Chronological Study Of The Hyaloid Vascular Regression After Administration Of MNU In Infant Mice

Background:The hyaloid vascular system consists of pupillary membrane and hyaloid vesselshyaloid artery,vasa hyaloidea propria and tunica vasculosa lentis.It is formed in the course of ocular morphogenesis and provides nourishment to the immature lens,vitreous and retina,and therefore a signaling network may exist between hyaloid vasculature and other ocular structures. It regresses at the late stages of gestation,generally completes by eight and half months,in human and within the first few weeks of postnatal development in rodents so that the transparent photopathway and normal ocular function is assured.The failure of regression of the hyaloid vessels in vitreous develops a disease called persistent hyperplastic primary vitreous(PHPV) or persistent posterior fetal fibrovascular sheath of the lens,an idiopathic developmental eye disease usually accompanied by other ocular malformations,such as retinal dysplasia, microphthalmia,and degenerative changes in lens.Objective:The maturation of retinal photoreceptor cells and the involution of embryonic hyaloid vessels occur contemporaneously during the early postnatal life in mice.Moreover,the single intraperitoneal injection of N-methyl-N-nitrosourea(MNU) in mice induces photoreceptor cell apoptosis.On the basis of these evidences,the present study aimed to explore the relationship between retinal development and hyaloid regression by investigating the influence of different dosages of MNU administration on developing retina and regressing hyaloid vessels in neonatal mice.Methods:A total of 350 newborn C57BL/6 mice were assigned to four different groups-Normal, MNU(40mg/kg) at P1,MNU(75mg/kg) at P1,and MNU(75mg/kg) at P9.Normal group mice were euthanatized on 0,1,2,3,4,6,8,10,12,14,16,18,20,22,24,28,32,36,42 and 44 days of age.The mice in MNU treated groups were injected with single dose of MNU intraperitoneally as indicated,and were sacrificed from the next day onwards at the same time points as for the normal group.At least 4 mice were sacrificed on each time point for every group.H&E sections were obtained from the left eyeballs for histological studies and wholemounts of hyaloid vessels prepared from the right eyeballs were stained with DAPI for quantitative analysis.Results:The regression of hyaloid vessels in normal mice completed by P28.Mice treated with low dose MNU(40mg/kg) at P1 showed similar pattern of hyaloid regression to that with normal mice,and their eye balls were found to have no ocular pathologies except in occasional cases. The histological examination in mice injected with high dose of MNU(75mg/kg) at P1 showed the features,such as microphthalmia,retrolenticular tissue,degenerative lens and retinal rosettes, which mimic the PHPV.Furthermore,retinal rosettes gradually decreased in number between P18 and P28,and only focal irregularities exhibited after P28 in outer nuclear layer.The wholemounts of hyaloid vessels were unable to prepare from these mice as the vitreous cavity was almost obliterated.In mice injected with MNU(75mg/kg) at P9 retinal degeneration and other ocular pathology were not manifested,however,HA stalk was demonstrated at all time points of observation.The quantitative analysis of hyaloid vessels in these mice indicated that the regression process slowed down after P20 and lasted till P42.Conclusion:This study demonstrated that 75mg/kg dose of MNU at P1 sufficiently induces retinal dysplasia for temporary period in newborn mice and its administration at P9 delays the hyaloid vascular regression,indicating the possible role of developing photoreceptor cells in the involution of hyaloid vessels.Future studies on physiological properties of developing photoreceptor cells in MNU treated neonatal mice could elucidate the current findings in detail.