Frog palate mucociliary apparatus: structure, function, and response to formaldehyde gas.

The upper respiratory tract mucociliary apparatus represents one of the first defenses against inhaled noxious materials. The frog palate has been widely used as a model to investigate the mode of action of this apparatus and to study its response to irritant gases. Video analysis was used here for the determination of mucus flow rate and flow patterns, ciliary beat frequency, and the nature of ciliary activity in the in vitro frog palate preparation. The results of studies of time-lapse video recordings were used in conjunction with light microscopic and ultrastructural morphologic investigations to determine functional interactions between cilia, the epiphase, and the periciliary fluid. It was concluded that the cilia enter the epiphase during the effector stroke, that waves may be produced on the under surface of this layer, and that the periciliary fluid is less viscous than, and moves in the same direction as, the epiphase. The response of the frog palate mucociliary apparatus to formaldehyde gas was also studied using an in vitro exposure system. There were distinct concentration-related responses to formaldehyde with initial stimulation, and at higher concentrations, subsequent inhibition of mucociliary function. Stimulation of mucus flow rate was due to increased ciliary activity, while inhibition of flow, which preceded ciliastasis, was attributed to direct effect of formaldehyde on the superficial mucus layer. Ciliastasis on the other hand was considered to provide evidence that the formaldehyde had penetrated the mucus layer and induced direct toxic effects on the underlying epithelial cells.