Papillary adenoma of the lung with lamellar and electron dense granules. An ultrastructural study.

An unusual papillary adenoma in the periphery of the lung was resected in a 25-year-old woman. Examination of the tumor showed a noninfiltrating cellular neoplasms consisting of cuboidal to columnar epithelial cells with eosinophilic granular cytoplasm and without nuclear atypia. Ultrastructurally the tumor cells had microvilli, infrequent cell junctions, and cytoplasmic dense granules and whorled lamellar membrane membrane inclusions. These findings suggest that this benign pulmonary neoplasm has morphologic characteristics consistent with a Clara Cell and alveolar type II pneumocyte differentiation. Although tumors of similar types can be experimentally induced in mice, and the light microscopic features of similar human neoplasms have been described, this is the first report of the ultrastructural characteristics of a benign human lung tumor with morphologic features resembling Clara cell and alveolar type II pneumocytes.