Sonic Hedgehog (SHH) protein, secreted from the zone of polarizing activity (ZPA) of the developing limb bud, establishes the anteroposterior axis. When SHH signalling is experimentally duplicated on the anterior margin of the chick wing bud, the result is:

• A Failure of digit formation on both sides
• B Posterior digit duplication without anterior involvement
• C Mirror-image duplication of the posterior digits on the anterior side ✓
• D Truncation of all digits to single phalanges

Explanation

SHH from the ZPA (posterior limb bud mesenchyme) creates a morphogen gradient that specifies digit identity from posterior (digit V/little) to anterior (digit I/thumb). When the ZPA is transplanted to the anterior margin, it creates a mirror-image gradient, resulting in supernumerary posterior-type digits appearing on the anterior side in addition to the normal posterior digits — a classic mirror-image duplication. This experiment established SHH as the morphogen of the ZPA. Loss-of-function mutations in SHH lead to preaxial polydactyly in humans.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.