Figure 1.

Natural barriers of the brain.

Two portals exist for accessing the brain – the BBB and the BCSFB. While endothelial cells comprise the primary cell type for both barriers, transmigration across the latter also involves transit across the ChP cells (see Box 1). Passage of soluble substances and ions across the BBB is modulated by adjoining endothelial cells connected by tight junctions. As the endothelial cells constituting the mature BSCFB are fenestrated, passage of soluble substances is regulated by a tight epithelium comprising ChP cells connected by tight junctions. Abbreviations: BBB, blood–brain barrier; BCSFB, blood–cerebrospinal fluid barrier; ChP, choroid plexus.