A combination of histological stains has been used to obtain maximum contrast between capillary endothelial cells, fibroblasts, and parenchymal cells in sections of a transplanted C3H mouse mammary tumor. Values of kinetic parameters were estimated by thymidine labeling techniques and autoradiography for each of these three cell populations.

The mean thymidine labeling index of carcinoma cells was 35%. Mitotic index, labeling index, and mean grain count per labeled cell decreased with distance of carcinoma cells from a capillary; there was migration of cells from regions of rapid proliferation near blood vessels to regions of slow or non-proliferation near necrosis. The duration of the cell cycle phases was estimated by a labeled mitoses experiment, and computer methods were used to analyze these results. The median cell cycle time was 13 hr, and the growth fraction was estimated from a repeated thymidine labeling experiment to be about 50%. Therefore, the turnover time of the carcinoma cells was about 22 hr.

The mean thymidine labeling indices of capillary endothelial cells and fibroblasts were 11.4 and 9.1%, respectively. These cells rarely were recognized in mitosis, and their turnover times were estimated from a repeated labeling experiment. The results were consistent with a turnover time of 50 to 60 hr for endothelial cells and 70 to 80 hr for fibroblasts, with a wide spread in the distribution of intermitotic times.

The mean labeling index of capillary endothelial cells remained quite constant between 10 and 40 hr after a single injection of tritiated thymidine; this result implies that capillary endothelial cells are not derived from a faster-proliferating precursor population. Extension of the capillary network in the growing tumor appears to depend mainly on the division of endothelial cells within the capillary walls.