From head to toe: conservation of molecular signals regulating limb and craniofacial morphogenesis

Recent evidence indicates that many molecules involved in generating and pa tterning the limbs also play a role during craniofacial morphogenesis. On t he surface, this is an unexpected finding given that these regions of the b ody have separate evolutionary origins, are composed of different embryonic tissues, and are quite dissimilar in their anatomy. Results from several e xperiments involving Sonic hedgehog and retinoic acid point to a remarkable conservation of the signaling pathways mediated by these morphogens across multiple organ systems. Moreover, mutants such as the extra-toes and doubl efoot mouse, and the talpid chicken also provide insights on common develop mental processes that underlie the formation of the limbs and face. The ide ntification of highly conserved aspects of morphogenesis is important for u nderstanding fundamental mechanisms of development, as well as for revealin g the common denominator of countless birth defects and providing new strat egies for their prevention and cure.