title: Cell-type homologies and the origins of the neocortex.
abstract: The six-layered neocortex is a uniquely mammalian structure with evolutionary origins that remain in dispute. One long-standing hypothesis, based on similarities in neuronal connectivity, proposes that homologs of the layer 4 input and layer 5 output neurons of neocortex are present in the avian forebrain, where they contribute to specific nuclei rather than to layers. We devised a molecular test of this hypothesis based on layer-specific gene expression that is shared across rodent and carnivore neocortex. Our findings establish that the layer 4 input and the layer 5 output cell types are conserved across the amniotes, but are organized into very different architectures, forming nuclei in birds, cortical areas in reptiles, and cortical layers in mammals.
authors.affiliation: Department of Neurobiology, University of Chicago, Chicago, IL 60637, USA.
authors.forename: Jennifer
authors.initials: J
authors.lastname: Dugas-Ford
authors.forename: Joanna J
authors.initials: JJ
authors.lastname: Rowell
authors.forename: Clifton W
authors.initials: CW
authors.lastname: Ragsdale
mesh_terms: D000818:Animals; D001483:Base Sequence; D005075:Biological Evolution; D001717:Birds / Q000033:anatomy & histology* / Q000235:genetics; D017931:DNA Primers / Q000235:genetics; D018076:DNA, Complementary / Q000235:genetics; D007091:Image Processing, Computer-Assisted; D017404:In Situ Hybridization, Fluorescence; D008322:Mammals / Q000033:anatomy & histology* / Q000235:genetics; D008856:Microscopy, Fluorescence; D008969:Molecular Sequence Data; D019579:Neocortex / Q000166:cytology*; D010802:Phylogeny; D016133:Polymerase Chain Reaction; D012104:Reptiles / Q000033:anatomy & histology* / Q000235:genetics; D017422:Sequence Analysis, DNA
chemical_list: D017931:DNA Primers; D018076:DNA, Complementary