Fig. 1. Survival of embryonic EPCs in the circulation and emergence of CECs. (A) Schemes illustrating, on the left, the preparation of E2 quail EPCs from somites (S) and splanchnopleural mesoderm (SpM) after enzymatic digestion (E), on the right, the intracardiac injection of cell suspensions in E2 chick hosts. A, aorta. (B-D) Transverse sections of chick embryos that received an intracardiac injection of quail somitic (B,D) or splanchnopleural cells (C). (B,C) Two days after injection, QH1⁺ECs (arrows) migrate in the mesenchyme close to the mesencephalic epithelium (m). Scale bar: 25 µm. (D) A QH1⁺EC (brown) is detected in a limb vessel (^*) 14 days after the injection. Scale bar: 30 µm. (E) Scheme of ex ovo parabiosis (see text for details). YS, yolk sac. (F) Transverse section in the region of contact between the two embryos: the broken line delimits the quail territory on the left, with QH1⁺ vessels (^*), and the chick territory on the right, with QH1^- vessels. Scale bar: 70 µm. The inset is a higher magnification of the outlined region: vascular connections are established as QH1⁺EC (arrow) and HCs (arrowheads) are present in the chick. Scale bar: 35 µm. (G) Transverse section in a chick embryo showing a QH1⁺EC (arrow) and a QH1⁺HC (arrowhead). CV, cardinal vein; N, notochord; ^*, aorta. Scale bar: 50 µm. (H) Yolk sac section from an E4.5 chick embryo that received an intracardiac injection of E3 quail blood cells: a QH1⁺EC reaches a vitelline vessel. Scale bar: 30 µm. (I-M) Parabiosis experiments. (I) Aspect of an E15 parabiosis in the egg. The chick (C) and quail (Q) embryos, together with their yolk sac (CYS and QYS, respectively), are wrapped into their respective CAM: the arrow indicates a chick CAM vessel; the arrowhead indicates a quail CAM vessel. Scale bar: 15 mm. (J) When dissected, the two CAMs appear fused (arrow). Scale bar: 20 mm. (K) Blood sample of an E8 chick embryo from a parabiosis: QH1⁺ cells (arrows) prove the establishment of vascular connections between the embryos. Scale bar: 15 µm. (L,M) Isolated morphologically distinct QH1⁺ECs present in a thymus (L) and in a lung capillary (asterisk in M) of E14 chick embryos. Scale bar: 25 µm.