THE INABILITY of certain materials to pass from the circulation into the parenchyma of the central nervous system (CNS) has been attributed to a special barrier surrounding the entire system—the blood-brain barrier. This barrier is not present around certain other tissue, such as muscle, as evidenced by the fact that the same material freely passes into them.
Certain areas of the brain, however, such as the choroid plexus and the area postrema, do not show a blood-brain barrier.1-3 In these regions, the capillary endothelium is quite different from the rest of the brain parenchyma in that the endothelial cells are fenestrated and peroxidase has been observed penetrating "diaphragmatic central channels of the pores."3 Thus, it seems reasonable to consider that the endothelium of the brain is at least one structural seat of the blood-brain barrier. Indeed, in the case of the barrier to the tracer, horseradish peroxidase, passage
Hirano A, Becker NH, Zimmerman HM. Pathological Alterations in the Cerebral. Arch Neurol. 1969;20(3):300–308. doi:10.1001/archneur.1969.00480090088013
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