Hydrodynamic model of directional ciliary-beat organization in human airways

In the lung, the airway surface is protected by mucus, whose transport and evacuation is ensured through active ciliary beating. The mechanisms governing the long-range directional organization of ciliary beats, required for effective mucus transport, are much debated. Here, we experimentally show on human bronchial epithelium reconstituted in-vitro that the dynamics of ciliary-beat orientation is closely connected to hydrodynamic effects. To examine the fundamental mechanisms of this self-organization process, we build a two-dimensional model in which the hydrodynamic coupling between cilia is provided by a streamwise-alignment rule governing the local orientation of the ciliary forcing. A phase transition from local mucus recirculations to a long-range unidirectional mucus flow allowing effective clearance is predicted at high ciliary density and a high mucus viscosity, as experimentally observed. In the latter case, we show that in a tube geometry mimicking a virtual bronchus, the transport direction spontaneously aligns with the distal-proximal axis.