Figure 2: Barrier interfaces in brain. (a) endothelial cells (endo) in the neurovascular unit have luminal tight junctions (shown by the arrow) that form the physical barrier of the interendothelial cleft. Outside the endothelial cell is a basement membrane (bm) which also surrounds the pericytes (Peri). Around all of these structures are the astrocytic end-feet processes from nearby astrocytes. (b) The endothelial cells of choroid plexus blood vessels are fenestrated and form a nonrestrictive barrier (shown by dashed arrows) between the cerebrospinal fluid (CSF) and blood vessel (bv). The epithelial cells (ep) have apical tight junctions (shown by arrows) that restrict intercellular passage of molecules. (c) In the meninges, the blood vessels of the dura are fenestrated and provide little barrier function (not shown). However, the outer cells of the arachnoid membrane (Arach) have tight junctions (shown by arrows), and this cell layer forms the physical barrier between the CSF-filled subarachnoid space (SAS) and overlying structures. The blood vessels between the arachnoid membrane and the pial surface (PiA) have tight junctions (not shown). (d) In early development, the neuroependymal cells are connected to each other by strap junctions (shown by arrows) that are believed to form the physical barrier restricting the passage of larger molecules, such as proteins, but not smaller molecules, such as sucrose. (e) The mature adult ventricular ependyma does not restrict the exchange of molecules (shown by dotted arrows). The neurovascular unit (a), blood-CSF barrier (b), and arachnoid barrier (c) are common between developing and adult brain, whereas fetal neuroependyma (d) differs from adult ependyma (e) (cited from [19] with permission from cell press).