Cell Cycle Kinetics in an in Vitro Tumor Model

Cell cycle kinetic parameters of multicell spheroids in vitro have been estimated using thymidine labeling techniques and autoradiography. Both the mitotic index and thymidine labeling index decreased in larger spheroids, whereas the duration of the cell cycle, as determined by two independent methods utilizing labeled mitoses or labeled cells, was essentially independent of spheroid size or age. These results suggest that the tumor-like growth exhibited by spheroids is the result of a decreasing growth fraction and a large apparent cell loss, rather than a general elongation of the cell cycle.