The chirality of ciliary beats

Many eukaryotic cells possess cilia which are motile, whip-like appendages that can oscillate and thereby induce motion and fluid flows. These organelles contain a highly conserved structure called the axoneme, whose characteristic architecture is based on a cylindrical arrangement of nine doublets of microtubules. Complex bending waves emerge from the interplay of active internal forces generated by dynein motor proteins within the structure. These bending waves are typically chiral and often exhibit a sense of rotation. In order to study how the shape of the beat emerges from the axonemal structure, we present a three-dimensional description of ciliary dynamics based on the self-organization of dynein motors and microtubules. Taking into account both bending and twisting of the cilium, we determine self-organized beating patterns and find that modes with both a clockwise and anticlockwise sense of rotation exist. Because of the axonemal chirality, only one of these modes is selected dynamically for given parameter values and properties of dynein motors. This physical mechanism, which underlies the selection of a beating pattern with specific sense of rotation, triggers the breaking of the left-right symmetry of developing embryos which is induced by asymmetric fluid flows that are generated by rotating cilia.