Bronchial metaplasia as a benign or premalignant lesion. I. Cytologic and ultrastructural discrimination between acute carcinogen effects and toxin-induced changes.

The respiratory tracts of hamsters were exposed to either of two different chemical compounds contained in cigarette smoke, the carcinogenic chemical benzo(a)pyrene or the noncarcinogenic chemical formaldehyde. At various times during or shortly after the exposure, the acute cytologic, histologic and ultrastructural responses to the two agents were compared. Both agents induced microscopically similar squamous metaplastic changes of the tracheobronchial epithelium. However, there were several cytologic and ultrastructural differences in the appearance of the chromatin, nucleoli and cytoplasm that could be used to separate carcinogen-induced from noncarcinogen-induced acute metaplastic changes. Furthermore, after cessation of the exposures, squamous metaplastic changes induced by the noncarcinogenic agent formaldehyde regressed to normal, while the acute carcinogen-induced changes were replaced by other, more persistent metaplastic changes.