Accelerated return of HSCs to quiescence in the absence of SPARC is niche dependent and specific to 5-FU. (A) WT and Sparc^−/− mice were transplanted with WT BM. Twenty weeks later, mice were treated with 5-FU (120 μg/g), and cell cycle status of HSCs was determined 10 days later by Ki67/Hoechst staining (N = 8-11 per group and time point; mean and SD; *P < .05; unpaired 2-tailed Student t test). (B) HSC (Lin⁻Sca-1⁺CD48⁻CD150⁺CD34⁻) numbers in WT and Sparc^−/− reverse chimeras 10 days after 5-FU. (C) WT and Sparc^−/− mice were treated with polyI:C at different time points prior to cell cycle analysis with Ki67/Hoechst (N = 6 per group and time point; mean and SD). (D-F) 10⁶ WT BM cells were transplanted into lethally irradiated WT and Sparc^−/− recipients, and (D) HSC cell cycle status was analyzed with Ki67/Hoechst after 4 weeks (N = 4-6 per group and time point; mean and SD; P values are shown) and HSC frequencies after (E) 2 and (F) 4 weeks. (G) Cell cycle analysis of HSCs of 19-day-old WT and Sparc^−/− mice using Ki67/Hoechst (N = 4 per group and time point; mean and SD). (H) Saturating (2 × 10⁵) and limiting (10⁴) numbers of CD45.1 WT BM cells were transplanted into irradiated (2 × 5 Gy) WT or Sparc^−/− recipient mice. Donor chimerism of surviving recipient mice was analyzed after 4 weeks.