Ultrastructure of neoplastic cells in different histologic subtypes of pulmonary adenocarcinoma

57 pulmonary adenocarcinomas were classified according to the WHO Classification of Lung Tumours. 55 cases were primary adenocarcinomas and two were metastases from the gastrointestinal tract. The acinar, papillary and bronchiolo-alveolar carcinomas were well or moderately differentiated and the solid adenocarcinomas were poorly differentiated tumours secreting mucus in intracytoplasmic vacuoles. All tumours, including the two metastases, were analyzed by transmission electron microscopy. Most of the acinar, papillary and all bronchiolo-alveolar adenocarcinomas contained a great or moderate number of secretory granules which were pale fibrillar or dense homogeneous. Among the solid adenocarcinomas there were no tumours with a great number of secretory granules. The surface fine structure of the metastatic carcinomas differed from those of the primary adenocarcinomas, having a brush-border instead of microvilli. The ultrastructure of the primary adenocarcinomas resembled the normal surface epithelial cells of bronchi and bronchioli, and two papillary adenocarcinomas contained lamellar bodies similar to granular pneumocytes. Instead of dividing into four different subtypes pulmonary adenocarcinomas can be classified according to histological grading.