Figure 5.

Average percentages of microglial cells vary across brain structures but correlate with numbers of non-neuronal cells of which they represent a constant ∼7%. A, Distribution of the percentage of all cells that are microglial cells in the three main brain structures per clade. The ranges of distribution and means of the percentage of all cells that are microglia differ across clades and brain structures (values in Table 5). B, Average percentages of total cells that are microglia differ across brain structures. Wilcoxon p < 0.0001 between Cb and both Cx and RoB in all mammalian species. Wilcoxon p = 0.4283 between Cx and RoB in all mammalian species. C, Average percentages of total cells that are microglia in brain structures across clades and species. In the Cb, significant differences are found between Marsupialia and both Artiodactyla and Carnivora (Wilcoxon, p = 0.0058 and p = 0.0020, respectively). In the Cx and RoB, no significant differences are found between clades, Wilcoxon p values of all pairs being ≥0.01. Within clades, significant differences are also found between the Cb and the Cx and RoB for each clade, except Afrotheria and Artiodactyla: Carnivora, between Cb and Cx Wilcoxon p = 0.0081 and between Cb and RoB Wilcoxon p = 0.0051; and Marsupialia, between Cb and Cx Wilcoxon p = 0.0003 and between Cb and RoB Wilcoxon p = 0.0003. D, Percentages of microglial cells among all cells across all brain structures and species correlate strongly with the neuronal cell density in the sample. The power function plotted in D applies to brain structures together and mammalian species with exponent (−0.378 ± 0.028, p < 0.0001, r² = 0.709). Spearman ρ values are indicated between brackets for each clade when significant (otherwise, indicated as nonsignificant) with respective p values falling to <0.01, except for Artiodactyla (p = 0.0386; nonsignificant) and Primata (p = 0.6932). E, Distribution of the percentage of non-neuronal cells that are microglial cells in the three main brain structures per clade. The ranges of distribution and means of the percentage of non-neuronal cells that are microglia are similar across clades and brain structures (values in Table 5). F, Average percentages of non-neuronal cells that are microglia represent a constant ∼7% across brain structures and clades in all mammalian species. Wilcoxon p values >0.01 for all pairs of structures. G, Average percentages of non-neuronal cells that are microglia in brain structures across clades and species. Across each structure, all Wilcoxon, p > 0.01. Within each structure across all clades, all Wilcoxon, p > 0.01. H, Percentages of microglial cells among non-neuronal cells across all brain structures and species fail to correlate significantly with the neuronal cell density in the sample. Spearman, all p values >0.01. D, H, 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. B, C, *Significance.