Immunohistochemical and ultrastructural characterization of cortical platemicrovasculature in the human fetus telencephalon

The blood-brain barrier (BBB) differentiation was investigated by immunohis tochemistry and electron microscopy in the radial microvasculature of the t elencephalon cortical plate (CP) of 12- and 18-week human fetuses. The BBB- specific glucose transporter isoform 1 (GLUT1) is expressed in both stages, with a main localization on the ablumenal and lateral plasma membranes of the endothelial cells. The endothelial cells are welded by short junctions with fusion points of the plasma membranes at 12 weeks and by extensive tig ht junctions at 18 weeks. The basal lamina is discontinuous beneath the end othelium-pericyte layer at 12 weeks and splits into two continuous layers c ircumscribing the pericytes in the later stage. The expression of laminin, a basal lamina glycoprotein, is continuous already at 12 weeks. The CP micr ovessels are tightly surrounded by processes of glial cells. Immunodetectio n of the cytoskeletal filament proteins, vimentin (VIM), and glial fibrilla ry acidic protein (GFAP), demonstrates that at 12 weeks the perivascular gl ial processes are mostly represented by VIM-stained fibers of the radial gl ia. At 18 weeks, GFAP-stained radial glia fibers, processes of VIM-stained astroblasts, and GFAP-positive astrocytes also build the perivascular envel opes. The results indicate that the vessel differentiation is already under way in the human CP at the midgestational age and entails the establishmen t of some barrier devices. The early relationship between perivascular glia coverage formation and endothelial barrier maturation suggests that also i mmature astroglial cells are involved in the setting up of the BBB. (C) 199 9 Academic Press.