During limb development, Sonic Hedgehog (SHH) is secreted from the Zone of Polarizing Activity (ZPA). What is the primary role of this signalling gradient in limb patterning?
- A Proximal-distal (shoulder-to-finger) axis specification
- B Dorsal-ventral axis specification
- C Anterior-posterior (thumb-to-little finger) axis specification ✓
- D Apical ectodermal ridge maintenance for outgrowth
Correct answer: C. Anterior-posterior (thumb-to-little finger) axis specification
Explanation
The Zone of Polarizing Activity at the posterior margin of the limb bud secretes SHH, creating a gradient that specifies the anterior-posterior axis (thumb = digit 1 = low SHH; little finger = digit 5 = high SHH). Proximal-distal axis is governed by the progress zone beneath the apical ectodermal ridge (AER) and FGF signalling. Dorsal-ventral patterning involves Wnt7a (dorsal) and engrailed-1 (ventral). AER maintenance involves FGF4 and FGF8.
Reference: BD Chaurasia's Human Anatomy, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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