Summary of the observed structural polarization and asymmetry in the axoneme and their possible importance for bending motion. The division of the axoneme into MTD groups based on the OIDLs structures. The group of MTD2–3–4 (dark green) has almost the same OIDLs in the proximal and central regions, whereas the group of MTD6–7–8 (blue) has extra OIDLs and density in the proximal region than in central regions.