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.

Distributions of Cells and Neurons across the Cortical Sheet in Old World Macaques

“…The neuron distributions follow the same posterior-to-anterior decrease in cortical neuron density that is typical in primates [Collins et al, 2010a[Collins et al, , 2016Cahalane et al, 2012;Young et al, 2013a, b], but average curve fit estimations suggest that the A-P gradient is not simply linear. Moving from posterior to anterior pieces, V1 values are the highest and are mixed with high V2 values, cortical pieces just anterior to these visual areas contain more moderate values, S1 contains pieces with high-tomoderate neuron densities, M1 contains pieces with lower-than-average neuron densities, and the most anterior pieces have moderate values.…”

“…The most detailed studies of the densities of neurons across the cortical sheet involve estimates made from many locations, most recently 742 pieces of cortex from a hemisphere of a chimpanzee [Collins et al, 2016], and previous counts from a single macaque, three baboons (and two epileptic baboons), one New World squirrel monkey, and two galagos [Collins et al, 2010a;Young et al, 2013b]. We extend these observations here to examine neuron surface density in detail across three species of Old World macaque monkeys.…”

“…Other studies have provided counts for a few locations across the cortical sheet, in support of the conclusion that cell densities are rather uniform across cortex, with the exception of a twofold increase in primary visual cortex of primates [Rockel et al, 1980;Carlo and Stevens, 2013]. In contrast, studies in rodents [Skoglund and Pascher, 1996;Herculano-Houzel et al, 2013], cats [Beaulieu and Colonnier, 1989], and primates [Collins et al, 2010a[Collins et al, , 2016Young et al, 2013a, b] have revealed consistent quantitative differences between cortical areas. Previous efforts in primates have confirmed that primary visual cortex (V1) and other secondary visual areas contain higher than average neuron numbers under a fixed amount of cortical surface area, while primary motor cortex (M1) contains lower than average neuron numbers.…”

“…All brains were bisected into the right (RH) and the left hemisphere (LH); the RH of case 10-50 was used in other studies, the LH of case 11-47 was processed in sections (as described below), and both hemispheres of case 12-58 were used in this study. The cortex was separated from the underlying subcortical structures, the pia mater was removed, and the sulci were opened with forceps to allow for manual flattening, as previously described [Gharbawie et al, 2011;Collins et al, 2016]. Each hemisphere was then placed in a glass dish with PBS, covered with a weighted glass slide, and left for a minimum of 12 h to completely flatten [Gharbawie et al, 2011].…”

Sculpting Cerebral Cortex with Serotonin in Rodent and Primate

“…In human and primate, the neuronal composition of the cerebral cortex is less homogeneous between areas with a higher level of arealisation than in rodent. Interestingly, the density of small interneurons appears very high in associative areas [60].…”