The 9.5-dpc cells sorted from yolk sac and P-Sp formed endothelial capillary-like tubes on Matrigel. Sorted Tie2-GFP⁺Flk1+CD41-cells from 9.5-dpc yolk sac and P-Sp and Tie2-GFP⁺ cells from adult heart were seeded on Matrigel-coated dishes. Capillary tube formation was induced in the presence of βFGF, VEGF, and ECGS (see “Materials and methods”) in 2% O₂, 5% CO₂ over a 7-day culture period. (A) Yolk sac-derived, (B) P-Sp-derived, and (C) adult heart-derived capillary-like tubes are depicted. Representative endothelial cells forming capillary-like tubes are highlighted in the photomicrographs by white arrowheads (A-C) at an original magnification of × 10. (D) Sorted Tie2-GFP⁺Flk1+CD41-yolk sac cells take up soluble Dil-Ac-LDL and coexpress CD31 in this composite confocal image as highlighted in 2 representative cells by white arrows (D-F). Panels E and F depict individual CD31 and Dil-Ac-LDL expression patterns. Bar represents 50 μm.