Endothelial cell divisions orient perpendicular to the nearest long axis. ES cell–derived vessels (WT; Tg PECAM-eGFP) were imaged for various lengths of time, and divisions in or around sprouts were scored for division orientation. In all series, the yellow cell is the scored endothelial cell just prior to division, the pink cells are the daughter cells, and the numbers in the lower right represent elapsed time in minutes. In all cases panel v diagrams the division that was scored, with the parent vessel long axis the broken blue line, the sprout long axis the dotted green line, and the division angle the broken orange line. (A) An endothelial division that occurred prior to nearby sprout formation. Although the sprout (arrows in panels iii and iv) migrated almost perpendicular to the vessel long axis, the division was oriented perpendicular to the parent vessel long axis (n = 2 of 2). (B) An endothelial division that occurred in the sprout field, at the base of a formed sprout but in the parent vessel. These divisions also oriented perpendicular to the parent vessel long axis (n = 6 of 6). (C) In contrast, an endothelial division that occurred within a formed sprout oriented perpendicular to the sprout vessel long axis (n = 5 of 5).