Morphology of blood-brain-CSF interfaces: (A) Schema of main CNS compartments and interfaces. The blood-brain and blood-CSF barriers are true barriers with tight junctions between endothelial and epithelial cells, respectively. The brain-CSF interface, because of gap junctions between ependymal (or pia-glial) cells, is more permeable than brain or spinal cord capillaries and choroid plexus. (B) Blood-CSF barrier. CP is comprised of one cell layer of circumferentially arranged epithelial cells. Plexus capillaries, unlike counterparts in brain, are permeable to macromolecules. (C) Blood-brain barrier: Endothelial cells are linked by tight junctions, conferring low paracellular permeability. Endothelial cell pinocytotic vesicle paucity reflects minimal transcytosis. (D) Brain-CSF interface: Ependymal lining in lateral ventricles permits relatively free diffusion of solutes between brain ISF and large-cavity CSF. Motile cilia at ependymal cell apex move CSF downstream to SAS. Reprinted with permission from Advanced Drug Delivery Reviews .