FIGURE 4. Representative temporal dependence of intracellular activity of ¹³¹IdU in labeled cells for case where 20 mBq per cell is taken up by V79 cells by end of uptake period. Area under the curve is proportional to cumulated decays in V79 cell nucleus. Time period of 0–14 h represents uptake of radiochemical into labeled cells. Time period of 14–86 h represents 72-h period where labeled cells were mixed in equal numbers with unlabeled cells and maintained at 10.5°C as a multicellular cluster. Finally, curved region corresponds to 1-wk colony-forming period where cellular activity has an effective half-time of 12 h (21). Inset shows corresponding cumulated activity as function of time during uptake (), cluster maintenance (), and colony formation (•) periods.