Brain barrier interfaces.
A. The outmost layer of the meninges, called the dura, contains fenestrated blood vessels that provide minimal barrier function. In contrast, the middle layer, the arachnoid layer, forms a physical barrier between the cerebrospinal fluid (CSF)-filled subarachnoid space (SAS) and overlying structures, by means of the tight junctions (shown by arrows) between the outer cells of the arachnoid layer. The innermost layer, the pia, produces the basement membrane (BM) covering the cortex.
B. The neurovascular unit exhibits the proper "blood-brain barrier". The cerebral endothelial cells feature luminal tight junctions (shown by the arrow) that form the barrier of the interendothelial cleft. The pericyte cells form close associations with the endothelium and astrocyte cell projections, called astrocytic feet.
C. The choroid plexus (CP) produces the CSF and forms the blood-CSF barrier. The blood vessels in the choroid plexus are fenestrated and form a non-restrictive barrier (dotted arrows), while the apical tight junctions of the choroid plexus epithelial cells restrict intercellular passage of molecules.
D. The fetal neuroependyma is composed of neuroependymal cells which are connected to each other by strap junctions, which form a barrier restricting the passage of larger molecules, such as proteins, but not smaller molecules, such as sucrose. In the adult ependyma, there are no junctions between the cells, thus, there is no restriction of the exchange of molecules.