Endothelial Vesicles and Protein Transport

Abstract

THE physiological mechanism involved in the transfer of water, solutes and macromolecules across endothelium remains obscure^1,2 though it has been subjected to considerable investigation. When permeability is increased, endothelial gaps, believed to be intercellular, are found in the endothelium^3–5. In electron microscopic examinations, tracer particles, injected into the blood stream, aggregate in the gaps, which are the sites of plasma leakage and the stigmata and stomata of light microscopy^6,7. However, in physiological states, with the exception of lymphatics^8,9 and such special vascular beds as the sinusoidal vessels of the liver^1,10–13, spleen¹⁴, bone marrow^13,15 and frog heart¹⁶, gaps do not occur in vascular endothelium. This report concerns the finding of a new type of vesicle in endothelium, and it is considered that these specialized micropinocytotic vesicles could be related to protein transport in the lymphatic and vascular endothelium of frog tissues.