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Fig. 2. NRP1 and TIE2 label extra-embryonic arterial and venous EC, respectively. (A,B) Whole-mount in situ hybridizations on yolk sacs of E3.5 chick embryos. (A) TIE2 is expressed on veins (arrows), but not on the parallel-running arteries (arrowheads). NRP1 (B) expression is the reciprocal. (C,D) Adjacent sections through the E6 chick allantois. VEGFR2 (C) labels EC of the umbilical artery (A) and vein (V). AS, allantoic stalk. (D) NRP1 (brown staining) is only observed in EC of the umbilical artery, while TIE2 (blue staining) is down-regulated in these EC (arrows) and transcribed in the veins. Mesenchymal cells surrounding arteries are also NRP1 positive (arrowheads). Bar in C, for C and D, 120 µm.