Oxygen induced retinopathy: the kitten model re-examined

Detection and Quantification of Retinal Neovascularization Using BrdU Incorporation

Purpose

The retina is a commonly used model for angiogenesis research due to its special characteristics. Oxygen-induced retinopathy (OIR) provides a useful model to study ischemia-induced neovascularization (NV) and to develop anti-angiogenic therapeutics. The purpose of this study was to develop a simple, accurate, and less-subjective quantification method for retinal NV in the OIR model.

Methods

To address this challenge, we combined the conventional vascular staining and BrdU labeling of newly formed vascular cells to detect and analyze retinal NV. With daily injections of BrdU, which was incorporated into the DNA of newly formed retinal vessels under the OIR condition, ischemia-induced retinal neovasculature with BrdU labeling was distinguished from pre-existing vasculature and accurately quantified using the ImageJ program.

Results

Compared with conventional quantification methods using isolectin B4 staining of the entire vascular network, BrdU labeling allowed us to distinguish newly formed vessels from the pre-existing vessels and to objectively quantify the newly formed vessels, which was verified in OIR mice with intravitreal injections of an antibody-neutralizing vascular endothelial growth factor.

Conclusions

BrdU labeling provides a useful and sensitive method for studying retinal NV and evaluating the therapeutic effects of medical interventions against pathological angiogenesis.

Translational Relevance

Quantitative, straightforward, and objective observation and evaluation of pathologic neovasculature are important to study the pathogenesis of NV and therapeutic effects using animal models.

Vascular endothelial growth factor in eye disease

Collectively, angiogenic ocular conditions represent the leading cause of irreversible vision loss in developed countries. In the US, for example, retinopathy of prematurity, diabetic retinopathy and age-related macular degeneration are the principal causes of blindness in the infant, working age and elderly populations, respectively. Evidence suggests that vascular endothelial growth factor (VEGF), a 40kDa dimeric glycoprotein, promotes angiogenesis in each of these conditions, making it a highly significant therapeutic target. However, VEGF is pleiotropic, affecting a broad spectrum of endothelial, neuronal and glial behaviors, and confounding the validity of anti-VEGF strategies, particularly under chronic disease conditions. In fact, among other functions VEGF can influence cell proliferation, cell migration, proteolysis, cell survival and vessel permeability in a wide variety of biological contexts. This article will describe the roles played by VEGF in the pathogenesis of retinopathy of prematurity, diabetic retinopathy and age-related macular degeneration. The potential disadvantages of inhibiting VEGF will be discussed, as will the rationales for targeting other VEGF-related modulators of angiogenesis.

Vitreal macrophages express vascular endothelial growth factor in oxygen-induced retinopathy

PURPOSE

The possibility of vitreal macrophages playing an angiogenic role in oxygen-induced retinopathy (OIR) was investigated. Oxygen-induced retinopathy was produced in newborn animals with the purpose of modeling the proliferative phase of human retinopathy of prematurity (ROP).

MATERIALS AND METHODS

To produce OIR in neonatal mice, litters at postnatal day 7 were placed in 80-90% oxygen for a period of 5 days and then returned to room air. Pups were killed on days 7, 12, 15, 17 and 20 over the postnatal period and were perfusion-fixed using a saline wash-out, followed by 4% paraformaldehyde and then India Ink. Eyes were enucleated and either whole-mounted, or snap-frozen and cryosectioned. Immunostaining procedures were used to visualize macrophages and vascular endothelial growth factor (VEGF) protein. The primary antibodies used were anti-F4/80 and antimouse VEGF, respectively. Vitreal macrophages closely associated with the vitreo-retinal interface (within 25 microm of the inner limiting membrane) were counted. In situ hybridization procedures were used to analyse for the presence of VEGF mRNA transcript in vitreal macrophages.

RESULTS

Macrophage numbers were found to significantly increase (P < 0.05) in eyes from oxygen-treated animals compared with those from age-matched controls. A close spatial relationship was observed between macrophages and vitreal neovascular sprouts. In addition, vitreal macrophages were also found to transcribe and express VEGF in the oxygen-treated animals during the vasoproliferative phase.

CONCLUSIONS

Our results raise the possibility that vitreal macrophages play a role in the pathogenesis of OIR and by inference, ROP.

Oxygen-induced retinopathy in the newborn rat: a scoring system for the evaluation of retinal vascular changes. Scoring system for OIR in the rat

The authors describe a scoring system they have developed for evaluating damage produced in the retinal vascular network of the newborn rat by exposure to oxygen. India ink-injected retinal flat mounts are used. The scoring system provides for division of the retina into three concentric zones with the optic disk as a center point. Each retinal quadrant is then examined for the presence of the following lesions of progressive severity: vaso-obliteration of the capillary network, loss of the periarteriolar capillary-free zone, newly formed capillary tufts. When more than one type of lesion is present in a quadrant, only the most severe lesion is considered. The numerical score corresponding to that lesion is then multiplied by the factor corresponding to the concentric zone in which the lesion lies to give the quadrant score. The total retinal score is represented by the sum of the four quadrant scores. This type of system, if adopted by other investigators, can facilitate precise and systematic description of the results of different experimental protocols.

