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

• A Proximal-distal axis of the limb (shoulder to fingertip)
• B Dorsal-ventral axis of the limb (knuckle to palm surface)
• C Apical ectodermal ridge maintenance and outgrowth
• D Anterior-posterior axis of the limb (thumb to little finger) ✓

Explanation

SHH is secreted from the ZPA located at the posterior margin of the limb bud and acts as a morphogen establishing the anterior-posterior (thumb-to-little finger, or preaxial-to-postaxial) axis. Its concentration gradient determines digit identity: low SHH → thumb, high SHH → little finger. The proximal-distal axis is controlled by FGFs from the apical ectodermal ridge (AER) and Wnt/BMP signaling. The dorsal-ventral axis is established by Wnt7a (dorsal) and Engrailed-1 (ventral). Polydactyly can result from ectopic SHH activation.

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

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Written and medically reviewed by the StethoPrep medical team.