Studies on the fine structure and the rhodopsin cycle of the rabbit retina in experimental degeneration induced by sodium iodate

Abstract

The authors investigated the ultrastructure and the rhodopsin cycle of the rabbit retina by experimental degeneration with sodium iodate. The first ultrastructural changes were observed in the pigment opithelium, whose cells showed early and severe involvement ending in complete destruction within a few days. A swelling of the outer side of the Muller cells and the degeneration of the outer segments of the visual cells followed the damage to the pigment opithelium. In the advanced stages of the treatment, the degenerated cells of the pigment epithelium were replaced by modified pigment cells. These new cells had peculiar cytological features and marked phagocytic activity toward fragments of the outer segments of the photoreceptors and of the disrupted pigmented cells. Similar degenerative changes developed in the animals treated under conditions of light and dark adaptation. In the light-adapted rabbits, however, the rhodopsin measurements revealed an early decrease of the rhodopsin concentration in the retinas treated with sodium iodate, whereas in the dark-adapted rabbits, normal levels of the photopigment were recorded. It is suggested, therefore, that the degeneration of the retinal pigment epithelium could block the process of resynthesis of the rhodopsin under light-adaptation conditions. This suggestion supports the hypothesis that important processes of the rhodopsin cycle, specially the isomerization of vitamin A and of retinene, can be performed only in this retinal layer. Moreover, the fact that the degeneration of the outer segments of photo-receptors always followed severe damage to the pigment epithelium may indicate the existence of some nutritional dependence of the visual cells on the pigmented ones. The experimental degeneration of the retina as a new tool for studying the retinal function is of great importance. In fact, the selective manner in which metabolic poisons affect the different retinal layers allows one to evaluate the morphological and functional effects caused by the blocking activity in these layers.