Organs such as the heart and gut are asymmetrical, but the mechanisms that control their development have been unclear. Two papers in 4 July Nature, show that beating of cilia and fluid flow in the embryos controls the position of the organs in the fully developed body.

Shigenori Nonaka and colleagues at Osaka University, Japan, used murine embryos cultured under an artificial extracellular (nodal) fluid flow. They observed that a rightward flow that was sufficiently rapid to reverse the intrinsic leftward nodal flow resulted in reversal of situs in wild-type embryos. The artificial flow was also able to direct the situs of mutant mouse embryos with immotile cilia (Nature 2002, 418:96-99).

In the second paper, Jeffrey Essner and colleagues at the University of Utah, Salt Lake City, show that the existence of node monocilia and the expression of a dynein gene that is implicated in...

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