Bronchiolo-alveolar carcinoma: a Clara cell tumor?

The cell of origin, and indeed the very existence, of bronchiolo-alveolar carcinoma have long been the subject of debate. Published cases have demonstrated the presence of type II pneumocytes in some tumors, while others have been composed of mucus-secreting bronchiolar cells. A case of bronchiolo-alveolar carcinoma is presented in which the ultrastructure indicates an origin from Clara cells, but in which some cells contain polymorphous and lamellar bodies, It is concluded that bronchiolo-alveolar carcinoma is a primary lung tumor derived from the terminal airways and in the majority of cases from the bronchiole. The value of electron microscopy in assessing such tumors is emphasized.