Pulmonary Adenoma of Strain A Mice: An Electron Microscopic Study2

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Abstract
“Spontaneous” and urethan-induced lung tumors from strain A mice were examined with the electron microscope. Most lung tumor cells appeared very similar to type B alveolar cells, one of the two recognized types of alveolar epithelial cells. In confirmation of the findings of other observers, it can be concluded that the mouse lung adenoma is derived from the type B alveolar cell. Morphologically unique cytosomes in both type B alveolar and tumor cells facilitated recognition and comparison of these cells. Some tumor cell cytosomes differed from those of the type B alveolar cell. They had a crystalloid component, which is either absent or not preserved in type B alveolar cells and in many tumor cells. Also a few tumor cells had glycogen, a substance absent in type B alveolar cells. The significance of these two differences is not known, but possibly these crystalloid-containing and glycogen-containing tumor cells represent new populations of neoplastic cells derived from the large mass of better differentiated tumor cells. No effect of urethan on type B alveolar cell ultrastructure was detected. No differences were noted between the urethan-induced tumors of very young mice and induced or “spontaneous” tumors of older mice.

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