HPP multilineage clonogenic progenitors and in vivo repopulating stem cells are maintained in short-term dlk-expressing cocultures. (A) Fetal liver stem cells were purified as described and assayed for their progenitor content immediately after purification and after culture on BFC-dlk-5 and BFC-Zeo-1. At day 4, the cultures were used for clonogenic progenitor (three experiments) and transplantation assay (two experiments). Bars represent data from three experiments with day 0 cells (Fresh) and day 4 cocultured cells (BFC-dlk-5 and BFC-Zeo-1), error bars represent SEM. ∗, P = 0.01 for total CFU-C from fresh stem cells compared with total CFU-C from BFC-dlk-5 cocultures at day 4; ∗∗, P = 0.001 for total CFU-C from BFC-dlk-5 compared with total CFU-C from BFC-Zeo-1 (Student’s t test). (B) Analysis of in vivo repopulating ability of purified fetal liver stem cells cocultured for 4 days on BFC-dlk-5, BFC-Zeo-1, and AFT024 monolayers. Results are from nine individual mice in two experiments (4–5 mice in each experiment) at 10 weeks after transplantation. P = 0.05 for BFC-dlk-5 vs. BFC-Zeo-1 (Student’s t test).