Clara cell granules of peripheral lung cancers

Thirty-five peripheral carcinomas of human lungs were morphologically examined in comparison with the bronchiolar nonciliated epithelium. Clara cell granules were found not only in 21 cases of papillary adenocarcinoma, including three of classical bronchiolo-alveolar carcinoma, but also two of acinar and four of solid carcinoma. The granules of tumor cells were identical to those of bronchiolar nonciliated cells on histochemical and ultrastructural levels. Most granules consisted of a monotonous and finely granular matrix, but there were variants containing fingerprint-like lamellae and small vesicles. These variants were also found in nonciliated cells of normal bronchiolar epithelium. The amount of granules was related to the degree of tumor differentiation. It is suggested that most peripheral carcinomas of the lungs have a potential for Clara cell differentiation, irrespective of whether they exhibit any histologic appearances.