Abstract
The respiratory tract of larger animals is cleared by sweeping bundled strands along the airway surface. These bundled strands can be millimetric in length and consist of MUC5B mucin. They are produced by submucosal glands, and upon emerging from these glands, the long axis of the bundled strands is oriented along the cilia-mediated flow toward the oral cavity. However, after release, the bundled strands are found to have turned orthogonal to the flow, which maximizes their clearance potential. How this unexpected reorientation is accomplished is presently not well understood. Recent experiments suggest that the reorientation process involves bundled strands sticking to MUC5AC mucus threads, which are tethered to the goblet cells. Such goblet cells are present in small numbers throughout the airway epithelium. Here, we develop a minimal model for reorientation of bundled mucus strands through adhesive interactions with surface goblet cells. Our simulations reveal that goblet cell interactions can reorient the bundled strands within 10 mm of release-making reorientation on the length scale of the tracheal tube feasible-and can stabilize the orthogonal orientation. Our model also reproduces other experimental observations such as strong velocity fluctuations and significant slow-down of the bundled strand with respect to the cilia-mediated flow. We further provide insight into the strand turning mechanism by examining the effect of strand shape on the impulse exerted by a single goblet cell. We conclude that goblet cell-mediated reorientation is a viable route for bundled strand reorientation, which should be further validated in future experiment.