PERIVASCULAR ASTROCYTES AND ENDOTHELIUM IN THE DEVELOPMENT OF THE BLOOD-BRAIN-BARRIER IN THE OPTIC TECTUM OF THE CHICK-EMBRYO

The role played by perivascular astrocytes in neural vessel maturation was investigated in microvessels of the chick embryo optic tectum. Th ree-dimensional reconstructions and quantitative analyses were made, a nd permeability was studied. On embryonic days 14-16, 12.5% of the mic rovessel wall is surrounded by astrocyte endfeet which, in most cases (82%), are located under endothelium junctions; the latter, at this st age, partly prevent the extravascular escape of the marker horseradish peroxidase. On days 18-21, the astrocyte processes form a nearly comp lete perivascular sheath enveloping 96% of the microvessel perimeter; the junctions of the endothelial cells are much wider and impermeable owing to extensive fusion of the endothelial plasma membranes. This in vestigation suggests a close relationship between the perivascular arr angement of glia and differentiation of the endothelium tight junction s and indicates that the morphofunctional maturation of the latter tak es place progressively during the prenatal organogenesis of the chick central nervous system.