Mucociliary clearance on the surface of the tracheal lumen is an important component of lung defense against dust mites and viruses. However, the axonemal structure that achieves effective ciliary motion and the mechanisms by which discretely distributed ciliary cells generate directional flow are unknown. In this study, we examined individual ciliary motion with 7–9-nm spatial precision by labeling the ciliary tip with quantum dots, and detected an asymmetric beating pattern. Cryo-electron tomography revealed that the densities of two inner dynein arms were missing from at least two doublet microtubules in the axonemal structure. Although the flow directions generated by individual ciliated cells were unsteady and diverse, the time- and space-averaged velocity field was found to be directional. These results indicate that the asymmetric ciliary motion is driven by the asymmetric axonemal structure, and it generates overall directional flow from the lungs to the oropharynx on sparsely distributed ciliated cells.

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