Left–Right Patterning: Breaking Symmetry to Asymmetric Morphogenesis

Vertebrates exhibit striking left–right (L–R) asymmetries in the structure and position of the internal organs. Symmetry is broken by motile cilia-generated asymmetric fluid flow, resulting in a signaling cascade – the Nodal–Pitx2 pathway – being robustly established within mesodermal tissue on the left side only. This pathway impinges upon various organ primordia to instruct their side-specific development. Recently, progress has been made in understanding both the breaking of embryonic L–R symmetry and how the Nodal–Pitx2 pathway controls lateralized cell differentiation, migration, and other aspects of cell behavior, as well as tissue-level mechanisms, that drive asymmetries in organ formation. Proper execution of asymmetric organogenesis is critical to health, making furthering our understanding of L–R development an important concern.