Schematic diagram of the structure of the blood–brain barrier (BBB). The BBB is created by the tight apposition of endothelial cells lining blood vessels in the brain, forming a barrier between the circulation and the brain parenchyma. (A) A thin basement membrane, composed of laminin, fibronectin, and other proteins, surrounds the endothelial cells, associated astrocytes, and pericytes, providing both mechanical support and a barrier function. (B, C) Electron micrograph of an endothelial type II tight junction (simple overlap; black arrows). Magnification is 9,000× (B) with a magnified inset of 60,000× (C). Scale bars are 500 nm respective to the magnification.