General Introduction to Barrier Mechanisms in the Central Nervous System

There are five exchange interfaces between the peripheral circulation (blood), the cerebrospinal fluid (CSF) and the brain: (i) meninges, (ii) blood vessels, (iii) choroid plexuses, (iv) circumventricular organs and (v) ependyma (neuroependyma in embryos). All five interfaces have distinctive morphological and physiological properties; the first three are characterised by intercellular tight junctions that provide important structural basis for limiting molecular exchange across their interfaces. Cells that form these interfaces are also sites of extensive exchange mechanisms (transporters) that control entry and exit of a wide variety of molecules into the brain. Secretion of CSF by the choroid plexuses which flows through the ventricular system, and the exchange of substances between the CSF and brain is an important mechanism for the control of the characteristic composition of the brain interstitial fluid. Understanding of the complexity of barrier mechanisms is essential for evaluation of the effects of inflammatory conditions affecting the brain, whether in the adult or during development.