The Effect Of Preterm Birth On Normal Retinal Development And Oxygen-induced Retinopathy In The Neonatal Rat

BACKGROUNDPreterm birth is a very common clinical phenomenon which leads to the high mortalityand morbidity of the neonates. Accumulating clinical evidences have indicated that therisk of visual impairment and ocular diseases is higher for preterm infants than full-terminfants. Furthermore, preterm birth seems to affect the structural and functionaldevelopment of the retinal vascular system resulting in a lower threshold for thedevelopment of vascular diseases. The most common disease is retinopathy of prematurity(ROP) clinically. ROP is a complex vasoproliferative disorder occurring as a result ofabnormal retinal vascular development in preterm infants. Less mature retinalvascularization is at an increased risk of developing more severe ROP. Small gestationalage and low birth weight are recognized as the most important risk factors of ROP. However, as a multi-factorial disease, the pathogenesis of ROP has not been completelyclarified yet.Oxygen-induced retinopathy (OIR) rat model is widely used in experimentalresearches on ROP based on the fact that the immature retinal vasculature of the neonatalrat is quite similar to that of the newborn preterm infant. However, the pathophysiologicconditions of the termly-born neonatal rat at birth and after birth used in OIR model aredifferent from that of the preterm infant. Researches which simulated postnatal growthrestriction of the preterm infants in neonatal rats found that postnatal growth determinesthe rate of normal retinal vascularization and the degree of abnormal vascularization inOIR. Pretermly-born rats would offer a good platform in combination of immaturity atbirth and possible poor growth after birth, and these features together are closer to theclinical experiences of preterm infants. Although the the preterm rat model has beenwidely used to study human developmental diseases, the effect of preterm birth on retinaldevelopment and OIR in neonatal rats has not been studied.PURPOSE①To investigate the effect of preterm birth on early neuroretinal and superficial vasculardevelopment of rat pups structurally and functionally;②To analyze the effect of preterm birth on the severity of retinopathy in the oxygenexposed neonatal rat as well as the expression of vascular endothelial growth factor(VEGF) and insulin-like growth factor-1(IGF-1).METHODS①Newborn Sprague-Dawley (SD) rats were delivered prematurely at day19of gestationby caesarean section from pregnant female rats. After birth, preterm pups andsurrogate mother were housed in room air. General state and weight change wereobserved. Naturally delivered term pups were used as control.②On postnatal day4,7,10and14, retinal cross-sections were stained with hematoxylinand eosin (H&E) and retinas were dissected, whole-mounted and stained withGS-isolectin B4. The morphology of retinal cells and superficial vessels were observed and the thickness of every layer of the neuroretina and the avascularized area weremeasured. On postnatal day14,21,28and35, electroretinogram (ERG) wasperformed on pups and the amplitude and latency of the waves were recorded.③Neonatal pups were reared in an oxygen chamber with fluctuating oxygen (80%/21%)for two weeks followed by4-8more days in room air. Retinas of pups on postnatal day14,18and22were dissected, whole-mounted and stained with GS-isolectin B4. Thevascular morphology was observed and the avascularized and neovascualr areas weremeasured. The expression of VEGF and IGF-1mRNA in the retina was tested byreverse transcription-polymerase chain reaction (RT-PCR). On day18, retinacross-sections were stained with H&E to identify the preretinal vascular endothelialcells.RESULTS①Preterm birth was induced successfully by the surgery of cesarean section with highsurvival rate of neonatal rat pups. Preterm rat pups had lower body weight thancontrols within the first3weeks after birth (P<0.001).②The morphology of retinal cells and superficial vessels was similar in the two groups.Preterm rat pups had significantly thicker retinas at early stage (P<0.05) and largerretinal avascular areas than controls on different postnatal days (P<0.001). On day14，the retinas kept the same structure in the two groups and retinal vascularizationcompleted. Functionally, the amplitude and latency of ERG waves in the two groupswere similar in two weeks after birth.③After exposure to fluctuating oxygen, preterm rat pups showed similar tortuosity offirst grade retinal vessels and significantly larger retinal vaso-obliterated areas on day14and18and larger retinal neovascular areas on day18than controls (P<0.001). Onday22, residual retinopathy of preterm pups was more than controls (P<0.01). Theexpression of VEGF mRNA in the retina was higher in the preterm rats than controlsand no obvious difference of the expression of IGF-1mRNA was noticed between thetwo groups. CONCLUSIONSPreterm birth induced by caesarean section had obvious negative effect on the bodygrowth as well as the neuroretinal and retinal vascular development of the neonatal rats.The immature retinal vasculature of the preterm pups was more easily susceptible to thedamage of oxygen resulting in more severe and durable retinopathy than term pups. Thisstudy may provide some basic data for studying the multiple effects of numerous riskfactors on the development of ROP in animal models.