General Characters of Endothelium
(1) Endothelial cells possess thin membranous bodies, except at the site of the nucleus, to enclose which the cell-body is thickened. (2) The intercellular substance is minimal in amount ; clear and homogeneous in character. (3) The cells are arranged, edge to edge, in a single layer. The wavy or denticulate edges of neighboring cells fit into each other, being separated by a mere line of the intercellular substance which in this tissue has received the name of " cement-substance ". It appears probable that the connection between neighboring cells is really much more intimate, and that what appears to be a homogeneous cement-substance is in reality a fluid derived from the lymph, and that the cells are connected with each other by exceedingly delicate cytoplasmic projections which join each other, the tissue fluid lying in the spaces between these cytoplasmic bridges. This arrangement is analogous to that described in connection with the prickle-cells which have for a long time been known to exist in the epidermis (see Stratified Epithelium).
Endothelium forms a thin membranous tissue composed almost exclusively of cells. It occurs in its most isolated form in the capillary blood vessels, the walls of which are simply tubes of endothelium, supported externally by the surrounding tissues and fluids and internally by the enclosed blood. It also covers the tissues surrounding the serous cavities of the body, where it serves both as a lining to the cavities and a smooth covering to the organs, ( The term " epithelial " is used here in its most inclusive sense to designate those tissues which cover surfaces, whether those surfaces are exposed to the outer world, as, for example, the skin and the mucous membranes, or are wholly enclosed, as are the inner surfaces of the blood vessels, lymphatics, and serous surfaces). Sometimes all these tissues are called epithelium and the term endothelium is discarded.
Other authors use endothelium to designate only the cells lining the blood vessels and lymphatics and similar cells occurring in connective tissue. The term endothelium is retained here to distinguish cells derived from the mesoblast, from the epithelium arising from the epiblast and hypoblast, and because there are morphological differences between these groups of tissues in the adult.
Ishing the friction resulting from their movements against each other. It does not occur in any situation where it would be exposed directly to the air.
The cells of endothelium vary somewhat in size and shape. They may be polygonal, rhomboid, or stellate in form, and during life are soft and extensible so that their sizes may be modified by stretching; or tension in one or more directions. The cell-bodies, or cytoplasm, are usually clear and apparently structureless or only slightly granular, but occasion ally some of the cells are smaller and more granular than the majority. This is especially marked in the cells surrounding minute apertures that are found here and there in the endothelial lining of the serous cavities. These stomata furnish a direct communication between the serous cavities and the lymphatic spaces in the tissues surrounding them. These openings virtually convert the serous cavities into enormous lymph spaces forming a part of the general lymphatic system.
The edges of contiguous endothelial cells are not everywhere in equally close approximation to each other. The points where they are more widely separated than usual are occupied by an increased amount of the cement-substance, or processes from cells in the underlying tissues are here intercalated between the endothelial cells, reaching the surface of the serous membrane. In either case these points of separation of the endothelial cells are not openings through the tissue, though they are spots where the tissue is relatively more previous than elsewhere. They are called pseudostomata, to distinguish them from the stomata already mentioned.
Functionally, endothelium appears to furnish a smooth covering for those internal surfaces of the body which are exposed to friction, as, for example, in the serous cavities and the inner surfaces of the vascular systems. In the capillary blood vessels and lymphatics endothelium forms the entire wall of the vessels, and itthinness permits the passage of fluids through those walls. Endothelium is developed from the mesoderm.