Blood-brain barrier: Physiological and functional considerations

The blood-brain barrier (BBB) is a physical frontier that separates the brain tissue from all the circulating substances in the bloodstream to keep the chemical microenvironment for the proper functioning of the brain. The BBB is formed by neurons, astrocytes, pericytes, and endothelial cells from the brain microvessels; the interaction and communication between these cell types give the BBB its low permeability. During development, the migration and phenotype acquisition of the endothelial cells is driven by factors produced by astrocytes and pericytes, triggering complex signaling pathways that lead to the developing of the barrier properties on the endothelial cells. In this chapter, we will review the events that lead to the formation of the BBB, how all the different cell types interact and communicate and how the BBB is affected during different brain pathologies.