Epithelial cell-particle contact

The mucociliary escalator functions when inhaled particles between 2 to 10 pm are deposited on the sticky mucous lining of the tracheobronchial tree and are propelled upward by the movement of this mucous layer in response to the beat of the cilia on the ciliated epithelial cells. This phenomenon is made possible because the mucous layer is biphasic, consisting of a watery solution in direct contact with the epithelia cells in which the cilia are free to beat. This watery solution is covered by a stickier, more adhesive gel layer that can trap and hold inhaled particles. The cilia beat in the wall layer at a rate between 1000 and 1500 strokes per minute such that at the point of their maximum upward velocity, the tips of the cilia come in contact with the gel layer to propel it upward. All recovery strokes subsequently occur in the water layer. This mechanism moves the mucus upward at a rate between 1 and 3 cm/minute. Ultimately, the mucus reaches the pharynx, where it is swallowed. Disease states that either alter the mucus-producing properties of cells in the tracheobronchial tree or decrease ciliary activity will obviously have a deleterious effect on this important clearance mechanism. 

Contact with airway or alveolar epithelial cells is a prerequisite for particle uptake, and such contact depends on three factors (1) particle deposition patterns (see Chap. 3) (2) penetration of particles through the surfactant and mucus layers that line the alveoli and airways (see Sec. VIII) and (3) removal or failure to remove particles by inflammatory cells. This section will concern itself with the relation between particle removal by inflammatory cells, particularly macrophages, and particle uptake by epithelial cells. 

The major conclusion to be drawn from these studies is that persistence of free particles in the airspaces or in the airways is one of the major determinants of epithelial particle nptake (64,81,95). Particle clearance mechanisms thns serve as a defense against prolonged contact of particles with lung epithelial cells, and this defense is important, for once contact occurs, cell perturbahon from surface... 

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