Fig. 1

Airway Surface Liquid (ASL) and Localization of the Major Mucins in Healthy Airways. a General schematic representation of mucus secretion from goblet cells and submucosal glands. The proposed structure of MUC5AC (threads in dark green) and MUC5B (bundles/strands in bright green) is shown. Mucociliary transport (MCT) of inhaled pathogens and particles (orange spheres of different sizes) is shown with a blue arrow. b Schematic representation of the generally-accepted structure of ASL. The periciliary layer (PCL) is estimated to be ~ 7 μm thick under normal conditions. Mucus layer thickness varies among individuals and in the different parts of the airway of the same individual (up to 70 μm) under normal conditions. c ASL gel-on-brush model with localization of large airway mucosal epithelium-expressed membrane-tethered mucins (MUC1 = purple, MUC4 = dark blue and MUC13 = pink and MUC16 = brown) and their interactions with secreted gel-forming (MUC5AC = dark green, MUC5B = bright green) and monomeric mucins (MUC7 = light blue; only depicted as incorporated in the gel-layer). Globular, non-mucin proteins that are secreted by different cells and incorporated within the gel mesh are represented in yellow dots in b) and c). MUC8 and MUC19 are omitted due to the sparsity of data on their secretion and localization in normal respiratory tissues. The MUC2 gel-forming mucin has also been omitted in this figure due to the very low levels of expression and secretion in normal airways (see text for references). MUC20, MUC21 and MUC22 were also omitted