Comparison of BM CRU ability with multilineage reconstitution supported by 9.5-dpc yolk sac- and P-Sp-derived endothelial cell cocultures. (A) The CRU activity in peripheral blood of recipient mice 1 and 6 months after transplantation with fresh or cultured donor stem cells. Freshly isolated Tie2-GFP⁺Flk1+CD41-yolk sac and P-Sp endothelial cells failed to result in any donor evidence of hematopoietic cell repopulation in mice that received competitive transplants. The 9.5-dpc Tie2-GFP⁺Flk1+CD41-yolk sac and P-Sp endothelial cell cocultures facilitated both short-term (1 month) and long-term (6 month) donor stem cell CRUs compared with fresh control CRUs (^*P < .05). The P-Sp endothelial cell coculture conditions markedly increased CRU activity of the donor stem cells as evidenced in the peripheral blood chimerism of recipient mice at 6 months after transplantation compared with donor stem cell Sca-1⁺c-Kit+lin-cell CRUs harvested from yolk sac endothelial coculture conditions (^**P < .05). Data are presented as the mean ± SD of 6 animals at each time point in each group. Representative peripheral blood analysis (B) of reconstitution of recipient mice receiving transplants with Sca-1⁺c-Kit+lin-cells following 7 days of coculture with Tie2-GFP+Flk1+CD41-yolk sac or P-Sp endothelial cells performed 6 months after the donor cell transplantation. Percent donor type CD45.2-expressing cells (y-axis) in recipient mice given transplants of 9.5-dpc Tie2-GFP⁺Flk1+CD41-yolk sac endothelial cell/Sca-1+c-Kit+lin-cocultures or 9.5-dpc P-Sp Tie2-GFP⁺Flk1+CD41-endothelial cell/Sca-1+c-Kit+lin-cocultures are depicted with B lymphocyte (CD45R/B220), granulocyte (Gr-1), and T-lymphocyte markers (CD4 and CD8) depicted on the x-axis.