Induction of additional limb at the dorsal-ventral boundary of a chick embryo

In the early chick embryo, an apical ectodermal ridge (AER) is formed from the overlying ectoderm of the presumptive limb bud region at the dorsal-ventral (DV) boundary. We report here that the ectopic DV boundary formed in the presumptive wing, flank, and leg fields induces an ectopic AER structure. Dorsal tissue (ectoderm and mesoderm) from the presumptive wing field of stage 10 to 17 embryos was inserted into a slit in the somatopleure of the future ventral side of host embryos. The same method was used to implant ventral tissue into the future dorsal side of host embryos. After the implantation, ectopic AER was induced and an additional limb or limb-like structure developed. In related experiments, ectoderm-free presumptive wing tissue was implanted, which resulted in a considerably decreased frequency of ectopic AER formation. Further analysis of chick and quail chimeras suggests that the ectopic AER was formed from the ectodermal cells overlying the boundary of host and graft mesodermal cells. These results indicate that the DV boundary organizes the AER structure in the limb bud field of early-stage chick embryos and that the ectoderm of the grafted tissues plays an important role in this process.