Ultrastructure of squamous cell carcinoma of the lung.

No extensive ultrastructural description of squamous cell carcinoma of the lung has been provided in the generally available literature thus far. The concise accounts of its ultrastructure usually presented may create the impression that squamous cell carcinomas of the lung show little more distinctive features than intercellular bridges, tonofibrils, prominent desmosomes, and paucity of cytoplasmic organelles. The process of keratinization in these tumors has only incidentally been briefly illustrated. In order to give a more detailed account of these tumors, we investigated an essentially unselected series of 40 lung tumors, diagnosed on light microscopy as squamous cell carcinomas, using World Health Organization criteria. Both at tissue and at cell level, the tumors showed highly variable ultrastructural details. Part of these, such as entrapment of sometimes poorly recognizable alveolar remnants, and the discrepancy sometimes found between prominence of tonofibrils and desmosomes on the one hand, and intercellular bridging on the other, may be of importance in diagnostic classification and grading. The emergence of unmistakable ultrastructural adenodiffentiation in tumor cells lining lumina formed on the basis of necrosis or keratinization, described for the first time, demonstrates that squamous and adenodifferentiation are even more closely intermingled than usually appreciated. The possibility is envisaged that the variability found in these tumors may correlate with differences in clinical behavior, not necessarily parallelled by differences in light microscopical grading.