Types of Endothelium in Normal and Neoplastic Brain Tissue
The junctions between adjacent endothelial cells of the cerebral capillaries in normal vertebrates determine whether or not proteins can leave the capillary lumen to move into the parenchymal interspaces. In rodents (mice and hamsters), these junctions are of the tight variety, such as that shown in Fig.2(TJ), and are typical of endothelium in normal brain tissue. Such tight junctions appear to form circumferential belts of occlusion between contiguous endothelial cells. A protein such as horseradish peroxidase, when injected into the blood, is thereby prevented from leaving the capillary lumen to enter the extracellular clefts of the neuropil. The same tight junctions also stop peroxidase from entering the capillary lumen after this protein has been injected into the cerebral ventricles and has moved throughout the extracellular clefts.