The cell biology of the blood-brain barrier

The blood-brain barrier (BBB) is formed by brain capillary endothelial cells (ECs). In the late embryonic and early postnatal period, these cells respond to inducing factors found in the brain environment by adopting a set of defined characteristics, including high-electrical-resistance tight junctions. Although the factors have not been identified definitively, a great deal of information about brain ECs has been obtained, especially recently. This review concentrates on a cell biological analysis of the BBB, with an emphasis on regulation of the specialized intercellular junctions. The development of these junctions seems to depend on two primary processes: the appearance of high levels of the tight junction protein occludin and intracellular signaling processes that control the state of phosphorylation of junctional proteins. Recent studies have revealed that the BBB can be modulated in an ongoing way to respond to environmental stimuli.