Figure 1. Overview of the multi-scale structures at play in motile cilia.

The cilia structure and the anatomy of ciliated epithelia are well known, but how these modules give rise to well defined beating dynamics is still poorly understood. (a) Electron micrograph of a motile cilium cross section, showing the 9+2 microtubule ultrastructure, and dynein arms [48]. The cilia are approximately 200 nm in diameter. (b) Schematic of the ultrastructure, highlighting the dynein motors that run all along the axoneme [49]. At this scale, models have been proposed [14] to describe the cooperative binding/unbinding of motors, to give rise to a waveform on the cilia. (c) Fluorescence optical microscopy of a single cell from respiratory tract, with cilia labelled in green and nucleus in blue [18]. These cilia are approximately 10 μm in length. (d) Stained cross-section of an epithelial cell layer from culture, showing the cilia “carpet” at the cell/fluid interface [37].