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Science 338 (6104): 206-207

Copyright © 2012 by the American Association for the Advancement of Science

Cilia Discern Left from Right

Dominic P. Norris, and Daniel T. Grimes

Although the human body shows left-right (L-R) mirror symmetry when viewed externally, the placement and patterning of the internal organs and associated vasculature are strikingly asymmetrical. In the mammalian early embryo, L-R symmetry is broken by the action of rotating cilia—small hair-like protrusions on the surface of cells—in a pit-like structure called the node (see the figure).These cilia generate a unidirectional flow of fluid across the node from right to left (1). This nodal flow breaks L-R symmetry by driving asymmetries in gene expression and Ca2+ signaling in cells at the periphery of the node. On page 226 of this issue, Yoshiba et al. (2) reveal a mechanism by which the mouse embryo senses nodal flow.

MRC Mammalian Genetics Unit, Oxfordshire OX11 0RD, UK.

E-mail: d.norris{at}

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