Fig. 3. Intra-embryonic expression of NRP1, TIE2 and VEGFR2. (A,B) Adjacent transverse sections of an E3 quail embryo at the level of the limb bud hybridized with the indicated probes. (A) NRP1 is expressed in motoneurons (arrow) of the neural tube (NT), in future dorsal root and sympathetic ganglia (arrowheads), and in EC of the aorta (Ao). EC of the posterior cardinal vein (cv), omphalomesenteric vein (omv) and umbilical vein (uv) are negative. (B) TIE2 labels all EC. (C,D) Adjacent sections through the hindbrain of an E4 embryo at the level of the otic vesicle (OV). (C) NRP1 is expressed in the internal carotid artery (ica), but not in the anterior cardinal vein (cv). (D) TIE2 shows the complementary expression. (E,F) Adjacent sections through aortic arches (AA) 3, 4 and 5 of an E6 chick embryo hybridized with the indicated probes. (E) VEGFR2 is detectable on the majority of EC in both the aortic arches and the anterior cardinal vein (cv). (F) NRP1 (brown staining) is only detectable in aortic arch EC, few of which (arrows) also express TIE2 (blue staining). (G) Section through the leg bud of an E6 chick embryo. NRP1 (brown staining) is expressed in EC and some mesenchymal cells of the aorta and femoral artery (fa) and is strong in regions of arterial branching (arrows). TIE2 (blue staining) is expressed weakly or not at all in the femoral artery, but is expressed in the cardinal vein (cv) and the peripheral venous plexus of the limb (arrowheads). (H) Higher magnification of the boxed region in G shows the femoral artery EC expressing NRP1, while the underlying venous plexus is TIE2 positive. Capillaries branching from the femoral artery express both markers (arrows). N, notochord. Bars, (A,B,E,F) 130 µm; (C,D) 65 µm; (G) 290 µm; (H) 65 µm.