The blood-brain barrier and astrocytes

The basis for the blood-brain barrier in mammals is the selective transport properties of brain capillary endothelium, including the elaborate system of tight intercellular occluding junctions that occur between apposed membrane faces of these cells. This unique specialization of brain capillary endothelial cells appears late in development and has been postulated to be under the inductive influence of astrocytes in the central nervous system. To examine this astrocytic contribution to endothelial cell monolayer permeability, we employed the cocultures of bovine endothelial cells (aortic or brain capillary endothelial cells, BBEC or BAEC) with astrocytes in a double chamber system. In system 1, where astrocytes were separated from endothelial cells, a 40% reduction in L-glucose permeability of the BBEC monolayer, but not the BAEC monolayer, was observed by cocultivation with astrocytes. By contrast, in system 2, where respective endothelial cells and astrocytes layered on the upper and lower surfaces of a membrane, the permeability of both BAEC and BBEC monolayers was reduced by cocultivation with astrocytes. The obtained results suggest that primary cultured BBECs, which had been primed by astrocytes in vivo, retain a higher sensitivity to astrocytes possibly through an astrocytic soluble factor (s) to exhibit BBB-specific phenotypes, and that even BAEC from extra-neural tissues, when cultured with astrocytes in close proximity in vitro, may acquire the similar phenotypes and serve for an extensive use of BBB model in vitro.