Sonic Hedgehog (SHH) secreted from the zone of polarizing activity (ZPA) in the developing limb bud establishes which pattern?

• A Proximal-distal (shoulder to finger) axis
• B Anterior-posterior (thumb to little finger) axis ✓
• C Dorsal-ventral (back to palm) axis
• D Left-right axis of the whole embryo

Explanation

The ZPA at the posterior margin of the limb bud secretes SHH, which patterns the anterior-posterior axis of the limb — determining digit identity from thumb (anterior, low SHH) to little finger (posterior, high SHH). The proximal-distal axis is governed by the apical ectodermal ridge (AER) and FGFs. The dorsal-ventral axis is patterned by Wnt7a (dorsal) and Engrailed-1 (ventral). Left-right asymmetry involves nodal cilia and Nodal signalling, not SHH from the ZPA.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.