TBR1

Abstract: The developing neocortex contains two types of progenitor cells for glutamatergic, pyramidal-projection neurons. The first type, radial glia, produce neurons and glia, divide at the ventricular surface, and express Pax6, a homeodomain transcription factor. The second type, intermediate progenitor cells, are derived from radial glia, produce only neurons, and divide away from the ventricular surface. Here we show that the transition from radial glia to intermediate progenitor cell is associated with upregulation of Tbr2, a T-domain transcription factor, and downregulation of Pax6. Accordingly, Tbr2 expression in progenitor compartments (the subventricular zone and ventricular zone) rises and falls with cortical plate neurogenesis. The subsequent transition from intermediate progenitor cell to postmitotic neuron is marked by downregulation of Tbr2 and upregulation of Tbr1, another T-domain transcription factor. These findings delineate the transcription factor sequence Pax6 --> Tbr2 --> Tbr1 in the differentiation of radial glia --> intermediate progenitor cell --> postmitotic projection neuron. This transcription factor sequence is modified in preplate neurons, in which Tbr2 is transiently coexpressed with Tbr1, and in the direct differentiation pathway from radial glia --> postmitotic projection neuron, in which Tbr2 is expressed briefly or not at all.

Abstract: Pallial and subpallial morphological subdivisions of the developing chicken telencephalon were examined by means of gene markers, compared with their expression pattern in the mouse. Nested expression domains of the genes Dlx-2 and Nkx-2.1, plus Pax-6-expressing migrated cells, are characteristic for the mouse subpallium. The genes Pax-6, Tbr-1, and Emx-1 are expressed in the pallium. The pallio-subpallial boundary lies at the interface between the Tbr-1 and Dlx-2 expression domains. Differences in the expression topography of Tbr-1 and Emx-1 suggest the existence of a novel “ventral pallium” subdivision, which is an Emx-1-negative pallial territory intercalated between the striatum and the lateral pallium. Its derivatives in the mouse belong to the claustroamygdaloid complex. Chicken genes homologous to these mouse genes are expressed in topologically comparable patterns during development. The avian subpallium, called “paleostriatum,” shows nested Dlx-2 and Nkx-2.1 domains and migrated Pax-6-positive neurons; the avian pallium expresses Pax-6, Tbr-1, and Emx-1 and also contains a distinct Emx-1-negative ventral pallium, formed by the massive domain confusingly called “neostriatum.” These expression patterns extend into the septum and the archistriatum, as they do into the mouse septum and amygdala, suggesting that the concepts of pallium and subpallium can be extended to these areas. The similarity of such molecular profiles in the mouse and chicken pallium and subpallium points to common sets of causal determinants. These may underlie similar histogenetic specification processes and field homologies, including some comparable connectivity patterns. J. Comp. Neurol. 424: 409 ‐ 438, 2000. © 2000 Wiley-Liss, Inc.