Conservation of glial density and cell-type ratios within a brain region across mammals

Glial cells in the brain are as numerous as neurons, but their organization is less understood. One view suggests that glial organization is similar across brain regions. To better understand glial organization, we examined functionally and architecturally diverse neural circuits in humans, other primates, carnivores, and rodents using histology and single nucleus RNA sequencing datasets. We focused on microglia, oligodendrocytes, and astrocytes, which constitute the major glial cell types. Across mammals, while glial volume densities and proportions of glial cell types are preserved within a region, they vary across regions in the brain, suggesting that glial organization is not uniform but circuit-dependent. Additionally, the ratio of glia to neurons increases with brain volume according to a $\frac{1}{4}$ power law in the primate frontal cortex, the neocortex, the piriform cortex, and the cerebellum. These findings show that glial and neuronal development and function are tightly coupled, and a circuit's function depends as much on its glia as its neurons.