Cytoplasmic chromatin fragments in senescent cells. (A) Cytoplasmic chromatin fragments (CCFs) in senescent cells are strongly positive for histone H3. Yellow arrow marks CCF. (B) Increased proportion of cells with CCFs in RS or OIS. Mean ± SEM, n = 3; P < 0.01 (OIS), P < 0.002 (RS). Assays were performed 10 d after induction with tamoxifen (OIS) or 4 wk after the last passage and >90% SA β-gal⁺ (RS). (C) CCFs in OIS cells are lamin A/C negative. (D) Cells from C were scored for lamin A/C⁺ and lamin A/C⁻ CCFs. Note that the majority of micronuclei in control cells are lamin A/C positive. Mean ± SEM, n = 3; P < 0.0006 for lamin A/C negative CCF con vs. OIS; P < 0.6 for lamin A/C positive CCF con vs. OIS. (E) CCFs in OIS are strongly positive for heterochromatic histone mark H3K27me3 but not for euchromatic mark H3K9ac. (F) Cells from E were scored for indicated histone modifications in CCFs. A single representative experiment is shown and at least 100 cells were scored per slide. Note the majority of micronuclei in control cells are positive for both marks. (G) CCFs in RS contain γ-H2AX but not 53BP1 (top). Intranuclear γ-H2AX foci colocalize with 53BP1 (bottom). (H) CCF⁺ and CCF⁻ cells from G were scored for colocalizing intranuclear γ-H2AX and 53BP1 foci. Mean ± SEM, n = 3; P < 0.019. Bars (in IF panels), 10 µm.