Practice question

Dorsal-ventral patterning mediated by Wnt7a from dorsal ectoderm specifies dorsal mesenchyme fate via induction of LIM-homeodomain factor Lmx1b. Dorsal Lmx1b-positive cells form hairy skin, nails, and extensor tendons, while ventral ectoderm expressing En1 suppresses Wnt7a allowing ventral flexor and pad differentiation. Loss of Wnt7a ablates Lmx1b expression, producing double-ventral phenotype with ventral footpads duplicated onto dorsal surface and absence of dorsal nails. Zone of polarizing activity and SHH signaling remains intact, confirming Wnt7a exclusively instructs dorsoventral polarity and dorsal identity specification.