Endothelial cell divisions are randomly oriented flt-1^−/− ES cell–derived vessels. Mouse ES cells (flt-1^−/−; Tg PECAM-H2B-GFP) differentiated to day 7 to 8 were imaged for several hours prior to fixation and staining for PECAM-1. (A) Time lapse images of a representative movie from 0 minute (i) to 240 minutes (vi), showing the H2B signal in vessels. Panel iii was used to calculate the angle of division relative to the vessel axis, according to the drawn yellow lines. The numbers in the lower right represent elapsed time in minutes. (B) Calculation of division angles relative to the vessel long axis. WT endothelial division angles are shown in blue, and flt-1^−/− endothelial division angles are shown in purple. Ninety degrees is perpendicular to the vessel long axis, and 0 degree is parallel to the vessel long axis; n = 125 divisions for WT vessels (same data as shown in Figure 2) and n = 93 divisions for flt-1^−/− vessels. (C) Representation of endothelial division angles, with the vessel long axis diagrammed by the long horizontal lines. Each shorter line represents 3 angle measurements that were close or equivalent to each other. For the video of panel A, see Video S2.