Mouse fetal liver V⁺P⁻ stromal cells support the expansion of CD41⁺CD42⁺MKs from human HSCs. Human HSCs (500 CD34⁺CD38⁻CD45RA⁻) were cocultured with sorted stromal cells for 7, 9, 11, and 14 days and assayed for MK production. MKs were identified as CD41⁺CD42c⁺ cells, and MK progenitors were identified as CD34⁺CD41^dim cells. (A) Total number of cells produced from 500 HSCs in the absence of stroma or with the different stroma. Values represent mean ± SD from 4 independent experiments. (B) Total number of MKs, identified as cells expressing CD41 and CD42c, produced from 500 HSCs. Values represent mean ± SD from 4 independent experiments. (C) Total number of MK progenitors, identified as CD34⁺CD41^dim cells, produced from 500 HSCs. *P < .05, **P < .01, and ***P < .001 vs no stroma; ##P < .01 and ##P < .001 vs V⁺P⁻. Other comparisons were not statistically significant. MKp, MK progenitor.