The Blood-Brain Barrier

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Abstract
"Attempts to define a blood-brain barrier for all substances in terms of a single anatomical structure have failed." This review has discussed the possibility that there may be no morphological evidence of a structural barrier; that comparisons of the fates of intravenous and intracisternal injections may be invalid; that distinctions between the brain''s behavior and that of other organs may not be meaningful; that reliance on in vitro methods to investigate a peculiarly in vivo relationship is often misplaced; and that the use of dyestuffs for these purposes should be discarded. It has been suggested that the blood-brain relationships for naturally occurring substances may be explicable in terms of central nervous system metabolism; and that the metabolic changes associated with growth and development, and with pathological lesions, may be sufficient to account for most, if not all, the changes in the relationship observed in these circumstances. The blood-brain relationship for each and every substance would, on this hypothesis, be different, and it is therefore unwise to assume an impediment for any one molecule by analogy with any other. The plea is repeated that until the rate of entry . of a metabolite into the brain can be satisfactorily demonstrated to be a restriction on its utilization, it should be assumed that it is a "reflection of, rather than a limiting factor, in, in vivo cerebral metabolism.".