Immunohistochemical detection of extravasated fibrinogen (fibrin) in human diabetic retina.

In diabetic retinopathy, breakdown of the blood-retinal barrier is an early functional disorder that can cause retinal edema, which in turn results in visual disturbance. Hard exudates, composed mainly of lipid and proteinaceous material, are one sign of chronic retinal edema caused by long-standing leakage from the vessels due to breakdown of the blood-retinal barrier. Utilizing diabetic retinas in which hard exudates were present, we performed immunohistochemical staining for fibrinogen. Because fibrinogen is a serum protein, its extravascular localization implies the existence of blood-retinal barrier breakdown. Our studies showed that the extravasated fibrinogen from blood-retinal barrier breakdown accumulated in the hard exudates and in areas of hemorrhage found primarily in the outer plexiform layer and was then phagocytosed by macrophages.