncytium, being a modified part of it, limiting the blood islands. Thus, for the entire vascular system, the endothelial tube, consisting of a single layer of epithelial cells, is the primary foundation, while all other elements of the vascular tube are functional adaptations developing from the surrounding mesenchyme. Narrow slits (stomata) are described at the boundaries between individual endothelial cells, through which supposedly the exit of formed elements and other blood components into the surrounding tissue occurs. However, there are substantial assumptions that these stomata are an artifact during histological processing and do not exist as morphologically preformed formations. In particular, not only in many capillaries, but also in the human peritoneum, the presence of such stomata between endothelial cells cannot be detected. From the point of view of the functional significance of E. as a boundary membrane between the liquid medium and tissues, carrying out exchange between them, the presence of stomata is in no way a mandatory morphological substrate for this exchange, and a more plausible view is that of E. as a syncytium. The fine structure of endothelial cells has been little studied. Recent works establish different structures of them in different areas of the body and even in functionally different parts of the same organ (glomeruli and interstitium of the kidney, cerebral cortex and subcortical ganglia, medullary and cortical substance of the adrenal glands, etc.). Moreover, these same works establish great variability of endothelial cells depending on general and local exchange conditions (Rapoport). This is connected with the difference in functions of endothelial cells in connection with the difference in structures and functions of the tissues surrounding them. Endothelial cells play a leading role in regulating exchange between blood and tissues. They are the main element of the hemato-encephalic barrier and other histo-hematic barriers (Stern and Rapoport), playing the role of a selective filter that passes some substances from the blood into the tissues and retains others. The regularities of physiological and pathological exchange between blood and tissues cannot be exhausted only by the laws of diffusion and osmosis, the law of membrane equilibrium of Donnan, oncotic pressure, etc., but assume the active participation of the endothelial cells themselves in this process. From this point of view, the different structure of endothelial cells in different areas of the body is the morphological basis for the difference in exchange and permeability of endothelial cells. Endothelial cells are the main structural element of the capillary wall, and the contractility of the latter is connected (for now hypothetically) with the contractility of endothelial cells. As an organ of exchange, endothelial cells are part of the reticulo-endothelial system. Endothelial cells have great proliferative ability, providing material for regeneration of vessels, proliferating with various pathological irritations, sometimes forming the main mass of the cellular proliferate in so-called endothelioses, resp. reticulo-endothelioses. Endothelial cells often circulate in the peripheral blood, especially with infectious, septic diseases (typhus, sepsis lenta, etc.).