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 e stablishment of its dorsoventral polarity, we decided to focus our att ention on an earlier stage (HH13). Using the quail-chick chimera syste m, we first show that the presumptive wing mesoderm occupies the media l half of the somatopleure at the level of somites 15-20. The correspo nding ectodermal area, however, will only give rise to the apical ecto dermal ridge. The rest of the limb bud ectoderm originates from the ec toderm overlying the paraxial and the intermediate mesoderms for its d orsal aspect and the lateral somatopleural mesoderm for its ventral as pect. We next used five experimental paradigms to show that the dorsov entral polarity of the presumptive limb is determined by its environme nt. Thus, presumptive limb regions flanked on two sides by rows of som ites give rise to bidorsal limb buds, indicating that the somites prod uce a dorsalizing factor. In addition, insertion of filters laterally to the presumptive limb region also results in bidorsal limb buds, sug gesting that the lateral somatopleure produces a ventralizing factor. We propose a model in which the polarizing activity of these two signa ls is mediated by the morphogenetic movements of the presumptive dorsa l and ventral ectoderms, which carry the dorsoventral information over the limb bud mesenchyme.