Practice question

Chick recombination experiments revealed epithelial-mesenchymal interaction principles underlying limb patterning. Limb type information resides in lateral plate mesoderm, while AER provides permissive outgrowth FGFs. Saunders and Zwilling transplanted leg mesenchyme beneath wing AER ectoderm; resulting limb developed as leg with toes and scales, not wing. Reciprocal graft produced wing. This proved mesenchyme instructs identity expressing TBX5 versus TBX4-Pitx1 program, while AER sustains proliferation. Such chimeras elegantly separated instructive versus permissive roles and demonstrated mesenchyme determines limb morphology during tetrapod development.