The blood-brain barrier (BBB) is unique because it has selective permeability (which also changes in some diseased conditions). The BBB is a complex system made up of a structurally distinct, continuous endothelial cell layer separating the blood from the extracellular fluid of the brain. The luminal plasma membrane of the endothelial cells is directed towards the blood, while the abluminal plasma membrane faces the brain. The presence of adhesion molecules and tight junctions between endothelial cells and the low density of pinocytes are some of the key structural features that make the BBB selectively permeable.
