LIGHT DAMAGE IN ROD OUTER SEGMENTS - THE EFFECTS OF FIXATION ON ULTRASTRUCTURAL ALTERATIONS

Purpose. In this electron microscopical study, we compared effects of chemical fixation versus cryofixation on the ultrastructure of acute r od outer segment alterations in the rat retina. Methods. The alteratio ns were induced by toxic levels of diffuse white light. Albino rats we re exposed to 2000 lux for 30 min. Samples from one eye of each animal were fixed by high pressure freezing and samples from the other eye w ere fixed by standard glutaraldehyde procedures. Results. Light expose d retina showed major differences in their rod outer segments, inner s egments and photoreceptor synaptic regions in chemical fixation. In pa rticular gross vesiculations of outer segment membranes were produced in light exposed experiments. In contrast, in cryo-fixed samples such prominent changes were not observed in outer segment membranes. There was, however, occasional formation of small vesicles and a reduction o f the cilium diameter in response to light damage. In the dark adapted control group the morphology of chemically fixed and cryo-fixed photo receptors was similar. Conclusions. We conclude, that cryo-fixed sampl es better represent the living state of the retina, because high press ure freezing is a purely physical method and acts much faster than che mical fixation.