C3 deficiency results in increased angiogenesis in ROP. WT or C3^−/− mice were subjected to the ROP model. (A) Retinal neovascularization was quantified on day p17 in WT and C3^−/− mice as described under “Hypoxia-induced retinal vascularization, retinopathy of prematurity model.” Retinal neovascularization is presented as the number of epiretinal neovascular nuclei per section. Data are mean ± SEM (n = 12-16 pups per group) and are shown as percentage of control. Number of epiretinal nuclei in WT mice represents the 100% control. *P < .001. (B) Paraffin-embedded axial sections (6 μm) of the retina were stained with PAS and hematoxylin. Pathological neovascularization invades into the vitreous cavity anterior to the internal limiting membrane. C3^−/− mice subjected to the ROP model had more neovascular regions anterior to the internal limiting membrane compared with WT mice. The arrows indicate the neovascular tufts anterior to the internal limiting membrane. Scale bars, 150 μm.