A schematic diagram of the major differences between general capillaries and brain capillaries. The main point is that the endothelium of brain capillaries has very tight junctions between the cells, and lacks intercellular clefts, fenestrations (holes), pinocytes (membrane vesicles that fuse with the membrane, engulf the extracellular molecules, invaginate and then transport the extracellular molecules into the brain). The feet of the adjacent astrocytes are believed not to contribute to the barrier but to provide structural support. With permission, from Oldendorf, 1977 (Fig. 2, p. 179). Copyright 1977 by Academic Press.