Experimental Study Of Posterior Vitreous Detachment Induced By Plasmin In Rats

Posterior vitreous detachment(PVD) is a separation of the cortical vitreous gel from the inner limiting lamina as far as the posterior border of the vitreous base.It is a age-related pathological process.But found clinically,complete PVD has an impact on the course of various vitreoretinal disorders including diabetic retinopathy,macular hole,retinal vein occusion and so on.And complete removal of the cortical hyaloid is often a primary aim of vitreretinal surgery.Despite improved surgical technique and skills,complete mechanical removal of the vitreous remains difficult.Surgical manipulation may cause retinal breaks and haemorrhage,particulary in children and young adults.Moreover,remnants of the poeterior hyaloid provide a scaffold for subsequent cellular proliferation and fibrocellular contraction.,resulting in epiretinal membrane formation and proliferative vitreoretionpathy.The term "pharmacological vitreolysis"has recently been introduced for the use of reagents that alter the molecular organization of the vitreous thereby providing an alternative to, or an adjunct to, mechanical vitrectomy.A number of enzymes have been used experimentally to modify vitrous structure and/or to induce PVD . Plasmin is one of them. Plasmin is a serine protease that mediates the fibrinolytic process and modulates the extracellular matrix.It hydrolyze a variety of glycoproteins , including laminin and fibronecin,both of which are present at the vitreoretinal interface and are thought to play a key role in vitreoretinal attachment.Plasmin dose not degrade collagen typeâ…£,a major component of basement membranes and the ILM.Therefor , plasmin holds theoretical promise to induce PVD without damaging the ILM and the retina . Plasmin injected intravitreally shortly before vitrectomy was sufficient to induce PVD in rabbits without damaging the highly sensitive retina(Verstraeten et al.,1993).It is the frist time that plasmin was used to induce PVD,and from that time on plasmin become "pharmacological vitreolysis"study focus continually.Earlier reports demostrate that only in combination with vitrectomy or intravitreal gas injection does plasmin facilitate the cleavage between the vitreous cortex and the ILM in procine eyes and rabbit eyes .But recently reports show without mechanical approach such as vitrectomy or additional gas injection,plasmin alone was sufficient to achieve complete seperation at the vitreoretinal junction in procine and human eyes. This study use rat as model, investigate the efficacy,dosage andmorphological changes of intravireal injection of plasmin in producing PVD and the mechanism of plasmin in inducing PVD. Healthy wistar rats were divided into A,B two groups randomly.Plasmin(2.38ug or 4.76ug) was injected into the vitrous cavity of the right eyes as experimental groups.The fellow eyes received balanced salt solution(BSS) as control.According to 3 different observation time,5 rats of each group were killed afer 1 hour,24 hour and 7 days.Light microscopoy examin the retinal pharmacologic changes and account the number of the retinal gangion cells;scanning electron microscopoy (SEM) were performed in all eyes to investigate the cleaving effect of plasmin at the vitroretinal interface ;transmisson electron microscopoy were performed in one rat (plasmin, 4.76ug 7d) to investigate the ultrastructure of the vitroretinal interface. SME revealed that plasmin cleaved the attachment of the vitrous collagen fibrils to the ILM in all experimental eyes but all control eyes showed a dense network of collagen fibrils covering the ILM,this demonstrate the efficacy of plasmin in separating the PVC from the ILM.But remnants of cortical vitreous were diffirent in experimental eyes.there was a direct correlation between both the concentration of palsmin used and the length of exposure and the degree ofvitroretrtinal separation. After treatment with 4.76ug for 7days,there were no remments of collagen fibrils left at the ILM at the posterior pole and at the equator in all five experimental eyes.TEM of one eye with 4.76ug for 7days showed on residul collagen fibrils on the retinal surface, the cellular anatomy of the retina were unchanged .There was no diffirence in the lamina densa,the lamina lucida and the adjacent Muller cells between the plasmin treated and the control eyes. Light microcopoy revealed in all plasmin treated eyes the morphology of the retina was no abnormalities compared with the control eyes and the ILM were continuous membrane and well preserved.The number of the RGCs was not only no significant difference between the experimental groups and the control groups statistically (P >0. 05),but also among the different plasmin treated experimental groups in the same time .Therefore this study demonstrate the morphology of the retina and the number of the RGCs was unchanged in the dosage we used and the time we observed. Limited by the experimental condition and time,we didn't examin the toxic effect of plasmin to the function of the retina which was valued by ERG,FFA and so on. In conclusion, plasmin injected intravitreally has cleaving effect between the PVC...