Differential distribution of an endothelial barrier antigen between the pial and cortical microvessels of the rat.

An endothelial barrier antigen (EBA), reported to be a marker for endothelial cells (EC) displaying blood-brain barrier (BBB) characteristics, was probed with a monoclonal antibody in pial and cortical microvessels in rat brain. In contrast to the uniform labelling of EC in cortical vessels, pial microvessels showed a heterogeneity in EBA expression. Most pial vessels consisted of a mixture of EBA positive and EBA negative cells whereas a smaller number of vessels were either completely negative or uniformly positive. Significantly, in vessels showing incomplete expression it was typically EC furthest from the brain surface that did not express EBA. Although the function of EBA is unknown, the variable expression in pial microvascular EC may be related to their incomplete barrier characteristics.