Tetraploid ↔ ES cell–derived embryos and hemangioblast colony analysis of Flk1 heterozygous, Flk1 null, and MFps-expressing Flk1 null ES cells. (A-F) X-gal analysis of tetraploid ↔ ES cell–derived embryos. E7.0 to E7.5 embryos derived totally from Flk1^lacZ/+ ES cells (A), Flk1^lacZ/lacZ ES cells (B-C), MFps-expressing Flk1^lacZ/+ ES cells (D), or MFps-expressing;Flk1^lacZ/lacZ ES cells (E-F). Flk1^lacZ/lacZ hemangioblast progenitors showed limited migratory potential (B-C). In contrast, MFps-expressing Flk1^lacZ/lacZ-derived hemangioblast progenitors showed enhanced migratory activity (E-F). MFps-expressing Flk1^lacZ/+ cells demonstrated the largest increases in X-gal–stained hemangioblast progenitors (D). Hemangioblast colony formation analysis was conducted on Flk1^GFP/+ and MFps-expressing Flk1^GFP/+ ES cell–derived embryoid bodies (G), as well as Flk1^GFP/GFP and MFps-expressing Flk1^GFP/GFP ES cell–derived embryoid bodies (H). C-chorion membrane, lines in panels B, C, E, and F represent extent of X-gal staining progenitor cell migration. The numbers in panels G and H represent the ES clone number used in the analysis. Original magnification × 20 (A-F).