**4.3 Blood-Cerebrospinal fluid barrier**

A third barrier represented by the blood-Cerebrospinal fluid barrier also serves to prevent indiscriminate entry of substances in the CNS [41]. This barrier is made up of choroid plexus epithelial cells. The blood-Cerebrospinal fluid barrier is made up of choroid plexus epithelial cells, which have smaller tight junctions than the BBB endothelia. The blood-Cerebrospinal fluid barrier prevents the entry of macromolecules into the Cerebrospinal fluid. The active transport systems of the BBB actively remove therapeutic organic acids from the Cerebrospinal fluid [42].
