Practice question

Dorsal ectoderm Wnt7a induces Lmx1b to dorsalize limb mesenchyme and overlying skin. Knockout of Wnt7a removes dorsal cue, allowing ventral program driven by Engrailed-1 and BMP to expand dorsally, resulting in biventral limbs with ventral skin, volar foot pads on both sides, absent nails, and ventralized musculature and tendons resembling double sole phenotype. Limb still forms because outgrowth depends on FGF10-FGF8 loop, not Wnt7a. Digit fusion results from BMP antagonism defects, not Wnt7a loss. Thus absent dorsal structures characterize Wnt7a deletion, illustrating DV axis failure and Lmx1b misregulation downstream.