Postnatal retinal angiogenesis is delayed in β3ΔRGT mice, but not in β3/β1(EGK) mice. Retinas on postnatal day 3 (P3) from β3^+/+ and the indicated β3 mutant littermates were stained with anti-CD31 (to mark blood vessels) (A-D) or with fluorescein isothiocyanate–isolectin B4 (for vessels) and anti-Ki67 (for cell proliferation) (E-J) (supplemental Figure 5), and cells were imaged as described in “Methods.” (A,C) Representative images of whole mount retinas with ×10 oil-dipping objective. Bar represents 500 μm. (B,D) Quantification of retinal vessels based on radial length from optic nerve to the periphery. (E,G) Quantification of endothelial tip cells at the retinal vascular front. (F,H) Quantification of endothelial cell filopodial extensions per 25-μm vessel length. (I,J) Quantification of Ki67-positive endothelial cells at the retinal vascular front. (B,D) Data from 12-14 retinas; (E,F,I), data from 8 retinas and 4 mice; (G,H,J) data from 4 retinas and 2 mice. Data are means ± SEM. *P < .05; **P < .01; NS, not significant (unpaired Student t test).