Spatial heterogeneity of cilia contributes to directed flow generation

In living organisms arrays of thousands of micrometer-scale motile cilia coordinate over centimeters to transport fluid. Tissues need to accommodate a variety of specialized cell types, thus cilia do not cover surfaces uniformly. However, how the density and localization of cilia impact flow generation is unknown. Here we combine measurements of cilia organization in the mouse airway with a reduced order hydrodynamic model, to study how spatial organization of cilia integrates across scales to produce long-range flows. Our measurements show that ciliated cells are uniformly distributed but occupy only a fraction of the total surface of the tissue. Furthermore, we measure basal body alignment and tissue-scale cilia orientation (from nm to cm) and find large variations in the local and global orientation of cilia. Despite this spatial heterogeneity, flow measurements show that ciliated cells produce large-scale directed steady flows. Using our model we explore the robustness of the flow to changes in density and orientation of cilia. We find that a fractional coverage of the area by ciliated cells allows the flow to be robust to changes in cilia orientation. Altogether our results highlight the importance of collective cilia properties for flow generation by cilia arrays.