Reporter gene analysis of Flk-1 gene regulatory elements in transgenic mouse embryos. The LacZ reporter gene was fused to regulatory elements derived from the mouse Flk-1 gene and tested for β-galactosidase expression in transgenic mouse embryos. (A) E10.5 transgenic mouse embryo expressing LacZ under the control of a 939bp promoter fragment in combination with a 2.3 kbpXhoI/BamHI fragment of the first intron spanning the region from +1677 bp to +3947 bp of the Flk-1 gene. Most if not all developing vascular structures show β-galactosidase expression, for example the endocardium of the heart, the dorsal aorta, intersomitic vessels or vessels of the developing brain. (B) E11.5 embryo of the transgenic mouse line 2603 that was established with the same construct. (C) E11.5 Flk-1/LacZ knock-in embryo in which the LacZ gene is expressed from the endogenous Flk-1 locus shows a highly similar staining. However, β-galactosidase expression was absent in small blood vessels of the yolk sac. (D to F) Paraffin sections of the embryo from (B) show β-galactosidase expression in the paired dorsal aortae (D), a venous vessel connected with the heart (E), and capillaries invading the neural tube (F). (G) LacZ staining of a 15 μm cryostat section of a P5 transgenic mouse (Line 2603) kidney. (H) Adjacent section of G, immunolabeled with anti-PECAM1 antibody. The LacZ expression in G colocalizes with PECAM1 expression in H. (I) β-galactosidase expression in a transgenic embryo containing thetk promoter in combination with the 2.3 kbpXhoI/BamHI fragment of the Flk-1 first intron. (J) β-galactosidase expression in a transgenic embryo containing a construct with Flk-1 promoter sequences (−640 bp/+299 bp) in combination with a 510 bp fragment of the first intron (+3437 bp to +3947 bp) of the Flk-1 gene. Bars, 100 μm (D to H).