In a chick embryo, the apical ectodermal ridge is surgically excised from a developing wing bud on day 3. Which outcome is expected?

• A Complete absence of the entire limb bud
• B Dorsalisation of the limb bud mesenchyme
• C Mirror-image duplication of distal digits
• D Proximal skeletal elements form but distal structures are missing ✓

Explanation

The AER produces FGF8 and FGF4, which maintain the underlying progress zone mesenchyme in a proliferative state and drive proximodistal outgrowth. Removal of the AER halts elongation, so proximal elements already specified (humerus) form while progressively more distal elements are lost. Mirror-image digit duplication follows an ectopic ZPA graft, not AER removal.

Reference: Langman's Medical Embryology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.