Cortical cell and neuron density estimates in one chimpanzee hemisphere

Abstract

The density of cells and neurons in the neocortex of many mammals varies across cortical areas and regions. This variability is, perhaps, most pronounced in primates. Nonuniformity in the composition of cortex suggests regions of the cortex have different specializations. Specifically, regions with densely packed neurons contain smaller neurons that are activated by relatively few inputs, thereby preserving information, whereas regions that are less densely packed have larger neurons that have more integrative functions. Here we present the numbers of cells and neurons for 742 discrete locations across the neocortex in a chimpanzee. Using isotropic fractionation and flow fractionation methods for cell and neuron counts, we estimate that neocortex of one hemisphere contains 9.5 billion cells and 3.7 billion neurons. Primary visual cortex occupies 35 cm(2) of surface, 10% of the total, and contains 737 million densely packed neurons, 20% of the total neurons contained within the hemisphere. Other areas of high neuron packing include secondary visual areas, somatosensory cortex, and prefrontal granular cortex. Areas of low levels of neuron packing density include motor and premotor cortex. These values reflect those obtained from more limited samples of cortex in humans and other primates.

Keywords: flow fractionator; isotropic fractionator; neuron density; primate neocortex; visual cortex.

Fig. 1.

(A) Lateral view of intact, adult female chimpanzee brain. (B) Complete dissection map, illustrating the 742 pieces of tissue dissected from the chimpanzee cortex. Total cell density in millions of cells per square centimeter of cortical surface is illustrated on the cortex flat map, with the darkest shading indicated areas of high cell density and lighter shading indicating low cell density. The total cell density includes all types of cells in cortex; that is, neurons, glial cells, and epithelial cells. Areal boundaries are estimated for V1, V2, somatosensory (areas 3b, 3a, 1, and 2), motor (areas 4 and 6), and premotor cortex. (C) Complete dissection map illustrating the 742 pieces of tissue dissected from the chimpanzee cortex. Total neuron density in millions of cells per square centimeter of cortical surface is illustrated on the cortex flat map, with the darkest shading indicating areas of high neuron density and lighter shading indicating low neuron density.

Fig. 2.

Pairwise comparison results of estimated marginal means based on the original scale of the dependent variable, either neuron (A) or cell (B) density in square centimeter, with mean differences significant at P < 0.05 and robust SE bars shown. All significant pairwise comparisons are highlighted by the significance bars. (A) V1 estimated marginal means of neuron density are significantly higher than all other cortical regions, whereas V2 is significantly higher than motor, premotor, and somatosensory blocks. Somatosensory predicted values are also significantly higher than motor and premotor blocks. Motor and premotor cortices do not significantly differ from one another, but are both significantly lower than all other areas of cortex. (B) V1 and the frontal region estimates show no significant differences in cell density, but each region independently contains higher means than every other area. Somatosensory, premotor, and motor cortex do not significantly differ from one another.

Fig. 3.

The cell and neuron densities of all 742 pieces are plotted based on their anterior–posterior position in the flattened cortex, and designated to a cortical area (shown in small map). Neuron (A) and cell (B) packing densities follow the same pattern, in which densities are highest in the most posterior positions and lower in more anterior positions, with the exception of a block of frontal cortex that contains higher densities. For both cell and neurons, although the correlation is low, cubic models provide a better correlation (neurons: R² = 0.424; cells: R² = 0.066) compared with linear (neurons: R² = 0.303; cells: R² = 0.011) or quadratic (neurons: R² = 0.414; cells: R² = 0.035) models. Most outliers are cell- or neuron-dense pieces in the posterior part of the brain.