Ink4a/arf is necessary to maintain IR-induced senescence of BM stromal cells. (A) OB-SC populations derived from wild-type (WT) or Ink4a/arf-deficient (KO) mice were established in vitro and their proliferation after exposure (IR) or not (CTR) to 10 Gy of IR is shown. A representative experiment (n = 3 independent stromal cell isolations) of cumulative population doubling determined over time is shown. (B) Proportion of WT or ink4a/arf-null OB-SCs in culture that have incorporated BrdU (3-day pulse) 7 days after exposure or not to IR. Up to 16 independent stromal cell isolations were performed. ***P < .001 by Student t test. (C) Representative photographs showing SA-β-gal (blue) of OB-SCs 10 days after exposure or not to 10 Gy of IR. Scale bar represents 100 μm.