The –8 region (E1) enhances endothelial activity of the promoter region (E3) in transgenic mice, and also targets the developing ear and lung. (A) Transgenic reporter constructs in relation to the endoglin locus. The exons and enhancer fragments are drawn to scale and are represented as black and green rectangles, respectively. The 5-kb and 1-kb scale bars apply to the endoglin locus and the LacZ constructs, respectively. (B) Representative E11.5 embryos stained with X-Gal for LacZ expression (blue). (Bi) The wild-type embryo shows no staining. (Bii) A E3/LacZ transgenic embryo showing weak LacZ expression (blue) in the vascular network (black arrow). (Biii) A E1/E3/LacZ transgenic embryo showing strong LacZ expression in the vascular network and around the developing ear (white arrow). (C) Histologic analysis of tissue sections from X-Gal–stained transgenic embryos in panel B. (Ci-v) Tissue sections from E3/LacZ embryos. (Ci) Somites showing occasional endothelial staining (arrow in inset). (Cii) The heart shows no staining. (Ciii) The fetal liver showing occasional endothelial staining (arrow). (Civ) The developing otocyst and (Cv) lung bud show no staining. (Cvi-x) Tissue sections from E1/E3/LacZ embryos. (Cvi) The dorsal aorta and intersomitic vessels showing strong endothelial staining (arrows). (Cvii) The heart showing strong staining in the endocardium. (Cviii) The fetal liver showing staining in the endothelial cells lining the hepatic sinusoids. (Cix) The otocyst and facioacoustic ganglion showing strong staining in the mesenchyme adjacent to the otocyst. (Cx) The lung bud showing staining in the bronchial epithelium and parenchyma (arrow). (Cxi-xv) Frozen sections of an E11.5 embryo stained with an anti–mouse endoglin antibody. Endoglin-positive cells stain a brown color. (Cxi) Section through blood vessels showing endoglin expression in the endothelium (arrow). (Cxii) Section through the heart showing staining of the endocardium. (Cxiii) Section through the fetal liver showing staining of cells lining the hepatic sinusoids. (Cxiv) Section through the otocyst showing staining of cells in the surrounding mesenchyme. (Cxv) Section through the fetal lung bud shows no staining within the bronchial wall but staining in the endothelial lining of a blood vessel (arrow). b indicates bronchus; da, dorsal aorta; g, ganglion; and o, otocyst.