Plasmalemmal vesicles represent the large pore system of continuous microvascular endothelium.

In the capillary physiology literature, molecules and particles larger than 10 nm are assumed to leave the plasma mostly through large pores located at the level of intercellular junctions in microvessels lined with a continuous endothelium. In morphological studies of similar microvessels, outgoing particles > 10 nm were detected in endothelial plasmalemmal vesicles not in intercellular junctions. Because the probes may not be found in transit through the junctions because they may be swept away by strong currents generated by Starling forces, we have examined a large number of junctions in arteriolar, capillary, and venular segments of bipolar vascular fields of mouse diaphragms collected after perfusion with large pore probes. The results presented in this study indicate that 1) the perfused probes accumulate in the luminal introits of the junctions as filtration residues that decrease in size and frequency from arterioles to venules, and 2) large pore probes move across the endothelium exclusively through plasmalemmal vesicles.