Limb development in vertebrates, particularly in the chick embryo model, is controlled by interplay between the limb mesenchyme and the apical ectodermal ridge (AER). The mesenchymal cells carry intrinsic information about limb identity—whether forelimb or hindlimb—while the AER acts mainly as a signaling center promoting limb outgrowth.

Outcome of Transplanting Leg Mesenchyme Beneath Wing AER

• The AER provides signals (notably FGFs) that maintain mesenchyme proliferation and allow continued limb growth. However, the identity of the limb (forelimb vs hindlimb) is determined by the mesenchymal cells themselves.

• When leg (hindlimb) mesenchyme is transplanted beneath the wing (forelimb) AER, the limb formed at that site exhibits hindlimb characteristics in the distal structures.

• This means distal hindlimb structures such as digits and bones appear at the end of the forelimb, reflecting the original identity of the transplanted mesenchyme.

Why the Other Options Are Incorrect

• (2) A complete hindlimb forms at the forelimb site: This would imply full replacement of the forelimb by a hindlimb, which is not observed. The proximal part often reflects host origin, yielding a mixed identity, not a full limb swap.

• (3) The forelimb forms normally: Forelimb structures are altered in the distal region due to the hindlimb mesenchyme influence, so this is false.

• (4) Neither limb forms due to predetermined cells: Limb development shows plasticity and interaction; cells are not irreversibly determined, and limb outgrowth proceeds with signals from AER, so this option is incorrect.

Classic Experimental Evidence

• This conclusion traces back to landmark embryological studies that demonstrated mesenchyme’s instructive role in limb identity, with the AER supporting limb bud outgrowth but not specifying limb type.

• The apical ectodermal ridge acts permissively, while mesenchymal signals are instructive in specifying limb pattern.

Conclusion

In chick embryos, transplantation of leg mesenchyme beneath wing AER results in the development of distal hindlimb structures at the forelimb site, reflecting that limb identity rests largely within the mesenchymal cells. The AER promotes outgrowth but does not override mesenchymal identity, demonstrating the dual roles in limb development.

The correct answer is:
(1) Distal hind limb structures develop at the end of the limb.