[Morphological and ultrastructural bases of the lymphatic vascular system]

The lymphatic vascular system plays a pivotal role in the control of the interstitial homeostasis. This review illustrates the morphological and ultrastructural features of precollecting, pre- and postnodal collecting and absorbing peripheral vessels. A special attention is devoted to the endothelium of the absorbing lymphatic, its cytological features, kinds of intercellular contact, contractile properties and occurrence of valves. The role of the different mechanisms involved in transendothelial transport, namely membrane diffusion, vesicular route, open junctions and intraendothelial channels, are considered. The morphological reality of "intercellular openings" and "intraendothelial channels" is also discussed. The latter are "dynamic units" which carry on a remarkable task in the drainage, toward the lymphatic lumen, of macromolecules (chylomicra, glycoproteins, etc.) and cellular elements (lymphocytes and polymorphonuclear leukocytes). On the other hand, macrophages follow a transcellular pathway. The factors which possibly underlie these two modes of transendothelial migration are also discussed.