Mucociliary transport and epithelial morphology with elongation and collapse in rat trachea.

Mucociliary clearance in the trachea depends on a close relationship between structure and function of the components of its mucociliary system. Current explanations of normal mucociliary function denote narrow tolerances to change in factors such as the depth of secretions lining the lumen and the distribution of cilia. This study centered on changes in airway length under physiological conditions in the rat. Following extension of the head, the length of the trachea increased 50% without change in diameter. The rate of mucociliary clearance did not change with head position, averaging 3.79 +/- 1.03 mm/min in the flexed and 3.81 +/- 1.10 mm/min in the extended position. Extension of the head caused surface epithelial cells to elongate longitudinally and to decrease in height. These changes were greater in ciliated cells than in mucus-containing cells, thus accentuating the degree of protrusion of the secretory cells into the lumen of the stretched trachea. It is proposed that, during head extension, the relative protrusion of secretory cells into the lumen added bulk to the volume otherwise occupied by luminal secretions, and that these consistent changes in cell geometry served to adjust the height of ciliary tips within the periciliary fluid, thus mitigating fluctuations in the depth of the fluid. It was concluded that the mucociliary system of tracheas subjected to substantial change in configuration supported clearance because spatial relationships of essence to mucociliary function were maintained.