Randomized endothelial division orientation is rescued by a sflt-1 transgene that rescues flt-1^−/− vessel dysmorphogenesis. ES cell–derived vessels were wild-type (WT), flt-1^−/−, or flt-1^−/− with a sflt-1 transgene (flt-1^−/−;Tg PECAM sflt-1#26, indicated as sflt26; or flt-1^−/−;Tg PECAM sflt-1#33, as sflt33). Cultures were differentiated to day 8, fixed, and stained for PECAM-1 (green) and phosphohistone H3 (red). (A) Representative vessels of the indicated genotypes, with white arrows pointing to endothelial divisions that were scored. (B) Graphic representation of division angles from the different genetic backgrounds. Ninety degrees is perpendicular to the vessel long axis, and 0 degree is parallel to the vessel long axis. WT is blue (n = 19), flt-1^−/− is purple (n = 29), sflt26 is yellow (n = 25), and sflt33 is light green (n = 23). (C) Representation of endothelial division angles, with the vessel long axis diagrammed by the long horizontal lines. Each shorter line represents a single angle measurement.