V⁺P⁻ stromal cells promote the expansion of hematopoietic progenitors. HSCs (50 SLAM cells) were plated in low-cytokine serum-free medium in the presence or absence of stromal layers. After 3, 5, and 7 days, the cells produced were subjected to a clonogenic assay for CFU-MKs. The graph bar represents the number of CFU-MKs (dark red) and undifferentiated CFU colonies (light red) produced per 50 SLAM cells. Values represent mean ± SD from 3 independent experiments. ***P < .001 vs the nonstroma group for the same day; ##P < .01 comparison for the number of CFU-MKs at day 7 in the stroma V⁺P⁻ vs other stroma groups. Other comparisons were not statistically significant. D, day.