Simple mechanosense and response of cilia motion reveal the intrinsic habits of ciliates

Beating status of T. pyriformis cilia near the glass substrate. (A) Snapshots of T. pyriformis contacting the glass plate. The cell repeatedly touched and slid on the wall. (Scale bar, 20 μm.) (B) Snapshots of ciliary beating. (Top) Normal beat. (Bottom) Sticking beat touching the wall. The snapshots were taken at 10-ms intervals. (Scale bar, 10 μm.) (C) Snapshot of a cell swimming adjacent to a wall. The retained cilia standing perpendicular to the wall are identified as black areas in phase-contrast microscopy. The black points indicated by red circles are cilia stopping their stroke in several frames around the picture. (Scale bar, 10 μm.) (D) Comparison of the beating speeds in the bulk with those near the wall. The beating speeds of the cilia in bulk were estimated from the beating frequencies, and the mean cilia length was 3.2 μm (20 individual cilia). The beating speeds of the cilia contacting the wall were measured at the black points, as shown in C. (E) Flow field around the swimming cell was estimated by PIV analysis. The bold black arrow indicates the swimming direction of the cell. The analysis detected the motion of 0.5-μm silica beads at a height less than 1 μm from the bottom wall. (Scale bar, 20 μm.) (F) Intensity map of flow velocity in E. The velocity between the cell body and wall was very small but not zero.

Schematic illustrations of the simulation, indicating the geometries of the SBA and parameters. (A) Beating thrust forces (reaction force due to cilia strokes) along the surface driving the swimmer. We defined the SBA as the gray area on the bottom wall shown in the figure. If the surface is included within the SBA, the beating thrust force vanishes. (B) Parameter setup of the simulation. The shape of the force swimmer was determined by the ratio of the main axis length L1 defined by the propelling direction to the waistline diameter L2. The parameters were set as L1=2 and L2=1 or 2. L1/L2=1 corresponded to a spherical shape, and L1/L2=2 corresponded to an ellipsoidal shape. The SBA range was the defined length a. The initial angle was θ0, and the initial height was fixed at h0=2.0.

Snapshots of simulation results reproducing swimmers approaching the wall. The initial angle is θ0=60°. The nonslip wall is located at z=−2. The gray area indicates the SBA. (A) a=0.0; the spherical swimmer without SBA approached the wall and then swam away from the wall. (B) a=0.0; the ellipsoidal swimmer without SBA swam away from the wall, similar to the spherical swimmer in A. (C) a=0.3; the spherical swimmer with SBA did not swim away but stopped on the wall. (D) a=0.3; the ellipsoidal swimmer with SBA swam adjacent to the wall.

Comparison of z and x positions and swimming angle. Red lines represent trajectories without the SBA, where length a=0.0, and blue lines indicate a=0.3. (A, C, and E) Spherical swimmer. (B, D, and F) Ellipsoidal swimmer. The gray area in A and B represents the SBA. From A and B, both the spherical and ellipsoidal swimmers showed similar changes in z position. From C and D, while the spherical swimmer stopped on the x axis when a=0.3, the ellipsoidal swimmer stably slid on the wall. From E and F, the swimming angles indicate asymptotic values. The angles below 0.0° indicate swimmers swimming away from the wall. The terminal angle of the spherical swimmer was 90.0°, which corresponds to a swimmer oriented vertically against the wall. The terminal angle of the ellipsoidal swimmer was 11.9°.