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All organisms with a complex nervous system have a well-developed blood-central nervous system barrier. In the vertebrates the central nervous system (CNS) lies behind the protective blood-brain barrier (BBB) and blood-cerebrospinal fluid barrier (BCSFB). In all mammals the BBB is formed at the level of the cerebral capillary endothelial cells and the BCSFB by the choroid plexus epithelium.
Begley et al. (Wed,) studied this question.