Tie2 signaling in blood vessels and lymphatic endothelial cells protected from apoptosis. (A) Expression of PECAM-1 and LYVE-1 in ES cell–derived cells was analyzed by flow cytometry at days 2, 4, and 6 of culture. The percentages of cells in each quadrant are indicated. (B) Addition of the caspase inhibitor Z-VAD-fmk (50 nM) to the culture media rescued the number of LYVE-1⁺ colonies from Tie2^-/- ES cells (▪). (C) The number of PECAM-1⁺ colonies from Tie2^-/- ES cells (▪) was also rescued in the presence of Z-VAD-fmk (50 nM). Results are expressed as mean ± SD. (D) In the presence of Z-VAD-fmk, the size of Tie2^-/- lymphatic endothelial cells was unchanged, although the number of lymphatic colonies was partially rescued. Scale bars, 20 μm.