Fig. 4. Vessel enlargement is associated with increased numbers of endothelial cells and endothelial cell proliferation. Retinal vessels in wholemounts of PBS- or ANG1^4FD-treated mice, stained with BrdU and Cy3-labeled secondary antibody. (A) In control mice, BrdU label (red) is apparent near the retinal periphery where active angiogenesis occurs in P14 mice, as well as in veins (arrows). (B) In ANG1^4FD-treated mice, intense BrdU labeling is apparent in veins (arrows) and in microvessels near veins. (C) Higher magnification of retinal vessels (labeled with isolectin B4, green) shows occasional BrdU-labeled (red) endothelial cells in veins (arrow), and few BrdU cells in arterioles (arrowhead) in control mice. (D) By contrast, in retinal vessels of mice treated with ANG1^4FD, BrdU-labeled endothelial cells are abundant in venous capillaries and veins (arrows). (E) Quantification of BrdU labeling shows approximately four times as many BrdU-labeled endothelial cells in retinal vessels from mice treated with ANG1^4FD as in PBS-treated mice.