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Structural and biochemical changes in vitamin A--deficient rat retinas.

The levels of rhodopsin and opsin were investigated in relation to the maintenance of retinal structure in retinas of vitamin A--deficient rats in low levels of cyclic illumination (1.5 to 2 foot-candles). Rhodopsin levels decreased in the deficient retinas to approximately 20% of control at 9 weeks, and this level was retained through 39 weeks on the deficient diet. Opsin levels decreased at a slower rate but reached about 20% of control levels at 32 weeks. Despite the decrease in rhodopsin levels, obvious deterioration of disc structure was not observed until 16 weeks of deficiency, when opsin levels had already decreased to 60% to 70% of control. The structural disruption of photoreceptor outer segments was localized initially in discs of the distal third. Rod cell degeneration preceded cone cell degeneration in vitamin A--deficient retinas. Most of the rods and cones persisted in the posterior retina at 23 weeks on the deficient diet; however, by 40 weeks, only 11% of the rod nuclei remained. In contrast, about 63% of the cone nuclei were present at 40 weeks of deficiency. The photoreceptor cells were affected by the deficiency to a greater extent in the inferior hemisphere than in the superior hemisphere of the eye.