Sickle cell disease is caused by a single point mutation in a hemoglobin gene. The defect results in red bloods cells that are abnormally shaped and non-pliable causing anemia, hemolysis, sluggish blood flow and vaso- occlusion. Resultant vaso-occlusions in retina have been most often observed in arterioles, causing ischemia in large areas of the retinal vascular tree. Neovascularization can eventually develop distant from the vaso-occlusion, followed by hemorrhage and fibrosis, and ultimately retinal detachment. The precise pathogenesis of the vaso-occlusions remain obscure. The goal of this proposal is to investigate, in cadaver eyes, the role of three potential contributing factors in the vaso-occlusive process in retina: platelet aggregation, sickled RBC's, and altered fibrinolysis. Platelets in sickle cell patients are not known to be more adhesive, but local factors like adenosine diphosphate can stimulate in situ aggregation. The concentration of ADP is controlled by endothelial and smooth muscle ADPase. Local ADPase will be visualized by enzyme histochemistry in whole retina from one eye by flat embedding reacted retinas in glycol methacrylate. The ADPase activity, predominantly in retinal vasculature, is visualized en bloc by dark field microscopy, can be quantitated by image analysis, and areas of interest can be sectioned. The 2 um sections permit high resolution visualization of ADPase activity and morphometric analysis of the constituents in and adjacent to vaso-occlusions with special stains(RBC, fibrin, WBC, endothelium). Sickled and non-sickled erythrocytes will be sought at the sites of occlusion. Where possible, the proportion of sickled and non-sickled forms will be identified. The fellow eye will be fresh frozen and cryosectioned for immunohistochemical analysis of the fibrinolytic system: tissue plasminogen activator (tPA), plasminogen activator inhibitor, fibrin, and factor VIII-related antigen (for intrinsic clotting cascade and the condition of endothelial cells). We have already observed a decrease in retinal vascular tPA in diabetic retinopathy, perhaps due to a decrease in blood flow analogous to that which occurs in sickle cell disease. Immunohistochemical analysis of basic fibroblast growth factor and transforming growth factor B will also be performed because of their role in controlling the fibrinolytic system in endothelium and potentially affecting the neovascular processes which follow ischemia. This study should help elaborate the mechanism of retinal vaso-occlusion in sickle cell disease and the primary determinant of eventual neovascularization, hemorrhage, fibrosis, retinal detachment, and blindness.
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