A conserved principle of glial organization in the paleocortex and neocortex

The organizational principles of glia remain largely unknown despite their vital role in nervous system function. Previous work has shown that the number of glia per unit volume of neocortex is constant across mammalian species. We hypothesize that the conservation of glia volume density within brain regions might be a governing principle of organization across species. To test this hypothesis, we used stereology, light microscopy, and data available in the literature to examine five brain regions: the cerebral cortex and four brain regions that differ from the cerebral cortex and each other - the anterior piriform cortex, the posterior piriform cortex, the entorhinal cortex, and the cerebellum. We discovered two orderly relationships: First, glia volume density within a brain region was constant across species, including humans, although it significantly differed between regions, suggesting that glia density might constitute a region-specific marker. Second, the ratio of glia to neuron increased with brain volume according to a [1/4] power law in the primate frontal cortex and the neocortex, the mammalian paleocortex, and the cerebellum. These relationships show that the development of glia and neurons are coupled, and suggest that what a neural circuit computes depends as much on its glial components as on its neurons. Main PointsO_LIThe volume density of glia (i.e., number of glia per unit volume) within a brain region is con-served across mammalian species including humans. C_LIO_LIThe ratio of glia to neuron increases with bigger brains. C_LIO_LIThe volume density of glia is significantly different across functionally and architecturally dif-ferent brain regions and could function as a region-specific marker. C_LIO_LIGlia obey scaling constraints that are different from scaling constraints for neurons. C_LI