Illustration of the sites of brain barriers in the developing and adult brain. A. The blood-CSF barrier. A barrier between choroid plexus blood vessels and the CSF. The blood vessels in the choroid plexus are fenestrated and form a non-restrictive barrier (dotted arrows). The choroid plexus epithelial cells (cpecs) are joined by functional tight junctions towards their apical surface that stop the movement of molecules (arrows). B. The blood-brain barrier. A barrier between the lumen of cerebral blood vessels and the brain parenchyma. The endothelial cells have luminal tight junctions (arrow) that form the physical barrier stopping the movement of molecules out of the vasculature. Astrocytic endfeet are in close association of the cerebral blood vessels and form what is known as the 'neurovascular unit'. The endfeet are not necessary for blood-brain barrier integrity. C. The inner CSF-brain barrier, present only during early development. A barrier between the CSF and the brain parenchyma. The neuroependymal cells lining the ventricular wall (orange) are connected by 'strap junctions' , halting the exchange of large molecules such as proteins between the CSF and brain (arrows), but not of smaller molecules like sucrose. This barrier is not present in the adult brain due to a loss of strap junctions. There is no restriction of movement at this time. D. The outer CSF-brain barrier. A barrier between the CSF-filled subarachnoid space (sas) and overlying structures. The blood vessels in this area are fenestrated and provide little by way of a barrier, but the outer cells of the arachnoid membrane (arach) are connected by tight junctions. Abbreviations: arach, arachnoid membrane; cpec, choroid plexus epithelial cells; dura, dura mater; nu., nucleus; pia, pia mater; sas, subarachnoid space. Adapted with permission from .