Many internal organs in vertebrates have conserved left-right (LR)orientations that are established by asymmetric gene expression during early development. The nodal flow model proposes that this `lopsided' gene expression is initiated in mice by cilia on cells producing a flow of extracellular fluid in the late gastrula node. Now, Joseph Yost's group report that a similar mechanism initiates LR asymmetry in zebrafish (see p. 1247). Using videomicroscopy, the researchers show that cilia on the dorsal forerunner cells, which form a ciliated epithelium inside the fluid-filled Kupffer's vesicle, are motile and create a fluid flow just before asymmetric gene expression starts in the embryo. Other results provide the first direct evidence that impairing cilia function in derivatives of the dorsal organizer affects LR development and identify genes upstream of cilia function. Kupffer's vesicle, the researchers conclude, is a transient embryonic organ of asymmetry in teleosts.

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