Endothelial cell divisions are oriented perpendicular to the vessel long axis in retinal vessels in vivo. Rat retinas were harvested on days P3 to P5 and processed for staining as described with Griffonia B4 isolectin (green) to visualize vessels and phosphohistone H3 (red) to visualize DNA in mitotic cells. (A) Several examples of divisions in retinal vessels that were scored for division angle. (B) Calculation of division angles relative to vessel long axis. Ninety degrees is perpendicular to the vessel long axis, and 0 degree is parallel to the vessel long axis; n = 86 divisions. (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.