Oxygen-induced retinopathy: ultrastructure of vitreous new vessels in the kitten model

Oxygen-induced retinopathy was produced by exposing 3-day-old kittens to 80% oxygen between 60 and 105 hours. They were then allowed to survive in room air until day 15, 21 or 28 when they were sacrificed after perfusion with colloidal carbon. Specimens were prepared for transmission electron microscopy. Ninety separate vitreous capillaries from oxygen-treated animals were examined. A total of 235 intercellular junctions were examined, 116 of them from the 15-day old animals. In the 15-day old animals, five junctions of 116 were open and the remainder were tight. No open junctions were seen in 21- or 28-day-old animals. In one capillary from a 15-day animal, fenestrated endothelium was seen in an aberrant, intraluminal loop of endothelium which formed no part of the blood/tissue barrier. The wall thickness of the vitreous new vessels seemed to decrease and the number of vesicles and vacuoles appeared to increase with increasing age. The basement membrane of the vitreous new vessels was scanty. In some sections, cells, presumably macrophages, were seen outside the new vessels. It is concluded that the increased permeability of the vitreous new vessels in 15-day-old animals can be explained by, and is possibly totally due to, the presence of open endothelial junctions.

Oxygen-induced retinopathy in the rat model

Identification of a suitable animal model is essential for the continued study of retinopathy of prematurity (ROP). Since 1984 we have used the newborn rat for the study of oxygen-induced retinopathy (OIR). The rat retina is highly immature at birth. Like those of humans, the retinal vessels arise from mesenchymal precursors, but contrary to that which occurs in humans, canalization of the rats inner retinal vessels is not related to the presence of cystoid spaces. In addition, only immature Stage I photoreceptors are present around the optic disk at birth. This extreme immaturity makes the rat retina highly susceptible to direct damage from oxygen. Oxygen-induced retinopathy can be produced by exposing the newborn rat to 80% oxygen for the first 7-10 days of life. We have demonstrated that OIR does not develop when oxygen is administered under conditions of moderate hyperbarism (+1.8 atm). It is possible that hyperbarism exerts a protective effect on the immature retinal vessels by inducing a vasoconstrictive response which reduces the amount of oxygen transported from the choroid to the inner retina during hypoxia. I recently hypothesized that this vasoconstriction might also affect the ciliary body, thus reducing the quality of aqueous produced, and we are currently studying the relationship between development of the immature retinal vessels in the rat and production and drainage of the aqueous. The question we are attempting to answer is whether a condition of relatively increased intraocular pressure is capable of promoting the development of OIR.

Retinopathy of prematurity

Over the last decade major advances have been made in the understanding of the pathogenesis and evolution of retinopathy of prematurity (ROP). The increased survival of very small premature infants in modern neonatal intensive care units has led to the resurgence of this potentially blinding disease. ROP appears to be a multifactorial disease, the prevention of which is probably impossible even now, with the most accurate methods of blood gas monitoring and oxygen restriction. In addition to oxygen, there are a number of significant risk factors, such as birth weight and gestational age, ventilator hours, hyper and hypocarbia, hypoxia and acidosis, xanthine therapy and probably bright light. Current data suggest that the level of antioxidants in the immature retina is relatively low and therefore oxygen radicals which accumulate in the preterm baby's retina may play an important role in the pathogenesis of ROP. The treatment of the disease in both its "active" and "cicatricial" stages emphasizes the need for a new classification which could serve as a common international language through which results may be compared. Vitamin E was suggested in some studies to be helpful in preventing the severe stages of the disease, but its efficacy has yet to be proved. Treatment modalities such as photocoagulation, cryotherapy and vitrectomy are being tried as a means of therapy in the more advanced stages of the disease. Preliminary results of a large multicenter study support the efficacy of cryotherapy.

Laminin-like immunoreactivity in induced new vessels in kitten and mouse eyes

Laminin is a basement membrane glycoprotein, widely distributed in body tissues. It has been found on capillary basement membranes in both developing and adult retinal vessels. Oxygen-induced retinopathy was produced in kittens and mice by exposing them to a high oxygen environment during the newborn period. When the animals developed a proliferative retinopathy after a period of survival in room air, they were sacrificed and the eyes enucleated and embedded in paraffin wax. Cross-sections were cut and de-waxed. After preliminary digestion with pepsin, we used a fluorescein isothiocyanate (FITC)-labelled double antibody technique to identify laminin-like immunoreactivity (LLIR). LLIR was found on both intravitreal and intraretinal new vessels in the kitten and mouse, indicating that it is probably secreted by endothelial cells during the formation of pathological new vessels.

Cytogenesis in the developing retina of the cat

Cytogenesis in the cat retina was studied using tritiated (3H) thymidine autoradiography and nuclear stains. Three zones of cell division were recognized. In the first zone cell cleavage occurs at the outer limiting membrane. The distribution of these mitotic figures is uniform as early as an embryonic age of 29 days (E29) until E50. At about E50 a "cold spot", made apparent by the absence of mitotic figures, is evident at the site of the developing area centralis. This spreads to encompass the whole retina by postnatal day 10 (P10). A second zone of cell division was recognized by the presence in the developing inner nuclear layer of 3H-thymidine labelled nuclei which do not migrate to the outer limiting membrane (ventricular surface) to divide. Some of these labelled nuclei are located in regions of the retina where cytogenesis at the ventricular surface has ceased. A third zone was observed in the optic nerve fibre/ganglion cell layers from about E54 until beyond the first postnatal month. This activity gives rise to vascular endothelial cells in the nerve fibre/ganglion cell layers. Once established, the developing vascular cells invade the inner plexiform and inner nuclear layers to form the deeper capillary net.

Current concepts of the effect of oxygen on the developing retina

The role of oxygen in the development of retrolental fibroplasia and the mechanism of oxygen changes in the developing retina are discussed. The primary effect of oxygen is one of Vascular closure and the secondary response, which occurs after removal from increased oxygen, is retinal neovascularization. A brief discussion of recent observations on the potential role of vitamin E in the prevention of retrolental fibroplasia is presented.