CD41 and Flk1 are expressed in Tie2-GFP⁺cells in the 9.5-dpc yolk sac and P-Sp. Flow cytometric histograms depicting Tie2-GFP⁺ cells from 9.5-dpc yolk sac (A) and P-Sp (E) tissues isolated from transgenic mice. Autofluorescence of the cells is illustrated by the faint line; GFP expression is indicated by the bold line. The percentage of GFP-expressing cells (of the total for each tissue) is indicated in the upper right corner of the histogram for each population (A,E). CD41 expression in yolk sac (B) and P-Sp cells (F) is illustrated by the bold histograms, whereas isotype-binding control cells are indicated by the faint line. Dot plots of cells coexpressing Tie2-GFP⁺ and Flk1 in 9.5-dpc yolk sac (C) and P-Sp (G) indicate that the percent of total cells simultaneously expressing these fluorochromes is 2% or less. Further analysis of the yolk sac (D) and P-Sp (H) cells coexpressing Tie2-GFP, Flk-1, and CD41 revealed a 10-fold greater frequency of coexpressing cells in the yolk sac-derived cells (D) compared to P-Sp cells (H). These results are representative data of 10 experiments where 30 to 40 yolk sac or P-Sp tissues were analyzed in each experiment.