Pericytes are required for blood–brain barrier integrity during embryogenesis

The blood–brain barrier is a gatekeeper between the central nervous system and the rest of the body, and is made up of vascular endothelial cells. Previous work upheld the notion that the barrier was formed postnatally as a result of signalling from non-neuronal cells called astrocytes to endothelial cells. Now, two independent studies demonstrate that the barrier is in fact formed during embryogenesis, with the critical factor being the interaction between blood-vessel-surrounding cells called pericytes and epithelial cells. A better understanding of the tight relationship between pericytes, neuroendothelial cells and astrocytes in blood–brain barrier function will contribute to our understanding of the breakdown of the barrier during central nervous system injury and disease. The blood–brain barrier (BBB) is made up of vascular endothelial cells and was thought to have formed postnatally from astrocytes. Two independent studies demonstrate that this barrier forms during embryogenesis, with pericyte/endothelial cell interactions being critical to regulate the BBB during development. A better understanding of the relationship among pericytes, neuroendothelial cells and astrocytes in BBB function will contribute to our understanding of BBB breakdown during central nervous system injury and disease. Vascular endothelial cells in the central nervous system (CNS) form a barrier that restricts the movement of molecules and ions between the blood and the brain. This blood–brain barrier (BBB) is crucial to ensure proper neuronal function and protect the CNS from injury and disease1. Transplantation studies have demonstrated that the BBB is not intrinsic to the endothelial cells, but is induced by interactions with the neural cells2. Owing to the close spatial relationship between astrocytes and endothelial cells, it has been hypothesized that astrocytes induce this critical barrier postnatally3, but the timing of BBB formation has been controversial4,5,6,7,8,9. Here we demonstrate that the barrier is formed during embryogenesis as endothelial cells invade the CNS and pericytes are recruited to the nascent vessels, over a week before astrocyte generation. Analysing mice with null and hypomorphic alleles of Pdgfrb, which have defects in pericyte generation, we demonstrate that pericytes are necessary for the formation of the BBB, and that absolute pericyte coverage determines relative vascular permeability. We demonstrate that pericytes regulate functional aspects of the BBB, including the formation of tight junctions and vesicle trafficking in CNS endothelial cells. Pericytes do not induce BBB-specific gene expression in CNS endothelial cells, but inhibit the expression of molecules that increase vascular permeability and CNS immune cell infiltration. These data indicate that pericyte–endothelial cell interactions are critical to regulate the BBB during development, and disruption of these interactions may lead to BBB dysfunction and neuroinflammation during CNS injury and disease.