Abstract
The blood-brain barrier (BBB) isolates the CNS from the rest of the organism. The anatomic foundation of this structure is a syncytium of cerebrovascular endothelial cells sealed together by tight junctions. The endothelial basal lamina, located at the abluminal side of endothelium, borders the perivascular (Virchow-Robin) space. The perivascular space is continuous with the subarachnoid compartment and is itself bordered by the glia limitans, formed by astrocytic and microglial end feet. Within the perivascular space, on either side of the endothelial basal lamina, dwell a unique population of phagocytic cells variously termed perivascular macrophages, perivascular microglia, perivascular c:ells or pericytes. These cells form a first line of defense once the BBB is breached. Astrocytes are also important for BBB development and function: their processes attach to the basement membrane shared with the endothelial cells and perivascular cells. The close contact between astrocytes and brain endothelial cells suggests that both cell types interact in maintaining the function of the BBB, a bidirectional interaction that is mediated at least in part by cytokine-mediated signaling (Selmaj, 1996: Johansson, 1990).