Figure 6.

Constant average microglial density across brain structures, mammalian clades, and species. A, Distribution of microglial density in the different brain structures per clade (top), average across all samples (middle), and average per clade (bottom). Values are given in Table 5. Densities vary 8.5-, 5.0-, and 5.3-fold across species in Cb, Cx, and RoB, respectively (top; values in Table 1). Invariant average microglial density across brain structures in all mammalian species together (middle; Wilcoxon p values >0.01 for each pair) and across mammalian clades (bottom; Wilcoxon p values across clades are >0.01 across brain structures and clades). B, Relationship between the density of microglial cells and the number of microglial cells across brain structures and mammalian species. p values for Spearman correlations when all structures are analyzed together, except for Primata, Cx only, are all >0.01. C, Relationship between structure mass and number of microglial cells across brain structures and mammalian species. Function plotted in C applies to all structures and species with exponent (0.962 ± 0.024, p < 0.0001, r² = 0.952). p values for each clade separately are all p < 0.0001. p values for Spearman correlations all fall to <0.01. D, Relationship between microglial density and structure mass across brain structures and mammalian species. p values for Spearman correlations when all structures are analyzed together, except for Primata, Cx only, are all superior to 0.01. E, Relationship between microglial density and neuronal density across brain structures and mammalian species. p values for Spearman correlations when all structures are analyzed together, except for Primata, Cx only, are all nonsignificant (p > 0.01). D, E, Each data point represents the values found per brain structure examined for each species colored by clade as in the key. Filled circles represent the Cx (including the hippocampus). Squares represent the Cb. Triangles represent the RoB. C, Values are exponent ± SE.