Three-dimensional beating pattern of the ciliary tip in the live ciliate Tetrahymena.

Ciliates utilise motile cilia, which are highly dynamic organelles protruding from the cell surface, to swim helically in a three-dimensional (3D) space. The 3D nature of their swimming behaviour and rapid ciliary beatings make its quantitative analysis difficult. Here, we quantified the 3D motion of a microbead bound to a ciliary tip in a live immobilised Tetrahymena thermophila cell using 3D tracking optical microscopy. We found that the tip of individual ciliate cilia, consisting of the 9+2 structure of the axoneme, shows semicircular counterclockwise rotation in a single plane when looking down on the cilium. The rotational trajectories of the tip consist of fast and slow strokes, with the tip path during the fast and slow strokes being an arc and linear, respectively. The direction of the fast stroke of the ciliary tip, with respect to the cell body, was from the right-anterior to the left-posterior region, which is consistent with the direction that would induce right-handed helical swimming of the Tetrahymena.