Percentage of BM-homed G₀/G₁ or S/G₂+M cells undergoing apoptosis 40 hours AT compared with the level of programmed cell death in ex vivo cultures maintained with and without cytokine supplementation.

CD34⁺ cells were isolated from one fresh MPB and 2 BM samples and cultured in vitro as described in “Materials and methods.” Between days 5 and 7, cells were harvested, washed, stained with Hst, and sorted to obtain cells in G₀/G₁(light bars) and S/G₂+M (dark bars). Both G₀/G₁ (87.5 ± 5.3 in G₀/G₁, n = 3) and S/G₂+M (28.5 ± 4.5 in G₀/G₁, n = 3) fractions were stained with CFSE and transplanted into conditioned NOD/SCID mice. Samples of G₀/G₁ and S/G₂+M cells were maintained in culture with or without cytokine supplementation as described in the legend of Figure 2 for an additional 40 hours. BM cells were recovered from individual mice and stained with Annexin V as described in “Materials and methods.” CFSE⁺, PI⁻ cells expressing Annexin V were considered apoptotic, and their percentage was calculated from total CFSE⁺ cells. Each bar represents the mean ± SD of 4 measurements for transplanted cells and 3 for cells maintained in vitro in 3 separate experiments.