blood brain barrier

The blood brain barrier is a cell layer of lining cells that separates the central nervous system from the bloodstream. It may be constituted by endothelial cells or by glial cells, and comparative evidence indicates that an endothelial barrier may have arisen independently several times during evolution. That variation is documented across fishes: in bichir and lungfish the barrier is formed by brain endothelial cells, whereas in sturgeon it is formed by a complex perivascular glial sheath in which no tight junctions have been detected. The barrier is therefore defined by its separating function rather than by a single cellular architecture. Two subordinate structures are recorded beneath this entry, and it is classified within the nervous system as a cell layer.