Expression of lacZ in Flk1-null embryos. (A) Lateral view of a Flk1-lacZ neo-out/EGFP embryo at LHF stage after 8 hours of X-gal staining. Scale bar is 100 μm; al indicates allantois; hc, heart crescent. (B) A transverse section of panel A. Note that lacZ signals are seen in amniotic (am), mesothelial (me), and chorionic mesoderm (cm) layer. (C) A transverse section of 8.5 dpc Flk1-lacZ neo-out/EGFP embryo at the level of heart. The lacZ⁺ cells are seen at splanchnic mesoderm (sm) and myocardium (mc). (D) Note that the dorsolateral part of somite is stained by X-gal. (E) A model for the differentiation of Flk1⁺ cells and the function of Flk1.