It is unsafe (and virtually impossible) to study heterotypic cell interactions and signaling events within the human preterm retina that cause the biologic features of severe retinopathy of prematurity. Because many newborn nonhuman mammals complete their retinal vascularization postnatally, animal models were developed to test the role of stresses in preterm infants on the pathogenesis of retinopathy of prematurity. The common neonatal animal models of oxygen-induced retinopathy use varying amounts of oxygen to examine the cellular and molecular mechanisms that drive the progression of pathologic changes in retinopathy of prematurity. All models of oxygen-induced retinopathy have limitations, because the animals in such models are not premature. Nonetheless, these models have substantially enhanced our understanding of the pathogenesis of retinopathy of prematurity.
Some current models of oxygen-induced retinopathy involve high levels of oxygenation, similar to those used in the 1940s when retrolental fibroplasia was first described. However, the oxygen stresses in preterm infants have changed greatly since those early days.4
The mouse model of oxygen-induced retinopathy13
is the most widely used, because genetically altered transgenic or knockout mice can be used to study the pathways involved in angiogenesis. However, the mouse model has limitations. First, 7-day-old mice are exposed to high oxygen levels continuously for 5 days, which can cause a partial pressure of arterial oxygen (PaO2
) of 500 mm Hg or more.14
The Extremely Low Gestational Age Newborns study15
tested the hypothesis that preterm infants who had blood gas disturbances on 2 of the first 3 postnatal days of life might be at risk for severe retinopathy of prematurity. That study showed that severe retinopathy of prematurity was more likely to develop in infants with a PaO2
in the highest quartile as compared with the lowest quartile. However, the median PaO2
was approximately 100 mm Hg on day 1 for all stages of retinopathy of prematurity finally analyzed, and on subsequent days, no infant had a PaO2
level as high as 400 mm Hg. Second, the oxygen level in preterm infants fluctuates on a minute-to-minute basis, but in the mouse model of oxygen-induced retinopathy, oxygen exposure is constant.16
Finally, the mouse model of oxygen-induced retinopathy causes vaso-obliteration (destruction of newly formed capillaries) in the central retina, followed by endothelial budding into the vitreous, and the retinopathy does not resemble that in most cases of severe retinopathy of prematurity seen today ().
Retinal Flat Mounts Stained with Griffonia Lectin to Visualize the Retinal Vasculature in Mouse and Rat Models of Oxygen-Induced Retinopathy
Several current models of retinopathy of prematurity recreate fluctuations in oxygen tension, which is recognized as a risk factor for severe retinopathy of prematurity.16-18
The most widely used model of oxygen fluctuations is in the rat, in which oxygen levels fluctuate between 50% and 10% every 24 hours.19
The advantage of the rat model is that it results in fluctuations in arterial oxygen concentrations in rat pups, the extremes of which mimic measured oxygen levels in infants in whom severe retinopathy of prematurity developed.16
The rat model recreates the appearance of severe retinopathy of prematurity with delayed physiologic retinal vascular development and subsequent vasoproliferation. The rat model also causes extrauterine growth restriction, another known risk factor for severe retinopathy of prematurity.20
A limitation of the rat model is that it is relatively difficult to manipulate the rat genome. Thus, most studies of oxygen-induced retinopathy in rats use pharmacologic methods or introduce viral vectors that contain nucleic acid sequences to silence or overexpress genes in order to study signaling pathways involved in the pathogenesis of retinopathy of prematurity. Despite this limitation, the rat model of oxygen-induced retinopathy remains the most clinically relevant model of retinopathy of prematurity, since its biologic features are most like those of severe retinopathy of prematurity in preterm infants ().
The development of the retinal vasculature in humans differs from that in many other mammalian species used as models of oxygen-induced retinopathy.21-24
Vasculogenesis in the human infant eye is ongoing until at least 22 weeks of gestation.25
After that time, it is unknown how retinal vascularization proceeds. On the basis of studies in animals, vascularization has been thought to progress by means of angiogenesis and the extension of existing blood vessels by proliferating endothelial cells that migrate toward a gradient of vascular endothelial growth factor (VEGF).26
Thus, several reasons support revisiting the two-phase hypothesis regarding the pathogenesis of retinopathy of prematurity in terms of vaso-obliteration and vasoproliferation, as described by Ashton in 1954. In human retinopathy of prematurity, there is first a delay in physiologic retinal vascular development rather than vaso-obliteration, with subsequent vasopro-liferation in some infants with severe retinopathy of prematurity (). Therefore, the delayed physiologic retinal vascular development of phase 1 reflects the zone of human retinopathy of prematurity, and the vasoproliferation of phase 2 reflects stage 3 of human retinopathy of prematurity (, , and ).