Abnormal lymphoid development in aging Polg^A mice. Polg⁺ and Polg^A mice were humanely killed for analysis at 9 to 13 months of age. Single-cell suspensions were prepared from thymus and bone marrow (BM), stained as indicated for each panel, and then incubated with the viability stain 4′,6-diamidino-2-phenylindole immediately before analysis. The individual graphs presented are representative of 5 independent experiments, in which a total of 6 Polg^A and 5 Polg⁺ mice were analyzed. The percentages shown with each region represent the mean frequency of that population, as a percentage of viable singlet events, across all 4 experiments. *P < .05, **P < .01, as assessed by Student t test, for regions with statistically significant differences in frequency between genotypes. (A) Thymus: The mean frequencies of CD4⁺CD8⁺ (double-positive) cells (oval) and CD4⁻CD8⁻ (double-negative) cells (rectangle) are shown. (B) Bone marrow: The mean frequencies of mature B cells (B220^hiCD43⁻, top left region), immature B/pre B cells (B220^intCD43⁻, bottom left region) and B220⁺CD43⁺ cells (oval), are presented. The B220⁺CD43⁺ region was further analyzed for expression of AA4.1 and CD19 (right panels). The mean frequency of B220⁺CD43⁺AA4.1⁺CD19⁺ pro-B cells as a percentage of viable singlet BM cells is presented.