IS THE PIAL MICROVESSEL A GOOD MODEL FOR BLOOD-BRAIN-BARRIER STUDIES

Pial microvessels have commonly been used as model systems for studyin g blood-brain barrier (BBB) properties instead of cerebral cortical mi crovessels. Since pial microvessels are relatively accessible they hav e been especially employed in electrophysiological and pharmacological studies. Measurements of electrical resistance across endothelial cel ls (EC) as a measure of their barrier properties have been made exclus ively from pial microvessels in in vivo BBB studies. Similarly the obs erved responses of microvessels to the application of pharmacological agents have commonly been made on pial microvessels as representative of BBB vasculature. In this review the properties of pial and cerebral microvessels are compared to determine whether the use of the pial mi crovessel as a model for BBB studies is valid. Similarities are descri bed in their ultrastructural features, permeability to electron dense tracers and molecular characteristics. Measurements of electrical resi stance from pial microvessels are compared with measurements from cere bral EC monolayers in tissue culture and indirect determinations for c erebral microvessels in situ. Two notable differences between pial and cerebral microvessels are described in the adult nervous system. Tigh t junctions between cerebral EC appear to consist of a uniform populat ion. In pial microvessels however tight junctions consist of two popul ations, in one the inter-EC tight junctions resemble those between cer ebral EC, with fusion of adjacent EC membranes. in the second populati on the inter-EC tight junctions differ with a discernible gap between adjacent EC membranes. The distribution of the endothelial barrier ant igen (EBA) is uniform between EC of cerebral microvessels. By contrast EC of pial microvessels form a heterogeneous population for EBA expre ssion which is related to the proximity of the EC to the astrocytic gl ia limitans. The role of astrocytes in the induction and maintenance o f the BBB characteristics is briefly reviewed. The possible significan ce of the lack of an astrocytic ensheathment of pial microvessels is a ssessed. In summary, caution is urged in employing pial microvessels i n BBB studies and the need for more information on possible pial micro vessel heterogeneity is stressed.