Regional differences in the stratified transitional field and the honeycomb matrix of the developing human cerebral cortex

The neurons of the cerebral cortex originate in the proliferative neuroepithelium and settle in the cortical plate during embryonic development. Interposed between these two sites is a large transitional field. We have earlier demonstrated experimentally in rats with H-3-thymidine autoradiography that this transitional field is a stratified structure composed of discrete layers of migrating and sojourning cells, and fiber bands. Here we show that the different layers of the stratified transitional field are identifiable without experimental procedures in the developing human cerebral cortex and that there are conspicuous regional differences in its stratification. At the peak of its development, the stratified transitional field contains three fibrous bands in an inside-out order: the commissural fibers of the corpus callosum, the thalamocortical and corticofugal projection fibers, and the expanding white matter. There are regional differences in the thickness of these fibrous layers as well as in the number and configuration of the perikaryal layers. This preview focuses on laminar differences of the transitional fields of the agranular frontal lobe and the granular parietal and occipital lobes. At the latter sites, but not in the frontal lobe, there is a distinctive multi-layered band, the honeycomb matrix, where radially oriented fiber columns are sandwiched between two perikaryal sublayers and are separated from one another by radially oriented cells. We postulate that the radial fiber columns of the honeycomb matrix are composed of topographically organized thalamocortical fibers and that the unspecified young neurons acquire their enduring topographic identity by making selective contacts with tagged fibers here before they resume their radial or tangential migration to the cortical plate.