The Difference in Vascular Volume between Cerebrum and Cerebellum is in the Pia Mater

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Previous studies using intravascular tracers have shown that the apparent vascular volume in the cerebellum is 10-60% higher than that in the cerebrum. We questioned whether the extravascular volume in the cerebellum could be accounted for by the vasculature of the pia mater that covers its highly infolded surface. Estimates of vascular volume were made using a previously reported point-counting method. Two counts were done: one in which only intraparenchymal vessels were included, and a second one in which both intraparenchymal vessels and pial vessels were included. We found no differences in intraparenchymal vascular volume between cerebellum and cerebrum. When the pial vessels are included, however, the cerebral vascular volume increases by less than 6%, whereas the cerebellar vascular volume increases by greater than 30%. We suggest that the higher cerebellar vascular volume measured using intravascular tracers is due to inclusion of the pial vasculature. Since pial vessels do not express blood-brain barrier characteristics as prominently as intraparenchymal vessels, we further suggest that estimates of barrier permeability in cerebellum should not be made using simple models developed for cerebral tissue.