Retinal alterations induced by continuous light in immature rats. II. Zinc iodide-osmium tetroxide (ZIO) reactive sites in the rod outer segments.

Immature albino rats were subjected to (a) continuous illumination for 5-9 days, or (b) continuous illumination followed by prolonged darkness. Their electroretinographic responses and the ultrastructural characteristics of the rod outer segments, as revealed by a mixture of zinc iodine-osmium tetroxide (ZIO) at different temperatures, were studied and compared with those of a control group maintained in a cyclic rhythm of light and darkness. Noteworthy differences in the distribution of ZIO reactive sites were observed in the rats exposed for 5-9 days to continuous illumination (no electroretinographic responses) as compared with normal controls. At 4 degrees C, ZIO staining was negative in the rods of illuminated rats whereas at 20 and 60 degrees C it was positive inside the tubular and vesicular structures. After prolonged darkness, in rats with a partial electroretinographic recovery and ultrastructural restoration, the ZIO reaction showed a similar pattern to that observed in the control group, ZIO deposits being found both in the intra- and extradiscal spaces.