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 endo thelial cells (EC) displaying blood-brain barrier (BBB) characteristic s, was probed with a monoclonal antibody in pial and cortical microves sels in rat brain. In contrast to the uniform labelling of EC in corti cal vessels, pial microvessels showed a heterogeneity in EBA expressio n. Most pial vessels consisted of a mixture of EBA positive and EBA ne gative cells whereas a smaller number of vessels were either completel y negative or uniformly positive. Significantly, in vessels showing in complete expression it was typically EC furthest from the brain surfac e that did not express EBA. Although the function of EBA is unknown, t he variable expression in pial microvascular EC may be related to thei r incomplete barrier characteristics.