Distribution of senescence-related γ-foci in mouse embryo fibroblasts. (A) Distribution of γ-foci on metaphases of primary mouse embryonic fibroblasts (MEF), low population doublings (PDs), high PDs in 20% O2 and 3% O2 (see Additional file 1 for experimental details). Proportion (%) of each type of damage is shown in each graph bar. Scoring is as in Figure 1B. (B) Scoring of γ-foci as along the chromatid arms proximal to telomere, along the chromatid arms distal to the telomeres, on the chromatid ends with fluorescence in situ hybridization (FISH) signal or on the chromatid ends without FISH signal. On average more than 10 metaphases were screened per point in independent experiments. (C) MEFs cultured in 20% O2 (triangle) were shifted at day 5 to 3% O2 (square), or maintained in 20% O2. The average of two cultures is shown. Note that the cultures transferred from 20% to 3% O2 in panel A were already pre-senescent, and also became senescent in 3% O2. The cultures transferred from 20% to 3% O2 in this panel were low PDs and accelerated growth in 3% O2. (D) Number of γ-foci in MEFs after oxygen transfer is shown. See Additional file 3 for experimental details. Error bars signify standard errors.