Confocal images of Flk1- and CD41-expressing cells detected in 9.5-dpc Tie2-GFP transgenic yolk sac vessels. (A) Image of a 9.5-dpc embryo proper (EP) and yolk sac (YS) with an insert panel to indicate the area depicted in panel B. (B) Tie2-GFP- (green), Flk1- (red), and CD41- (blue) expressing cells are seen in this composite image of day-9.5 YS tissue. The inset panel depicts the area visualized at higher magnification in panels C to F. (C) A higher power image of panel B that represents a composite of cells expressing Tie-2GFP (green), Flk1 (red), and CD41 (blue). Tie2-GFP expression (green) is primarily restricted to endothelial cells that coexpress Flk1 (red) as evidenced by cells highlighted by white arrows (C-E). Heterogeneity in the expression of Tie2-GFP and Flk1 leads to variation in the composite coexpression of these antigens leading to the appearance of bright yellow to orange aggregates (illustrated in the 2 cells indicated with white arrows in panel C). Few cells coexpress Tie2-GFP (D), Flk-1 (E), and CD41 (F), but 2 illustrative cells are indicated by white arrowheads in panels C to F. CD41 (blue) expression alone is restricted to round cells present within the lumen of vessels in the yolk sac (yellow arrow in panels C-F). Images were generated at projections of the z-stacks at 512 × 512 pixels. Panel A viewed at 4× with phase contrast. Bar in panel B represents 100 μm; bar in panels C-F is 50 μm.