Figure 1.

Schematic diagram of the respiratory mucus layer.

The airway surface liquid (ASL) overlays the respiratory epithelia and consists of two layers: the gel layer and the periciliary layer (PCL). The gel layer contains soluble mucins MUC5AC and MUC5B, secreted primarily from goblet cells and mucous cells within submucosal glands (not shown), respectively. The soluble mucins are major contributors to the viscosity and gel-like properties of this layer which enables the impediment of airway pollutants to be cleared by mucociliary clearance. Compared to the gel layer, the PCL – the height of which is approximately that of outstretched cilia – is free of soluble mucins and is therefore less viscous, which provides favorable conditions for ciliary beating. Cilia present on the epithelial surface are rich in transmembrane (TM) mucins (MUC1, MUC4, MUC16, and MUC20) which create a glycan meshwork that increases in density closer to the cell surface, aiding the exclusion of molecules and invading pathogens. This figure represents a general schematic representation of the ciliated respiratory epithelium, thus the term 'secretory cell' may refer to different cell types including club or dense-core granulated cells of the airway epithelium [76].