Diagram illustrating the location of tight junctions at the blood-brain barrier (BBB). Tight junctions between endothelial cells of the BBB limit the rate of paracellular transport of solutes between endothelial cells. These junctions are the most apical element of the junctional complex, and consist of several transmembrane and cytoplasmic proteins, including occludin, ZO-1, claudin-1 and -5, linked to an actin-based cytoskeleton. Tight junctions function as a boundary between the apical (blood) and baso-lateral (brain) side of the endothelium. Endothelial cells at the BBB are closely associated with astrocytic foot processes, which play a role in maintaining barrier integrity.