Permeability and vascularity of the developing brain: cerebellum vs cerebral cortex

Rats of various postnatal ages were utilized to investigate developmental changes in the short-term distribution of [3H]mannitol, [3H]inulin and [51Cr]-tagged erythrocytes in both the cerebellum (CER) and cerebral cortex (CC). As the CNS undergoes maturation the most precipitous decrease in the 1-h radiomannitol (and inulin) space in the CER takes place during the second postnatal week whereas the greatest reduction in the spaces for both radioisotopes in the CC occurs during the earlier period between 0.5 and 1 week. The volume of distribution in immature brain tissue (1 week) into which [3H]mannitol penetrates in 1 h is substantially less (by 75--90%) than that into which this tracer permeates in non-neural tissue such as heart and liver; thus, even prior to the proliferation of glial elements, there is a limited barrier effect. At 3 weeks the permeability of the blood--brain barrier in the CER to mannitol is comparable to that in the CC. The vascularity of the CER (as estimated by residual [51Cr]erythrocyte-volume) is greater than that of the CC at each postnatal age investigated. The uptake of radiomannitol by the choroid plexus (blood--CSF barrier) in immature animals is a reflection primarily of developmental changes in volume of extracellular fluid rather than in permeability of the basolateral face of the choroidal epithelium. Qualitative differences, neonates vs adults, in the distribution of [3H]mannitol between CSF and brain tissue are attributable to maturational changes in secretory and permeability phenomena associated with the blood--CSF and blood--brain barriers.