Résumé
The bronchial epithelium mucociliary function ensuring the lung continuous clearance relies on mucus rheological properties and cilia beating coordination. Our biophysical approach aims at understanding two points. First, the biochemical and physical parameters affecting the mucus viscoelastic properties and evaluating its use as a marker for the diagnosis of respiratory diseases. Secondly the cilia coordination and its coupling with the mucus.We performed rheological experiments on mucus extracted from Air Liquid Interface (ALI) cultures of bronchial epithelium reconstituted from bronchial biopsies. Our approach consists in combining standard macro-rheology to micro-rheology performed with optical tweezers, in order to quantify the mucus flowing behavior at different scales, from the cilia up to the mucus layer length scales.Macro- and micro-rheology give different results. At the macro-scale, we obtain an elastic plateau (1 to 2 Pa), a shear-thinning behaviour and a very low yield stress (0.05-0.2 Pa). While at the micro-scale, we obtain a viscous flow, with a very low viscoelastic modulus (10-3-10-2 Pa). Nevertheless, by applying local forces at the interfaces, we observe an important adhesion and elasticity. Mucus adhesion should be taken into account to understand the differences between these different length scales and more generally to characterize the mucus properties. We also perform original experiments using optical tweezers directly on the epithelium, to access the viscoelastic response in the various mucus layers. We obtain an increased elasticity in the vicinity to the epithelium. Moreover, comparison of physical properties of mucus from the culture and directly extracted from patient lungs, provides an additional validation of the ALI model.Finally, the specific methodology we developed to quantify cilia activity and coordination, allows us to map, the cilia spatial distribution, the beating frequency and orientation of each cilium, and their standard deviation within a cilia cluster (cell). Local quantification of the cilia activity coupled to the measurement of the mucus velocity field, should help to evaluate the efficiency of the mucociliary function and to understand the mechanisms of clearance.