Sonic hedgehog (SHH) protein secreted by the zone of polarizing activity (ZPA) of the developing limb bud controls which aspect of limb patterning?
- A Proximal–distal (shoulder-to-hand) axis
- B Dorsal–ventral (back-of-hand to palm) axis
- C Anterior–posterior (thumb-to-little finger) axis ✓
- D Long bone elongation via the apical ectodermal ridge
Explanation
The ZPA, located at the posterior (ulnar) margin of the limb bud, secretes SHH to establish the anterior–posterior axis. High SHH concentrations pattern posterior digit identities (digits 4 and 5), while low/absent SHH at the anterior margin specifies digit 1 (thumb). Loss-of-function SHH mutations cause oligodactyly with posterior digit loss; ectopic ZPA transplantation to the anterior margin creates mirror-image digit duplication. Proximal–distal axis is controlled by FGFs from the apical ectodermal ridge (AER) working with Hox genes. Dorsal–ventral axis is controlled by Wnt7a (dorsal ectoderm) and Engrailed-1 (ventral ectoderm).
Reference: BD Chaurasia's Human Anatomy, 8th ed.
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Written and medically reviewed by the StethoPrep medical team.