Autofluorescent structures in cultured WI-38 cells

Human fibroblasts (WI-38) were monitored for intracellular fluorescence at several passage levels. It was found that late-passage cells contained marked increments in particulate intracellular fluorescence when compared with early-passage cells. Intracellular hydrolysis of α-glycerophosphate at pH 5 was used to localize acid phosphatase activity. Most fluorescent structures contained acid phosphatase, and it was concluded that the fluorescence was associated with lysosomes. Autoradiographic analysis of labeled-thymidine uptake into nuclei was employed to distinguish dividing and nondividing cells in cultures of varying passage levels, and these results were correlated with intracellular fluorescence. It was found that cells containing high levels were unable to incorporate thymidine into nuclei. These results suggest a correlation between fluorescence accumulation in lysosomes during aging and inhibition of cell cycling.