Quantitative cytoarchitectonics of the posterior cingulate cortex in primates.

The cytoarchitecture of four cortical areas within the posterior cingulate region and the differences among them are quantitatively analyzed in 17 primate species. The transition from allocortex (area 29) to isocortex (areas 30, 23, and 31) is characterized by shifts in laminar proportions and cellular densities. The molecular layer and inner main lamina (layers V and VI) occupy a smaller proportion of the cortex in iso- than in allocortex, while conversely, the outer main lamina (layers II and III) makes up a larger proportion of the cortex in iso- than in allocortex. Layer IV is relatively larger in area 31 than in area 23. The packing densities of cell bodies were determined by gray-level index values. These values show that the isocortex has lower cell densities in the outer main lamina, but higher cell densities in the inner main layers. Furthermore, the granular layer of area 31 is more densely packed than it is in area 23. These results are discussed as structural correlates of a relative increase in the receptive and processing capacities of the isocortex over the adjacent allocortex.