Flk1 protein expression in heart and somite in 8.5 dpc wild-type embryo (7 somite pair stage). (A-C) Immunostaining of heart using anti-GATA4 antibody (A) and anti-Flk1 antibody (B) in a transverse section, and the merged image (C). Signals are seen in endothelial cells in endocardium (ec), and some myocardial cells (mc). Scale bar is 100 μm. (D-F) Immunostaining of heart using anti-Flk1 (D) and anti-sarcomeric MHC antibody MF20 (E) in a transverse section and the merged image (F). Scale bar is 50 μm. (G-I) Immunostaining of somite using anti-Flk1 (G-H) or anti-platelet endothelial cell adhesion molecule (anti-PECAM; I) in combination with anti-Pax3. ^*Nonspecific signal in visceral endoderm. Scale bar is 50 μm. (G) An Flk1⁺ cell coexpressing Pax3 is seen in the dorsolateral part of the somite. (H) Coexpression of Flk1 and Pax3 in a cell migrating out of somite (arrowhead). (I) Exclusive expression of PECAM and Pax3; da indicates dorsal aorta; nt, neural tube. (J-O) High-power magnification of Flk1-Pax3 double-positive cells. An arrowhead indicates an Flk1-Pax3 double-positive cell migrating out of the somite, and arrows indicate cells within the somite. Hoechst nuclear staining is shown in blue.