Endothelia

Abstract

The inner lining of blood and lymphatic vessels is a simple, squamous epithelium, designated as “endothelium.” The endothelium performs multiple tasks and is diversely developed according to the different functions in the various segments of the macro- and microvascular systems. It may be tight or more or less open for traffic between the lumen and the tissue outside in one or both directions, with specific transendothelial transport mechanisms existing in capillaries. Capillaries are exchange vessels and can be classified as three main types: continuous capillaries (panels a and b), discontinuous capillaries, and fenestrated types (cf. Figs. 122 and 148, respectively).