Figure 9.

Neuronal and glial mass fractions are well predicted by measured glial cell density. (A) Estimated neuronal mass fraction (f_n) in each brain structure in each species is plotted as a function of the inverse of the measured neuronal density in the structure (d⁻¹_nmes, left) and as a function of the inverse of the measured glial density in the structure (d⁻¹_gmes, right). (B) Estimated glial mass fraction (f_g) in each brain structure in each species is plotted as a function of the inverse of the measured neuronal density in the structure (d⁻¹_nmes, left) and as a function of the inverse of the measured glial density in the structure (d⁻¹_gmes, right). Notice that neuronal (or glial) mass fraction is well predicted by variations in glial cell density, but not in neuronal cell density. Inverse of cell densities given in picograms/neuron. Functions plotted are (A) f_n = 0.265 (d⁻¹_gmes)^{0.356 ± 0.019} (r² = 0.823, p < 0.0001) and (B) f_g = 2.008 (d⁻¹_gmes)^{−0.716 ± 0.026} (r² = 0.906, p < 0.0001). Cerebral cortex plotted as circles, cerebellum as squares, and rest of brain as triangles; eulipotyphlans shown in orange, primates in red, and rodents in green. Data from Herculano-Houzel et al. (2006, 2007, 2011), Azevedo et al. (2009), Sarko et al. (2009), and Gabi et al. (2010).