Autoradiographic Analysis of Cell Proliferation in Spontaneous Breast Cancer of C3H Mouse. II. Growth and Survival of Cells Labeled With Tritiated Thymidine2

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Abstract
Autotransplanted breast tumors of C3H/Jax mice were removed at various periods up to 4 weeks after intravenous injection of 50 or 100 µc. of tritiated thymidine. Autoradiographs of the tumors were counted in detail and the following indexes were determined: Cells/Field, Mitoses/100 Cells, Degenerates/100 Cells, Labeled Cells/100 Cells, Labeled Mitoses/100 Mitoses, Labeled Degenerates/100 Degenerates, Labeled Mitoses/100 Labeled Cells, and Labeled Degenerates/100 Labeled Cells. The indexes for the transplants derived from any of the primary tumors are consistent with each other and are distinct from those of transplants derived from other primaries. With these indexes it is possible to compare the relative size of the labeled and unlabeled cell populations and the rates of growth and degeneration in these populations at various periods after labeling. The data indicate that growth and survival are similar in labeled and unlabeled cells and are stable with time. These results have dual significance: (1) Under the circumstances of these experiments, tritium incorporated into the deoxyribonucleic acid of labeled cells has no demonstrable effect on growth or survival, and (2) as a passive cell marker tritiated thymidine is practical and effective for studying many aspects of tumor growth. To account for the ultimate fate of labeled cells, a model is suggested in which the number of labeled chromosomes in the population remains constant after the first division post labeling. The data from one tumor conform closely to the model in that they follow the prediction that the number of labeled cells does not increase beyond the sixth generation after labeling.