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Left-Right Patterning

Human anatomy is asymmetric with the heart located on the left side and the liver on the right. Asymmetry is a feature common to all vertebrates and even paired-symmetric organs such as lungs display asymmetries in the number of lobes. Evidence that nodal signaling is responsible for left-right specification comes from genetic analysis of organisms deficient in left-right specification. These genetic studies led to identification of mutations in components in the nodal signaling pathway such as ActRIIB, Criptic, and FoxH1 in mouse. These studies found that the left-right symmetry is created as a result of nodal antagonist expression on the right side of the embryo which is balanced by nodal upregulating itself on the other half of the embryo. The result is a nodal gradient that is high on the ventral side of the embryo and, through antagonist action, declines as a gradient to the midline. Studies on the nodal signaling pathway and its downstream targets such as PITX2 in other animals have shown it may also control left-right asymmetric patterning in sea squirt, amphioxus, sea urchin and [mollusc]] lineages.