Differences in lymphatic and blood capillary permeability: ultrastructural-functional correlations

The major structural features of lymphatic capillaries, as they contrast with blood capillaries and as they pertain to endothelial permeability, are reviewed briefly with special emphasis on intrarenal vessels. The most characteristic structural feature of lymphatic endothelium is the discontinuity of the basal lamina. Basal laminae of blood vessels, such as renal glomerular capillaries, are prominent and are known to play a role in preventing extravasation of plasma proteins. By analogy, the lack of a basal lamina around lymphatic capillaries can be considered to be of major functional importance in facilitating access of interstitial macromolecules to the abluminal surface of endothelial cells and thus to the transport pathways that provide entry to the lymph. Tracer studies with horseradish peroxidase, for example, reveal that the protein enters the intraendothelial cytoplasmic vesicular system suggesting that this system may provide a transport pathway. Tracer is also seen between adjacent endothelial cells but in the kidney, liver and thyroid these intercellular channels comprise relatively narrow spaces of about 20 nanometers or less and do not form prominent gaps such as are seen in lymphatics of the diaphragm and skin. Evidence that macromolecular transport across endothelial cells may be asymmetric, favoring movement from interstitium to lymph, is derived from 1) studies using isolated perfused lymphatics, 2) differential luminal and abluminal membrane staining with cationic stains, 3) the presence of charged microdomains on lymphatic endothelial cell surfaces revealed with macromolecules of different charges, and 4) studies on cultured monolayers of porcine arterial endothelial cells.(ABSTRACT TRUNCATED AT 250 WORDS)