Fig. 1

Maximum number of divisions reached by primary human muscle cells, and the number of divisions required to obtain sufficient cell numbers: a Distribution of the maximum number of divisions that human muscle cells can execute. Each point represents one sample. Based on this number, a safe window to analyze fully active and proliferative muscle cells is under 21 divisions. Light blue: age 20–30, dark blue: age 30–40, gray: 40–50, black: 50–75 years old. b Absence of correlation between age and the maximum number of divisions. c Table showing the number of primary muscle cells obtained at different phases of cell culture. Typically, 470,000 CD56+ve muscle cells can be purified from a muscle biopsy culture after ~10 divisions (first row, light green). The number of cells after each division and the number of divisions is given by row. Pink indicates the pre-senescence stage (based on the data in a) when cells may start to slow down their capacity to proliferate and then senesce (and rate of division drops from an average of 0.58 to 0.25). Importantly, for some subjects, the cells will not reach 30 divisions, as shown in plot 1a. Typical measurements of the number of days of expansion and of the average number of divisions per day are given on the right side of the table. d Myoblasts under 21 divisions were negative for beta-galactosidase. Top right panel: positive control of senescent cells positive for beta-galactosidase. Scale bar = 100 μm. e No cleaved PARP-1 was observed by Western blot, suggesting that myoblasts under 21 divisions do not show any sign of necrosis nor apoptosis