Tilt back to turn left

Operating an eggbeater vertically creates a vortex, but a slight tilt makes fluid move across a bowl. Julyan Cartwright (CSIC, Granada, Spain), Oreste Piro, and Idan Tuval (CSIC-UIB, Palma de Mallorca, Spain), now claim that a similar backward tilting of mouse cilia may create a flow that defines the left side of the embryo.

Flow had already been established as a determinant of left–right asymmetry, at least in mice. Leftwards flow is created by a group of ∼30 cilia in the mouse node, a fluid-filled region on the surface of the embryo. Interference with this flow creates a mirror image (situs inversus) of the normal left–right asymmetry of internal organs. Situs inversus happens in either half the cases (if mutation results in no flow and a random breaking of symmetry) or...