Expansion of fetal liver-derived G1ME cells in long-term culture. (A) Cell morphologies at various times in culture (May-Gr̈nwald-Giemsa stain). Chimeric fetal livers were cultured on OP9 stroma with Tpo, according to the strategies in Figures 1 and 5. At day 12, a population of large, multinucleated megakaryocytes was visible in both wt and Gata1^- cultures. By day 40, only the Gata1^- cultures were growing and contained predominantly immature blasts (right) with morphology similar to ES-cell-derived G1ME cells (compare with Figure 1C). Original magnification, × 200; inset, × 630. Photographs were taken by using a microscope (Axioskop 2; Carl Zeiss) equipped with a color digital camera (Axiocam; Carl Zeiss). (B) Flow cytometry analysis of the cultures in panel A. The hematopoietic blasts from GATA-1^- chimeric fetal liver cultures are predominantly cKit⁺ CD41⁺. These cells are also Ly9.1⁺ (data not shown), indicating that they are Gata1^- donor ES cell-derived.