Qualitative histological differences between transplacentally-induced lung tumors in young and aging mice.

Transplacentally-induced lung tumors arising in BALB/c mice 38-45 weeks old (late crop) were significantly larger and more invasive of the surrounding lung parenchyma than were the tumors appearing at 10-36 weeks (early crop). The late-crop tumors were also somewhat more papillary and heterogeneous. Development of the 2 crops of tumors from different cell types of origin is postulated. A specific association of lymphocytes with the pleural surfaces of some tumors was noted and this association was significantly more likely for tumors invading the parenchyma than for non-invasive ones.