The blood-brain barrier (BBB) is formed by the micro vasculature of the brain ( 1 ). The permeability properties, per se , of the BBB are regulated by the capillary endothelial cell ( 2 ). However, there are at least four different cells that comprise the brain microvasculature (Fig. 1 ), and all contribute to the regulation of the cerebral microvasculature and, indirectly, to the regulation of BBB permeability ( 3 ). The endothethial cell and the pericyte share a common capillary basement membrane. There is approximately one pericyte for every two to four endothelial cells. More than 99% of the brain surface of the capillaries is invested by astrocytic foot processes ( 4 ). There is innervation of the capillary by nerve endings of either intra- or extra-cerebral origin ( 5 , 6 ). The distance between the astrocyte foot process and the capillary endothelial cell and the pericyte is only 20 nm ( 7 ). Therefore, the interrelationships between the endothelium, the pericyte, and the astrocyte foot process are as intimate as any cell-cell interactions in biology. The space filled by the basement membrane and situated between the endothelium/pericyte and the astrocyte foot process forms the interface between blood and brain. Fig. 1.  The cells comprising the brain microvasculature are the capillary endothelium, the capillary pericyte, and the astrocyte foot process. In addition, nerve endings directly contact the capillary endothelial surface on the brain side of the microvasculature. From ref. 3 .