Figure 4.

Microglia/neuron ratio varies uniformly with neuronal density, but not with microglial density across cortical gray matter sites. A, Non-neuronal cells/neuron ratio (O/N) is plotted as a function of neuronal density in the gray matter sites across species and clades. p value for Spearman correlation for all clades together is p < 0.0001. Exponent, p value, and r² for the relationship plotted in A (−0.793 ± 0.009, p < 0.0001, 0.888). B, Microglia/neuron ratio is plotted as a function of neuronal density in the gray matter sites across species and clades. p values for Spearman correlations are all p < 0.0001, except for Artiodactyla (p = 0.1819). The power function plotted in B applies to all gray matter sites together and mammalian species with exponent (−1.020 ± 0.016, p < 0.0001, r² = 0.799). Exponents and p values for each clade and species are listed in Table 4. C, Microglia/neuron ratio is plotted as a function of microglial density in the gray matter sites across species and clades. p values for Spearman correlations are as follows: p < 0.0001 for all clades together and each clade separately except for Carnivora (p = 0.8414). Exponents and p values for each clade and species are listed in Table 4. Each data point represents the values found per cortical gray matter site examined for each species colored by clade as in the key.