Cortical cell and neuron density estimates in one chimpanzee hemisphere

Significance Chimpanzees are our closest relatives, and understanding the organization of their brains can help us understand our own evolution. Here we present a detailed examination of cell and neuron densities across the chimpanzee cortex. We found similarities to other mammals, including primary sensory areas with high neuron densities and a trend of decreasing neuron densities along the posterior to anterior axis of the cortex. However, we also found a prefrontal region with anomalously high neuron density that disrupts the trend of decreased neuron densities in frontal brain regions. The data reported here allow valuable comparisons among the brains of our close relative and those of humans and other primates. 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 cm2 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.

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