Endothelial cell proliferation is reduced in the absence of Apelin or APJ. (A-B) Whole mount immunohistochemistry for PH3 (red) on APLN^+/+ and APLN^−/− retinas counterstained with IsolectinB4 (green). Quantification in panel C shows that there are less PH3-positive ECs in APLN^−/− and APJ^−/− retinas compared with wild-type littermates. The number of PH3-positive cells in the non-ECs (NECs) is not significantly different from the wild-type. N = 7 mice for APLN^+/+ and APLN^−/− and N = 3 for APJ^+/+ and APJ^−/−. (D-K) apelin and apj (apj1A and B) were knocked down in zebrafish using specific morpholinos (MO). After 3 days of remodelling, the vascular area in the bony rays was quantified (G). (H-K) Quantification of PH3 positive ECs showed less ECs undergoing mitosis in the caudal fin electroporated with MOs against APLN and APJ. **P < .01. *P < .05. N = 4 experiments, 3 fish per group. Mann-Whitney test was used. Error bars indicate SEM. Scale bars indicate 80 μm (A-B) and 100 μm (D-F, H-J). Images of retinas were acquired on a Leica TCS SP5 confocal microscope using 10×/0.3 NA objective and images of zebrafish fins using a 10×/0.25 NA objective.