Left-right asymmetry of internal organs is widely distributed in the animal kingdom. The chick and mouse embryos have served as important model organisms to analyze the mechanisms underlying the establishment of the left-right axis. In the chick embryo many genes have been found to be asymmetrically expressed in and around the node, while the same genes in the mouse show symmetric expression patterns. In the mouse there is strong evidence for an establishment of left-right asymmetry through nodal cilia. In contrast, in the chick and in many other organisms left-right asymmetry is probably generated by an early-acting event involving membrane depolarization. In both birds and mammals a conserved Nodal-Lefty-Pitx2 module exists that controls many aspects of asymmetric morphogenesis. This review also gives examples of divergent mechanisms of establishing asymmetric organ formation. Thus there is ample evidence for conserved and non-conserved strategies to generate asymmetry in birds and mammals.

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