Studies on limb morphogenesis II. Experiments with the polydactylous mutant eudiplopodia

Abstract

The development of the rear limb buds in embryos homozygous for eudiplopodia was studied. Eudiplopod embryos are unique among polydactylous forms in having their supernumerary digits located dorsally to the normal toe complement, which is always present. Histological examination of eudiplopod limbs from stages 17 to 26 indicated that a secondary apical ectodermal ridge (AER) forms in the dorsal ectoderm at stages 22 or 23. Prior to these stages (17–21) the histological picture was normal in all respects. Reciprocal recombinations between eudiplopod and normal limb bud components indicated that the genetic defect in this mutant is localized in the ectoderm. Limb buds composed of eudiplopod ectoderm and normal mesoderm developed a two-plane duplication typical of eudiplopod limbs while those constituted from eudiplopod mesoderm and normal ectoderm developed into normal limbs. These results are discussed in context of the role of the AER in normal limb development and its interaction with the mesoderm in the formation of terminal limb parts.