Variations in the [³H]Thymidine Labeling of S-Phase Cells in Solid Mouse Tumors

To determine whether all tumor S-phase cells incorporate [³H]thymidine, we labeled the cells in three mouse tumors (MCa-11, colon-26, and colon-51) in vivo for V2 h with [³H]thymidine (10 μCi/g of body weight). Cells from the tumors, as well as control cells from the bone marrows of the tumor-bearing mice, were then placed onto slides and Feulgen stained. The positions of these Feulgen-stained cells were mapped with a computerized scanning stage, and their nuclear DNA content and nuclear areas were determined by absorption cytophotometry. Next, the slides were processed for autoradiography and exposed for 32 or 64 days to obtain plateau labeling. The cells were then relocated, and the areas of the autoradiographic grains over each nucleus were measured. We found that 99% of the S-phase bone marrow cells were labeled. The 5-mm tumors, however, showed a wide range of S-phase labeling, with 94, 89, and 85% of the MCa-11, colon-51, and colon-26 S-phase cells, respectively, being labeled. The same mice bearing both 5- and 20-mm MCa-11 tumors, however, showed 95 and 57% labeling of the S-phase cells in the small and large tumors, respectively. These results show that the [³H]thymidine labeling of S-phase cells varies greatly for experimental mouse tumors of different size and type, and they suggest that “labeling index” and flow cytometric DNA measurements may not give valid estimates of the actual proportions of cycling S-phase cells in such tumors.