TBI induces senescence but not apoptosis in HSCs. BMCs were harvested from control (CTL) and irradiated (TBI) mice 2 months after 6 Gy TBI as described. (A-B) Percentages of SA-β-gal⁺ cells (A) and Annexin V–positive apoptotic cells (B) in HPCs, LSK cells, MPPs, HSCs, ST-HSCs, and LT-HSCs are presented as mean ± SD (CTL: n = 4; TBI: n = 6). *P < .05, **P < .01, and ***P < .001, TBI vs CTL. (C) Fold increases in relative gene expression for various CDK inhibitors in sorted HSCs after TBI. Data from 3 independent experiments using sorted HSCs pooled from 3 to 4 mice per group are presented as mean ± SD (n = 3). **P < .01 and ***P < .001, TBI vs CTL. (D) SA-β-gal⁺ and SA-β-gal^– LSK cells were isolated from irradiated mice (TBI) 2 months after 6 Gy TBI by cell sorting according to the intensity of C₁₂FDG staining. Their differential SA-β-gal activities were confirmed by SA-β-gal enzymatic activity assay after isolation as shown in the inserted microscopic images. (E) The expression of p16 in SA-β-gal⁺ and SA-β-gal^– LSK cells and in control unirradiated LSK cells (CTL) was measured with qRT-PCR and expressed as fold increases compared with CTL. Data are presented as mean ± SD (n = 3). a, P < .05 vs CTL-LSK cells; b, P < .05 vs TBI-LSK β-gal^– cells. (F) Cumulative production of number of cells from wells with single TBI SA-β-gal⁺ and SA-β-gal^– LSK cells and control (CTL) LSK cells is shown as a representative of 3 separated assays.