Abstract
Rapid increases in microvascular permeability are associated with the appearance of openings in microvascular endothelium, which are believed to develop between the endothelial cells of venules. Reconstruction of these openings, from electron micrographs of serial sections of the endothelium reveal that many pass through the periphery of the endothelial cells close to intact intercellular junctions. Transcellular pathways are the principal type of opening induced in microvascular endothelium by the ionophore A23187, by VEGF, and by high transmural pressures. Some mediators induce the fusion of vacuoles with the luminal and abluminal surfaces of the endothelium, and it is suggested that the transcellular openings may develop from vacuolar channels.