Tie2 protein level is significantly reduced in p110α-deficient mice. (A-H) Anti–Tie2 IHC staining. (A-B) Whole-mount anti-Tie2 IHC of E9.5 embryos. (C-D) Transverse sections of anti-Tie2–stained embryos (at the level of branchial arches). DA indicates dorsal aorta. (E-F) E9.5 yolk sac membranes after whole-mount anti-Tie2 staining. Arrow in panel E points to a Tie2-positive blood vessel. (G-H) Histologic sections of anti-Tie2–stained yolk sac membranes. Arrowheads in panels G and H point to endothelial linings of vascular lumens. Green arrows mark the endoderm. Staining signals on the endoderm were the result of nonspecific antibody binding. (I-L) Anti–Flk-1 IHC staining. (I-J) Head vasculature (arrowheads). (K-L) Embryonic hearts. Arrowheads point to endocardial linings. Images were captured as described in Figure 1, except that methyl green was used as counterstaining dye for panels C, D, G, and H. Scale bars: (A-B) 500 μm; (C-D, G-H) 50 μm; (E-F, I-L) 200 μm.