Ontogeny of four blood-brain barrier markers: an immunocytochemical comparison of pial and cerebral cortical microvessels.

Pial and cortical microvessels possess many blood-brain barrier (BBB) properties in common, including impermeability to electron dense tracers, high transendothelial electrical resistance and specialised endothelial cell ultrastructural features. To compare pial and cortical microvessels further, a developmental, immunocytochemical study was undertaken of 4 BBB markers in the rat: OX-47, EBA, GLUT-1 and s-laminin. The appearance of the markers was monitored from embryonic d 16, to postnatal and adult stages. Each of the 4 markers appeared simultaneously in both pial and cortical vessels. GLUT-1 and OX-47 were present in endothelial cells of the BBB from E 16 to the adult. EBA and s-laminin appeared from postnatal d 7 through to the adult. Pial microvessels lack the ensheathment of astrocytes which may be involved in the induction and/or maintenance of BBB markers in the cortex. It is possible that astrocyte-derived factors diffusing from the brain surface are responsible for induction of BBB properties in the pial microvessels.