Blood brain barrier and blood CSF barrier

Comparison between the structure of the Blood-Brain Barrier (BBB) and the blood-CSF barrier. (A) BBB separates the lumen of the brain capillaries from the brain parenchyma. The main contribution to the BBB property of reduced permeability comes from the tight junctions (drawn in violet) among endothelial cells lining the capillaries. The so-called neurovascular unit also comprises the pericytes, a basement membrane surrounding both pericytes and endothelial cells and astrocyte end-feet processes from nearby astrocytes. As well as the undisputed role of the tight junctions in sealing the interendothelial cleft, all the elements of the neurovascular unit are likely to contribute to some extent to the augmented selectivity of the BBB. That said, their role is still controversial; (B) The Blood-CSF barrier is found in the choroid plexus of each ventricle of the brain. Unlike the endothelium in the brain parenchyma, capillaries of the choroid plexus have no tight junctions and are fenestrated. However, the choroid plexus is delimited overall by a monolayer of tight-junctioned epithelial cells. This particular epithelium is in direct continuity with the ependymal layer lining the ventricle, though the rest of the ependymal layer is much more permeable. Therefore, unlike the BBB, the blood-CSF barrier is located at epithelial level, while capillaries are relatively leaky and permeable to small molecules, thus allowing, among other processes, the rapid delivery of water through the bloodstream to the surrounding epithelial cells for CSF production in the choroid plexus. Similarly, to what can be found in other tissues of the body, also in the choroid plexus pericytes and a basement membrane wrap around the endothelial cells. Although in principle both the barriers serve the same defensive purpose for the CNS, their distinct structure allows the interchange of different substances between bloodstream and brain.