Cingulate area 32 homologies in mouse, rat, macaque and human: Cytoarchitecture and receptor architecture

Homologizing between human and nonhuman area 32 has been impaired since Brodmann said he could not homologize with certainty human area 32 to a specific cortical domain in other species. Human area 32 has four divisions, however, and two can be structurally homologized to nonhuman species with cytoarchitecture and receptor architecture: pregenual (p32) and subgenual (s32) in human and macaque monkey and areas d32 and v32 in rat and mouse. Cytoarchitecture showed that areas d32/p32 have a dysgranular layer IV in all species and that areas v32/s32 have large and dense neurons in layer V, whereas a layer IV is not present in area v32. Areas v32/s32 have the largest neurons in layer Va. Features unique to humans include large layer IIIc pyramids in both divisions, sparse layer Vb in area p32, and elongated neurons in layer VI, with area s32 having the largest layer Va neurons. Receptor fingerprints of both subdivisions of area 32 differed between species in size and shape, although AMPA/GABAA and NMDA/GABAA ratios were comparable among humans, monkeys, and rats and were significantly lower than in mice. Layers I–III of primate and rodent area 32 subdivisions share more similarities in their receptor densities than layers IV–VI. Monkey and human subdivisions of area 32 are more similar to each other than to rat and mouse subdivisions. In combination with intracingulate connections, the location, cytoarchitecture, and ligand binding studies demonstrate critical homologies among the four species.