ULTRASTRUCTURAL-LOCALIZATION OF LECTIN-BINDING SITES IN THE DEVELOPING BRAIN MICROVASCULATURE

The temporo-spatial patterning of lectin-binding sites was examined by lectin histochemistry and quantitative methods in the microvasculatur e of the optic tectum of 9-, 14-, 20-day-old embryos and 30-day-old ch ickens, Horseradish peroxidase and colloidal-gold-labelled lectins wer e used for detection of beta-D-galactose (RCA-I, Ricinus communis aggl utinin-I) and of N-acetylglucosamine and sialic residues (WGA, Wheat g erm agglutinin) at light and electron microscopical levels. At the lig ht microscopical level, RCA-I and WGA binding sites were detectable in the early embryonic capillaries in a diffuse staining pattern; in lat er embryonic stages and in adult animals, RCA-I labelling became locat ed on the abluminal surface of the vessels, while WGA staining was det ected on the luminal surface. Ultrastructurally, gold labelling for RC A-I was seen intracytoplasmically in endothelial cells in 9-day-old em bryos. In 14-to 20-day-old embryos and in chickens, binding sites for RCA I were detected in endothelial tight junctions and basement membra nes. In contrast, labelling of the gold-coupled WGA lectin was distrib uted almost exclusively on the luminal endothelial surface already in early embryos. The results indicate that the endothelial cells of the optic tectum acquire functional polarity early in their development an d that glycoconjugates containing beta-D-galactose residues are involv ed in the biochemical composition of the tight junctions and basement membrane, which are considered to be key structures in blood-brain bar rier (BBB) differentiation.