Sites of Runx1 excision. (A) Tek expression in the para-aortic splanchnopleure of E8.5 Tg(Tek-lacZ) embryos. (B) Dorsal aorta from boxed region in panel A, illustrating that β-gal⁺ cells are confined to the endothelium. (C) Vitelline artery, from boxed region in panel A, showing Tek-lacZ expression in endothelial and mesenchymal cells. (D) Section through the AGM region of an E10.5 Tg(Tek-lacZ) embryo. (E) Ventral aspect of the dorsal aorta from the region boxed in panel D. (F) Vitelline artery from boxed region in panel D (40 ×). (G) Transverse section through the AGM region of an E10.5 R26R Tg(Tek-cre) fetus. (H) Detailed view of the dorsal aorta from the region boxed in panel G. Examination of approximately 1000 endothelial cells from 30 sections determined that 52% were β-gal⁺ and had therefore undergone R26R excision by Tek-cre. (I) Vitelline artery of an E10.5 R26R Tg(Tek-cre) fetus, showing that excision occurred in both mesenchymal and endothelial cells. (J) Dorsal aortae of an E8.5 Runxl ^lz/+ embryo, showing β-gal⁺ cells are confined to the endothelium. By E10.5 many ventral para-aortic mesenchymal cells are also Runx1⁺.⁹  (K) Vitelline artery, E8.5 Runxl ^lz/+ embryo showing β-gal⁺ cells confined to the endothelium. (L) Vitelline artery, E10.5 Runxl ^lz/+ embryo (40 ×). Da indicates dorsal aorta; v, vitelline artery; m, mesenchymal cell; e, endothelial cell; hc, hematopoietic cluster.