Electron microscopic sub-classification of small cell carcinoma of the lung.

An electron microscopic study of 28 small cell carcinomas of the lung is presented. Cytoplasmic secretory granules, characteristic of endocrine cells of the human foetal lung were observed in a variable number of tumor cells. Two groups of tumors could be distinguished based on the morphology of the cytoplasmic secretory granules. Twenty-three tumors showed cells with granules resembling type 1 or P1 cells of the human fetal lung, and 5 tumors with granules resembling type 3 cells of the human fetal lung. No relationship was found between the light microscopic WHO classification of small cell carcinoma of the lung and the results obtained by electron microscopy. Increased serum calcitonin as well as inappropriate ADH secretion may be correlated with one of the two types of small cell carcinoma, but further investigations are needed.