E10.5 embryos were fixed, stained, and analyzed as described (33 (link), 63 (link)). Embryos were stained with biotinylated anti–PECAM-1 (BD Biosciences, 553371) and anti–c-Kit (BD Biosciences, 553352) antibodies. Samples were mounted in a 1:2 mix of benzyl alcohol (Sigma-Aldrich, 402834) and benzyl benzoate (Acros Organics, 105862500) to increase tissue transparency and visualized with a Nikon A1RS confocal microscope. Three-dimensional reconstructions were generated from Z-stacks (80 to 200 optical sections) using Fiji software.