Morphology of spontaneous and induced tumors in the bronchiolo-alveolar region of F344 rats.

The morphology of spontaneous and chemically-induced metastasizing carcinomas and adenomas in the bronchiolo-alveolar region of F-344 rats was studied. Histologically, the tumors were tubulo-papillary. Ultra-structurally, they consisted of cells which formed and secreted osmiophilic lamellated inclusion bodies, a marker of alveolar type II cells. Mitotic tumor cells also demonstrated such bodies. No cells of bronchial or bronchiolar origin were found in the tumors. We conclude that in F344 rats, lung tumors located in the bronchioloalveolar region consist of alveolar type II cells exclusively and are, therefore, alveolar cell adenomas and carcinomas, respectively.