The linear organization of cell columns in human and nonhuman anthropoid Tpt cortex

Neurons in the cerebral cortex are organized horizontally into laminae and vertically into columns and modules. Little is known about the structural variation of neuronal organization in the vertical (pia to white matter) dimension. We describe here a new computer-assisted methodology that quantifies the linear arrangement of cells and shows how cortical columns in a homologous region differ by species and age. Perikarya in eulaminate temporal cortex, Tpt, were segmented from the background on the basis of their optical densities and sizes in human, rhesus (Macaca mulatta), and chimpanzee (Pantroglodytes) brains. Within each lamina, the two-dimensional arrays of neurons were divided into repetitive, objectively defined vertical clusters. Following this, ratios and indices quantified the displacement of perikaryal centroids from the central axis and from the center point in each cell cluster. The extremely linear and vertical arrangement of cells in the prelaminated fetal cortical plate served as the template to which the other arrays were compared. In all species, the linear arrangements of perikarya in lamina III, and to a lesser extent, in lamina V, closely resemble that of the early fetal template, whereas perikaryal arrangements in layers II and IV diverge from the template formation. Corroborating subjective visualization, each lamina had its own 'fingerprint'. As expected, cell density is less in the species with larger brains, with most of the differences in density coming from increased spacing between cellular columns rather than among the cells within columns. Not all aspects of perik-aryal organization alter when bigger brains are compared with smaller ones. Although chimpanzee brains are about four times bigger than those of rhesus monkeys and human brains are about three times larger than chimpanzee brains, absolute measures of cellular linearity in chimpanzees and rhesus monkeys resemble each other more closely than the same measures do in humans and chimpanzees. After accounting for differences in interval widths, the parameters of linearity sorted on the basis of brain weight in pyramidal cell layers III and V, but not in the stellate cell layers II and IV. Human perikarya have the widest horizontal dispersion and this displacement is most pronounced in layer II, least in layer III.