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 most cases (if an artificial rightward flow is imposed).
Biologists came up with this model but lacked the fluid physics to explain how a bunch of spinning cilia could create a directional flow. The Spanish team took known data on the size, speed, and viscosity of the system and realized that viscosity would dominate over inertia, so varying the angular velocity in different parts of the cilia's rotation cycle would not give a directional effect. A set of cilia yields a set of vortices unless, and this was the group's insight, the cilia are tilted relative to the substrate. “Given that the cilia are cylindrical,” says Cartwright, “there's no other way to produce this flow.”
If cilia rotating clockwise are tilted toward the back of the embryo, the cilia sweep toward the embryo's left when they are closest to the outside of the embryo. The resulting coherent fluid flow is what Cartwright believes carries the unknown leftwards determinant. On its return journey back to the right, the cilia will be traveling closer to the substrate, in a stratum that the researchers suggest may be relatively devoid of the determinant. Although tilted cilia cannot be seen in the current images of the node, images that are higher in resolution and taken from the side rather than the top may change that situation. ▪