Notch signals lead to an enhanced cellular expansion of HSCs and CD45RA⁻cells with reconstitution potential in vivo. Cocultures were initiated with CD90^low cells (HSCs) placed on OP9-Ctrl, -DL4^hi, and -DL4^low cells and analyzed by flow cytometry at days 7, 14, and 21 of culture. The cellular fold expansion from the input number (5-10 × 10³) is shown for (A) CD90^low and (B) CD45RA⁻ cells. Bar graphs show mean and SE of 6 to 8 independent experiments. Statistical significance is indicated as (*) for P ≤ .05, and (**) for P ≤ .01. Cocultures were initiated with HSCs placed on OP9-DL4^hi cells for 14 days, (C) phenotypically defined pHSC/CD90^low, and (D) phenotypically defined pMPP/CD90⁻ cells were isolated by flow cytometry, and 2 × 10³ pHSC/CD90^low and 5 × 10⁴ pMPP/CD90⁻ cells were injected into sublethally irradiated NSG neonatal mice. Ten weeks after injection, BM was analyzed by flow cytometry for the expression of the indicated marker. Numbers within each plot indicate the percentage of cells in the indicated gates or quadrants. (E) Absolute numbers of CD14⁺, CD15⁺, and CD19⁺ cells recovered. (F) A linear correlation between the absolute numbers of CD45RA⁻ cells recovered from the BM and the number of freshly sorted HSCs injected into mice is shown (R² = 0.87). Absolute numbers of CD45RA⁻ cells recovered from (C) and (D) were plotted on the linear graph and the corresponding number of HSCs injected was extrapolated. A total of 8 mice were analyzed.