In Drosophila the decision processes between the neural and epidermal fate for equipotent ectodermal cells depend on the activity of proneural genes. Members of the Drosophila Iroquois-Complex (Iro-C) positively regulate the activity of certain proneural AS-C genes during the formation of external sensory organs. We have identified and characterized three mouse Iroquois-related genes: Irx1, -2 and -3, which have a homeodomain very similar to that of the Drosophila Iro-C genes. The sequence similarity implies that these three genes represent a separate homeobox family. All three genes are expressed with distinct spatio/temporal patterns during early mouse embryogenesis. These patterns implicate them in a number of embryonic developmental processes: the A/P and D/V patterning of specific regions of the central nervous system (CNS), and regionalization of the otic vesicle, branchial epithelium and limbs.
We describe a comparative lung expression analysis of the murine Irx1 and Irx2 genes. At embryonic day 8.5 (E8.5), the Irx1 and Irx2 expression starts in the foregut region, where the laryngo-tracheal groove will form. The expression is prominent in the lung epithelium during glandular development. It declines at the end of the canalicular phase. We further compare the Irx1 and Irx2 expression domains to Gli1, 2, 3 and Mash1. Their homologues in Drosophila melanogaster are known as regulative partners of the iroquois complex. The Irx and Gli genes are coexpressed in the developing lungs at the same time. Their transcripts are not localised in the same cells but adjacent to each other in either mesenchymal or epithelial structures. It is thought that the lung development is regulated by the mesenchymal/epithelial interactions.
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