Selective expansion of immature hematopoietic blasts from Gata1^- ES-cell in vitro differentiation cultures. (A) Summary of the culture method used. wt or Gata1^- ES cells were cultured on OP9 stromal cells for 5 days to generate multipotential hematopoietic progenitors. These were expanded and differentiated further on OP9 cells in the presence of Tpo. At about 3 weeks, nonadherent cells were removed from the OP9 stromal layer and transferred to liquid culture with Tpo. (B) Proliferation of nonadherent cells in differentiation cultures from Gata1^- and wt ES cells. Cumulative cell numbers are plotted against time. One of 3 representative experiments is shown. (C) Morphology of cells after various times in culture. May-Gr̈nwald-Giemsa stain. The blast cells derived from Gata1^- cultures are shown at day 40. We refer to cells at day 40 as GATA-1^- megakaryocyte-erythroid (G1ME) cells because they exhibit erythromegakaryocytic potential, as demonstrated in Figure 3. Original magnification of top (day 12), × 200; bottom left (day 40), × 200; bottom right (day 40), × 630. Photographs were taken by using a microscope (Axioskop 2; Carl Zeiss) equipped with a color digital camera (Axiocam; Carl Zeiss).