Sonic Hedgehog (SHH) is secreted from the zone of polarizing activity (ZPA) in the developing limb bud. Its primary function is:

• A Specifying dorsoventral axis (back of hand versus palm)
• B Specifying anterior-posterior (thumb-to-little-finger) axis of the limb ✓
• C Driving proximal-to-distal outgrowth of the limb bud
• D Inducing apoptosis in the interdigital mesenchyme

Explanation

SHH secreted from the ZPA (located in the posterior mesenchyme of the limb bud) specifies the anterior-posterior identity of the digits — it establishes the thumb-to-little-finger axis. Loss-of-function SHH mutations cause oligodactyly and loss of posterior digits. Dorsoventral axis is specified by Wnt7a (dorsal ectoderm) and Engrailed-1 (ventral ectoderm). Proximal-to-distal outgrowth is driven by FGFs secreted from the apical ectodermal ridge (AER). BMP signalling drives interdigital apoptosis.

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

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Written and medically reviewed by the StethoPrep medical team.