Practice question

Classic cross-species grafts proved mesenchymal instructive capacity for limb type. When wing lateral plate mesenchyme was transplanted beneath leg AER ectoderm, the resulting limb formed wing structures with feathers despite leg ectoderm, while leg mesenchyme under wing AER formed scales, claws and leg morphology. This showed AER is permissive and functionally equivalent along body axis, while underlying mesoderm contains instructive codes such as Tbx5 versus Tbx4 and Pitx1 defining forelimb versus hindlimb identity. Wnt7a or Lmx1b manipulations affect dorsoventral fate, Hox knockouts affect proximal-distal pattern but not whole limb identity determination.