Retinopathy is a common cause of blindness in all age groups. There are 15 million people in the United States with age-related macular degeneration (AMD)—20% of those aged 65 to 74 years and 30% aged 74 years—and 1.6 million of those have neovascular (wet) AMD. In the working age population there are 20 million (7% of the population) with diabetes, and 50% of patients with diabetes mellitus have diabetic retinopathy (DR) after 25 years. In children, retinopathy of prematurity (ROP) is a major cause of vision loss. Approximately 460,000 (11%) of infants per year are born prematurely, and there are 2000 infants per year with severe ROP, even with treatment. There is a high impact of blindness in children (0–70 years). Although fewer children are affected by retinal neovascularization than adults, ROP offers a window into understanding the pathophysiology of retinopathy. The study of ROP has
helped to develop nondestructive therapy for retinopathy. Research in ROP is easier in theory than that in DR or AMD, as there are clear and distinct comparisons to be made: normal in utero development versus development after premature birth. If we understand what factors change between the in utero retina where vascularization proceeds normally without retinopathy and the retina of infants born prematurely, then we may better understand normal vascular development, vessel loss, vessel repair, and neovascularization. ROP occurs over a much shorter time line (10–20 weeks) than does DR or AMD.1 ROP and other ocular diseases with pathologic neovascularization have two phases. The first phase consists of cessation of vessel growth and loss of vessels. In ROP, this phase has its onset at the time of premature birth and is associated with the loss of factors normally provided by the mother in utero. Phase I is also precipitated by the addition of factors in the extrauterine environment, notably oxygen that is above intrauterine levels even in room air. The relative hyperoxia is exacerbated by supplemental oxygen. This shift from the environment in utero leads to cessation of normal retinal vessel growth and vaso-obliteration, leaving peripheral retina avascular. In phase I, oxygen-regulated growth factors are suppressed by higher than normal levels of oxygen and others are lacking because they are normally provided by the mother in the third trimester of pregnancy. As the retina matures after birth, it becomes more metabolically active, and the avascular retina becomes hypoxic, leading to phase II of ROP. The hypoxia of phase II induces a rapid increase in hypoxia-inducible factor (HIF)-regulated growth factors that were suppressed in phase I. Factors missing from the mother may rise slightly if the fetal liver and other organs that produce them have matured sufficiently but still may be lower than in utero levels. To study both phases, we developed a mouse model of oxygen-induced retinopathy to take advantage of the genetic manipulation possible in mice.2 Proliferative retinopathy in the
mouse model develops reliably (and quantifiably) over 10 days, unlike rodent models of diabetes, in which proliferative disease does not develop. Neonatal mice are exposed to 75% oxygen from postnatal day 7 until day 12. Hyperoxia causes vessel regression, and the cessation of normal radial vessel growth occurs mimicking the first phase of ROP. The extent of vasoobliteration can be determined by measuring the nonperfused area in retinal wholemounts. On return to room air, the nonperfused areas of retina become hypoxic, thereby inducing the expression of angiogenic factors and resulting in retinal neovascularization. The neovascular phase in the oxygen-induced animal model is similar to the second phase of ROP in humans and, in addition, mimics many aspects of proliferative diabetic retinopathy and some aspects of neovascular AMD. Neovascular tufts are measured by quantifying endothelial cell nuclei extending into the vitreous in cross sections of retina, or by quantifying the area of vascular tufts in retinal flatmounts. The mouse model has been useful in delineating the molecular changes in both phases of the neovascular eye diseases.
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