Molecular basis of left-right asymmetry

In vertebrates visceral asymmetry is conserved along the left-right axis wi thin the body. Only a small percentage of randomization (situs ambiguus), o r complete reversal (situs inversus) of normal internal organ position and structural asymmetry is found in humans. A breakdown in left-right asymmetr y is occasionally associated with severe malformations of the organs, clear ly indicating that the regulated asymmetric patterning could have an evolut ionary advantage over allowing random placement of visceral organs. Genetic , molecular and cell transplantation experiments in humans, mice, zebrafish , chick and Xenopus have advanced our understanding of how initiation and e stablishment of left-right asymmetry occurs in the vertebrate embryo. In pa rticular, the chick embryo has served as an extraordinary animal model to m anipulate genes, cells and tissues. This chick model system has enabled us to reveal the genetic pathways that occur during left-right development. In deed, genes with asymmetric expression domains have been identified and wel l characterized using the chick as a model system. The present review summa rizes the molecular and experimental studies employed to gain a better unde rstanding of left-right asymmetry pattern formation from the first split of symmetry in embryos, to the exhibition of asymmetric morphologies in organ s.