Practice question

Limb outgrowth is sustained by reciprocal epithelial-mesenchymal feedback between mesoderm and ectoderm. Lateral plate mesoderm expressing Tbx5 Tbx4 under Wnt2b signals expresses FGF10, which signals via FGFR2b to overlying ectoderm, activating Wnt3a and beta-catenin to induce and maintain AER FGF8 expression. FGF10 itself is the crucial mesenchymal inducer that keeps AER functional; its loss causes FGF8 fading, progress zone arrest and severe truncation. Wnt3a induces AER initially, Gremlin antagonizes BMP to prolong AER, BMP4 terminates AER, but maintenance origin lies in continuous FGF10 signaling.