An additional limb can be induced from the flank of the chick embryo by FGF4.

To elucidate what initiates formation of the limb, we have attempted to induce an additional limb from the flank of the chick embryo by infecting retrovirus or implanting cells. We report here that an additional limb can be formed from the flank when we implant fibroblast growth factor 4 (Fgf4)-expressing cells into the lateral plate mesoderm at the pre-limb bud stage. In a newly formed limb bud, expressions of both Sonic hedgehog and chick Fgf4, which are authentic morphogenetic signals from the zone of polarizing activity and the apical ectodermal ridge, respectively, are induced by the implanted cells. Thus, it is concluded that the competence for limb development is present along the flank of the chick embryo and that FGF4 applied ectopically at the pre-limb bud stage can alter the developmental fate of flank cells to become limb cells. The present experimental system will contribute to a further elucidation on how the limb is formed.