Squamous metaplasia of the respiratory tract. Possible pathogenic role in asbestos-associated bronchogenic carcinoma.

Asbestos workers who smoke have a substantially greater risk of developing bronchogenic carcinoma than nonsmokers. Squamous metaplasia often replaces the mucociliary epithelium in the respiratory tract of chronic users of cigarettes. As a result, clearance mechanisms are altered. Using both scanning and transmission electron microscopy, we examined the interaction of chrysotile and crocidolite asbestos with the metaplastic mucosa found in the bronchi of cigarette smokers and produced in cultures of hamster trachea by enriched, serum-free medium. After deposition on the squamous epithelium, both "long" and "short" fibers were either phagocytosed by, or moved between, the cells of the mucosa. The interaction of asbestos with the mucociliary epithelium differed. Most long fibers were cleared, whereas short fibers were taken up by the mucosa. The apparent increase in uptake of long asbestos fibers by the metaplastic squamous mucosa could contribute to the synergism between this unique mineral and cigarette smoke in the causation of bronchogenic neoplasms.