Differences in cytoarchitecture of Broca's region between human, ape and macaque brains

Areas 44 and 45 have been identified in non-human primates as homologs of the human Broca region. Distribution of large and smaller pyramids and the ventro-lateral localization in the posterior frontal lobe enable their identification in non-human primates. Since only humans hold the ability of language, it has been hypothesized that differences in microstructure may, together with other anatomical factors, e.g., white matter tract connectivity, volumes of cortical areas and their molecular differentiation, be responsible for the lack (non-human primates) or ability (humans) of language. We sought to identify microstructural differences, by quantitatively studying the cytoarchitecture of areas 44 and 45 using layer-specific grey level indices (volume proportion of neuropil and cell bodies) in serially sectioned and cell body stained human, bonobo, chimpanzee, gorilla, orangutan and Macaca fascicularis brains. The main results are the interspecies differences in neuropil volume relative to cell bodies in all layers of both areas which allows a grouping of the different species into three major groups: Homo sapiens has the largest, great apes a markedly lower, and macaque the lowest neuropil volume. This indicates considerably more space for local and interregional connectivity in human brains, which matches recent studies of fiber tracts and spacing of cortical minicolumns because increasing connectivity also requires more space for axons and dendrites in the neuropil. The evolutionary enlargement of neuropil is, therefore, a major structural difference between humans and non-human primates which may correspond to the underlying functional differences.