Three-dimensional reconstruction of vesicles in endothelium of blood-brain barrier versus highly permeable microvessels.

In this study the three-dimensional organization of pinocytotic vesicles in mouse endothelia from permeable (choroid plexus, area postrema, and skeletal muscle) and blood-brain barrier (bbb) (cerebral gray and white matter) microvessels was examined. Reconstructions of 75 segments of endothelial cells from microvessels were done with very thin (less than 23 nm) serial sections and tracings. A total of 2,013 vesicles from five tissues were reconstructed for this study. Vesicles were classified as to whether they were attached to other vesicles (fused), connected to golgi or endoplasmic peticulum (tubule-connected), open to vessel lumen or ablumen (surface-connected) or isolated in the cytoplasm (free). The densities of tubule-connected vesicles and free vesicles were the same in all four types of vessels. It seems unlikely, therefore, that these vesicles are related to vascular permeability. Vesicular clusters and surface-connected vesicles were found in much higher densities in area postrema, choroid plexus, and skeletal muscle vessels than in bbb vessels. Single-vesicle transendothelial channels were found in attenuated endothelia of area postrema and choroid plexus. These results support the hypothesis that endothelial vesicles play a role in vascular permeability, possibility by transient fusion of vesicle clusters to the plasmalemma, to form transendothelial channels.