Practice question

Left-right asymmetry initiation involves ciliary flow-driven Nodal asymmetry conserved vertebrates from mouse to human. Motile monocilia at embryonic node generate leftward fluid flow concentrating Nodal vesicles containing Sonic hedgehog and retinoic acid. Nodal, TGF-beta family ligand, expressed left lateral plate mesoderm signaling via ALK4/7 ActRIIB receptors activating Smad2/3 and FoxH1 transcription factor inducing Pitx2 homeobox controlling organ situs dextra versus sinistra, heart looping direction. Right side antagonists Lefty1/2 and Cerl2 restrict spread via feedback. Hedgehog provides upstream node patterning but Nodal-Pitx2 cascade definitive executor. Mutations cause heterotaxy, situs inversus syndromes linking morphogen asymmetry.