Practice question

Evidence that mesenchyme determines limb-type identity derives from classic transplantation where leg mesenchyme was placed beneath wing AER in chick embryos. Despite being covered by wing ectoderm providing only generic FGF8 outgrowth signals, resulting appendage formed leg structures including toes, claws, scales, demonstrating instructive information resides in mesoderm expressing Pitx1, Tbx4 versus Tbx5. Reciprocal experiment produced wing. This separates permissive epithelial signal from instructive mesenchymal program and establishes lateral plate mesoderm as source of limb-type specific pattern, maintained throughout development and regeneration experiments.