CSF tracer localization after intracisternal and lumbar intrathecal infusion. (A-F) Localization of FITC-conjugated dextran (FITC-d500, MW 500kD) and Texas Red-conjugated dextran (TR-d3, MW 3 kD) 30 min after intracisternal infusion. Large molecular weight FITC-d500 remained restricted to perivascular spaces (arrows) surrounding penetrating arteries (A-B) and extending to the level of the terminal capillary beds (C-F, arrows). Small molecular weight TR-d3 moved quickly into the brain interstitium and was taken up by subpopulations of neurons (arrowheads). (G-L) 120 min after lumbar intrathecal infusion of tracers, large molecular weight FITC-d500 accumulated in perivascular spaces (arrows), but not as uniformly as observed after intracisternal infusion. Small molecular weight TR-d3 moved readily throughout the brain parenchyma. Inset (L) depicts IB4 staining and background green and red fluorescence in tissue from animals not injected with CSF tracer. GFAP: glial fibrillary acidic protein (astrocytic marker); IB4: isolectin B4 (vascular endothelial marker).