Comparison of the mammalian and avian telencephalon from the perspective of gene expression data

Pallial and subpallial morphological subdivisions of the mouse and chicken telencephalon were examined from the new perspective given by gene markers expressed in these territories during development. The rationale of this ap proach is that common gene expression patterns may underlie similar histoge netic specification and: consequently, comparable morphological nature. The nested expression domains of the genes D1x-2 and Nkx-2.1 are characteristi c for the subpallium (lateral and medial ganglionic eminences). Similar exp ression of these markers in parts of the mouse septum and amygdala suggests that such parts may be considered subpallial. The genes Pax-6, Tbr-1 and E mx-1 are expressed in the pallium. Complementary areas of the septum and am ygdala shared expression of these genes, suggesting these are the pallial p arts of these units. Differences in the relative topography of pallial mark er genes also define different regions of the pallium, which can be partial ly traced into the amygdala. Importantly, there is evidence of a novel "ven tral pallium" subdivision, which is a molecularly distinct pallial territor y intercalated between the striatum and the lateral pallium. Its derivative s in the mouse apparently belong to the claustroamygdaloid complex. Chicken genes homologous sequence-wise to these mouse developmental genes a re expressed in topologically comparable patterns during development. The a vian subpallium -the paleostriatum- expresses D1x-2 and Nkx-2.1; expression extends as well into the septum and anterior and medial parts of the archi striatum. The avian pallium expresses Pax-6, Tbr-1 and Emx-1 and also conta ins a distinct ventral pallium, formed by the neostriatum and ventral inter mediate parts of the archistriatum. The lateral pallium comprises the hyper striatum ventrale, overlying temporoparieto-occipital corticoid layer and p iriform cortex, plus dorsal intermediate and posterior archistriatum. The d orsal pallium includes the dorsal, intercalated and accessory hyperstriatum , plus the dorsolateral corticoid area. The medial pallium contains the hip pocampus and parahippocampal area. A dorsal part of the septum shares palli al molecular markers. Gene markers thus suggest common sets of molecular de velopmental determinants in either pallial or subpallial domains of the mou se and chicken telencephalon, extending all the way from the posterior pole (amygdala) to the septum. Ventral pallial derivatives identified as claust roamygdaloid in the mouse correlate with avian neostriatum and parts of the archistriatum.