Fig. 1. (A-C) Metaphase singlets in pre-senescent fibroblast cells aged in culture. Fibroblasts from human SR foreskin fibroblast cells at PD 18 (early passage) and PD 65–67 (pre-senescent) were subjected to FISH-IF using probes for the telomeric and control regions listed and an antibody to H3P, which serves as a G2/mitosis marker. The FISH signals are detected with FITC (green) and the anti-H3P antibodies with Cy-3 (red). (A) A pre-senescent cell hybridized to a probe for ß-globin, showing two doublet signals. Bar, 10 µm. (B) A pre-senescent cell hybridized to a 17p telomeric probe showing two singlet signals. (C) The percentage of chromosomes displaying metaphase singlets in pre-senescent cells aged in culture. The mean percentage (±s.d.) of chromosomes displaying a singlet signal is indicated, together with the P value of the binomial distribution probability analysis (^*P<0.0001, ^**P<0.001, ^***P<0.01). The number of chromosomes scored for each probe is indicated in the bar.