Components of the BBB. (A) Vascular cast of a spinal cord showing density of the CNS vascular network. (B) Electron micrograph (EM) of a cross section of a CNS vessel depicting a relationship among endothelial cells (ECs), pericytes (PCs), and astrocytes. (C) Magnified EM of ECs depicting a relationship among ECs (with tight junctions [TJ]), PCs, basement membranes (BMs), and astrocyte endfeet (AE). (D) Schematic representation of the cell types within the neurovascular unit. (E) Immunofluorescence micrograph depicting relationship of PCs (red) with ECs (green). (F) Micrograph depicting relationship of astrocytes (red-labeled with GFAP-cre; Rosa-tdTomato) with blood vessels (unstained). Astrocytes extend processes that ensheath the blood vessels, such that the outline of the blood vessels can be visualized by the endfeet of these processes (courtesy of Matthew Boisvert and Nicola Allen).