Chimeric analysis of Flk1^lacZ/+ and Flk1^lacZ/lacZ ES cells expressing the MFps protein. (A) E12.5 chimeric embryo containing Flk1^lacZ/+ ES-derived cells showing X-gal–stained vasculature throughout. (B) E12.5 chimeric embryo-derived from Flk1^lacZ/lacZ ES cells shows a lack of Flk1 null (X-gal stained) cell contribution to the vasculature. (C-D) Low-degree chimeric contribution of MFps-expressing; Flk1^lacZ/+ ES cells leads to normal vascular channel formation (arrows in D point to superficial abdominal and limb vasculature). (E-F) High-degree chimeric contribution of MFps-expressing; Flk1^lacZ/+ ES cells leads to aberrant vascular development in the yolk sac and vascular hemorrhage (arrowhead in F). (G-H) Hemangioma isolated from the small intestine of a 3-week-old MFps-expressing Flk1^lacZ/+ chimeric adolescent (arrowhead in G). Higher-power magnification of the hemangioma revealed extensive hemorrhaging and hypervascularity in the surrounding tissue of the intestine (arrow in H). (I-K) Low-degree chimera contribution of MFps-expressing Flk1^lacZ/lacZ ES cells rescued vascular contribution (arrows) to intraembryonic (I) and yolk sac sites (J-K). X-gal–staining MFps-expressing; Flk1^lacZ/lacZ-derived endothelial cells were found in intersomitic vessels in the embryo (arrow in I) and in morphologically normal yolk sac blood vessels (arrow in J). Histologic analysis shows X-gal–stained endothelial cells surrounding hematopoietic cells in the yolk sac vasculature (arrow in K). (L-N) High-degree chimera contribution of MFps-expressing Flk1^lacZ/lacZ ES cells lead to aberrant vascular development and hemangioma formation in the yolk sac (L-M). (N) Histologic analysis of the hemangioma seen in panel M demonstrated the MFps-expressing; Flk1^lacZ/lacZ null endothelial cells lining the blood filled hemangioma cavity (arrows in M-N). Panels G and H were stained by hematoxylin and eosin; panels K and N were stained by eosin. Original magnification × 8 (A-C, E); × 12 (D, F, L); × 20 (I, J, M); and × 200 (G, H, K, N).