Flt-1^−/− ES cell cultures have mitotic endothelial cells.

Day 7 (A-D) or day 6 (E,F) WT (A,C,E), andflt-1^−/− (B,D,F) attached cultures were labeled with antibodies to PECAM (green) and phosphohistone H3 (red), then stained with the nuclear marker topro-3 (blue). The arrowhead (B) shows a phosphohistone H3⁺ nonendothelial cell (PECAM⁻), whereas the arrow (B) points to a phosphohistone H3⁺ endothelial cell (PECAM⁺). Notice the increase in phosphohistone H3⁺/PECAM⁺ cells inflt-1^−/− cultures relative to WT cultures. All panels are confocal images at ×40 magnification.