The dorsoventral polarity of the presumptive limb is determined by signals produced by the somites and by the lateral somatopleure.

When it first appears at stage HH16, the wing bud is already polarized along the dorsoventral axis. To study the mechanisms leading to the establishment of its dorsoventral polarity, we decided to focus our attention on an earlier stage (HH13). Using the quail-chick chimera system, we first show that the presumptive wing mesoderm occupies the medial half of the somatopleure at the level of somites 15-20. The corresponding ectodermal area, however, will only give rise to the apical ectodermal ridge. The rest of the limb bud ectoderm originates from the ectoderm overlying the paraxial and the intermediate mesoderms for its dorsal aspect and the lateral somatopleural mesoderm for its ventral aspect. We next used five experimental paradigms to show that the dorsoventral polarity of the presumptive limb is determined by its environment. Thus, presumptive limb regions flanked on two sides by rows of somites give rise to bidorsal limb buds, indicating that the somites produce a dorsalizing factor. In addition, insertion of filters laterally to the presumptive limb region also results in bidorsal limb buds, suggesting that the lateral somatopleure produces a ventralizing factor. We propose a model in which the polarizing activity of these two signals is mediated by the morphogenetic movements of the presumptive dorsal and ventral ectoderms, which carry the dorsoventral information over the limb bud mesenchyme.