Pallial and subpallial derivatives in the embryonic chick and mouse telencephalon, traced by the expression of the genes Dlx-2, Emx-1, Nkx-2.1, Pax-6, and Tbr-1

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 D1x -2 and Nkx-2.1, plus Pax-g-expressing migrated cells, are characteristic fo r 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 t he Tbr-1 and D1x-2 expression domains. Differences in the expression topogr aphy of Tbr-1 and Emx-1 suggest the existence of a novel "ventral pallium" subdivision, which is an Emx-1-negative pallial territory intercalated betw een the striatum and the lateral pallium. Its derivatives in the mouse belo ng to the claustroamygdaloid complex. Chicken genes homologous to these mou se genes are expressed in topologically comparable patterns during developm ent. The avian subpallium, called "paleostriatum," shows nested D1x-2 and N kx-2.1 domains and migrated Pax-6-positive neurons; the avian pallium expre sses Pax-6, Tbr-1, and Emx-1 and also contains a distinct Emx-1-negative ve ntral 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 o f such molecular profiles in the mouse and chicken pallium and subpallium p oints to common sets of causal determinants. These may underlie similar his togenetic specification processes and field homologies, including some comp arable connectivity patterns. (C) 2000 Wiley-Liss, Inc.