FIG. 4.

Induction of apoptosis and necrosis in young and old human fibroblasts by various DNA-damaging agents. After 36, 48, and 72 h of treatment, the cells were stained with acridine orange and analyzed by FACS (see Materials and Methods), which allowed the measurement of both apoptosis and necrosis. The open and solid bars represent untreated young and old cells, respectively. The levels of apoptosis and necrosis in young untreated cells were too low to be seen on the graph. The hatched bars represent the level of apoptosis or necrosis in treated cells, and light and dark hatched bars represent young and old cells, respectively. All the experiments were repeated at least three times, and standard errors are shown.