The endoendothelial layer of normal and injured vascular endothelium--an electron microscope study

The ultrastructural appearance of the endoendothelial lining of normal and injured small blood vessels has been examined after staining with alcian blue, ruthenium red or colloidal iron hydroxide. An endoendothelial layer is constantly present in all types of small blood vessel but its appearance and staining reactions are variable and unpredictable. Although normally visible only on the luminal aspect of endothelial cells, study of inflamed vessels suggests that it also covers the deep aspect of endothelium. The endoendothelial layer persists apparently unchanged after mild or severe injury to endothelium. It appears to be to some degree movable and can be displaced by emigrating leucocytes, but rapidly regains its original position and appearance after the escaping cell has passed. No change in the appearance of the endoendothelial layer could be detected which might account for either the adhesion of leucocytes to vascular endothelium in areas of acute inflammation or of lymphocytes to the surface of high endothelial venules in lymphoid tissues.