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Lung mucus production and mucociliary clearance: methods of assessment.

The primary defence mechanism of the tracheobronchial tree against all types of inhaled particles is the mucociliary escalator. This is composed of ciliated epithelium extending from the larynx to the terminal bronchioles (16th division of Weibel, 1963), on which lies a layer of mucus, the mucus blanket. As well as ciliated columnar cells, the tracheobronchial epithelium contains goblet cells which secrete mucus, the ratio between the two being 5 to 1 with a relative decrease in the number of goblet cells from the trachea to the lung periphery. Each ciliated cell has on its surface about 200 cilia of average length 6 µm and diameter 0.2 µm. Epithelial serous and Clara cells are also secretory cells, the latter being more numerous in the bronchioles and on occasion capable of transition to goblet cells on irritation. The submucosal glands are simple tubuloalveolar structures present in the cartilage containing trachea and bronchial tree but not in bronchioles. Each gland contains both mucus and serous cells as well as myoepithelial cells which may serve to extrude secretions.