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The toxic effects on rabbit retina following intravitreally injected urokinase at different concentrations were studied with electron microscope. Saline was injected to the one eye of each animal as control. 1,000IU of urokinase produced mild swelling of the mitochondria in the inner retinal layer which appeared reversible within one week period. Following the injection of 3,000IU of urokinase, there were marked changes on first day; degeneration and vacuolization of the axon, degeneration of the synaptic vesicles and inner nuclear layer cells and fragmentation of the outer segment of the photoreceptor cells. The retinal pigment epithelium demonstrated many phagosomes and phagolysosomes. More advanced changes were progressed at one week after injection. With the increased dose of urokinase, more marked cellular changes took place in the inner and outer retinal layers. No evidence of regeneration was observed two weeks after the injection of 3,000IU of urokinase which indicates long lasting toxic effect of the urokinase onto the retina in large doses. Thus, the large dose of clinical application as the intravitreal injection over 3,000IU is much warned and further study on the long term follow up for the observation of the late complication is also suggested.
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