<P> Drug delivery to the brain is the process of passing therapeutically active molecules across the blood--brain barrier for the purpose of treating brain maladies . This is a complex process that must take into account the complex anatomy of the brain as well as the restrictions imposed by the special junctions of the blood--brain barrier . </P> <P> The blood--brain barrier is formed by special tight junctions between the epithelial cells that surround the brain tissue . All tissue is separated by this layer of epithelial cells, however only the brain epithelial cells have these tight junctions that do not allow larger molecules to pass between them . The structure of these tight junctions was first determined in the 1960s by Tom Reese, Morris Kranovsky and Milton Brightman . These tight junctions are formed through the tight apposition of the endothelial cells with one another . Furthermore, astrocytic, "end feet" "the terminal regions of the astrocytic processes surround the outside of the capillary endothelial cell". The astrocytes are glial cells that are restricted to the brain and spinal cord that work to maintain an appropriate environment for neural signaling . The connection between these glial and endothelial cells is not very well known but it "may reflect an influence of the astrocytes on the formation and the maintenance of the blood--brain barrier . </P>

The blood-brain barrier may prevent medications from reaching brain tissue