Abstract
A tumor-resistant strain of 10-week-old Swiss-Webster mice was exposed to an ozone concentration of 2.5 ppm for two hours daily for 120 days. Groups of ten animals were killed periodically and the tracheobronchial tree was studied by light and electron microscopy. After the 45th day of exposure the trachea showed progressive metaplasia toward a squamoid epithelium containing many cytoplasmic secretory vesicles, with accompanying decrease in number of ciliated cells. Papillomatous proliferation of the bronchiolar epithelium occurred, together with abnormal cilia and micronodular hyperplasia of Clara-like cells. The tracheal and bronchiolar changes appeared to be reversible 120 days after the last exposure. However, micronodular hyperplasia of Clara-like cells remained unchanged in frequency and morphologic appearance.
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