Neuronal migration and the evolution of the human brain

In non-primate mammalian species, telencephalic and diencephalic neurons originate from their respective local proliferative zones. Using vital dye labeling in organotypic slice cultures, we show that in human brain, a contingent of neurons from the ganglionic eminence of the telencephalon migrate to the dorsal thalamic association nuclei of the diencephalon. These neurons rely on homotypic-neurophilic guidance during their migration, are GABAergic, and express Dlx1/2 homeodomain-containing proteins. Similar experiments in a non-human primate and in rodent embryos did not reveal a similar migratory pathway. Migration assays demonstrated that the human dorsal thalamus attracts telencephalic cells, an effect not observed in the mouse, in which such migration is inhibited by chemorepulsive cues. These data suggest that modifications in the pattern of migratory guidance cues in the forebrain may underlie the appearance of a new migratory pathway and thus contribute to the evolutionary expansion of the thalamic association nuclei in the human.