Monocilia on cells of the primitive node rotate to generate a leftward flow of extracellular fluid. Disruption of this step leads to randomisation of organ laterality because the downstream asymmetric cascade, acting through Nodal and Lefty-2, fails to activate which transcription factor in the left lateral plate mesoderm?

• A SOX9
• B PITX2 ✓
• C PAX6
• D NKX2.5

Explanation

Leftward nodal flow at the primitive node initiates asymmetric expression of Nodal in the left lateral plate mesoderm; Nodal, together with its antagonist Lefty-2, induces PITX2, the transcription factor that executes left-sided morphogenesis of heart, gut and lungs. Loss of nodal ciliary function produces situs ambiguus or situs inversus. NKX2.5 is a cardiac progenitor factor expressed bilaterally, so it does not encode sidedness.

Reference: Larsen's Human Embryology, 5th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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