Practice question

Dorsal-ventral limb polarity originates from non-AER ectodermal signals rather than ridge. Wnt7a transcript localized specifically to dorsal ectoderm induces LIM homeodomain transcription factor Lmx1b in dorsal mesoderm, conferring dorsal characteristics including nails, dorsal tendon pattern, hair orientation and pigmentation. Ventral ectoderm expresses Engrailed1 repressing Wnt7a and activating BMP signaling promoting ventral fates like footpads. Ectoderm rotation experiments inverting DV orientation produce inverted limb polarity, confirming ectodermal instruction. Wnt7a also contributes to maintaining SHH posteriorly, integrating axes, but primary function remains dorsalizing mesenchyme through Lmx1b induction pathway.