Inequality made equal

Forces generating asymmetry give rise to left–right (LR) differences in internal organs such as the lungs and liver. Now, Yasuhiko Kawakami, Juan Carlos Izpisua Belmonte, and colleagues (Salk Institute, La Jolla, CA), and Julien Vermot and Olivier Pourquié (Stowers Institute for Medical Research, Kansas City, MO) show that these forces are buffered by the action of retinoic acid (RA) to ensure symmetry in vertebrae and muscle formation.

Vertebrae and muscle are derived from the somites, which form as symmetric segmentations along the anterior–posterior axis. These segments form near the node, a mass of cells that provides positional information to organize the body plan. In mice and chicks, the node contains ciliated cells that generate fluid movement to produce LR asymmetry.

The Salk group shows that this ciliated system is conserved in zebrafish. They...