The occurrence of cone inclusions in the ageing human retina and their possible effect upon vision: An electron microscope study

During normal ageing, photoreceptors of the human retina undergo various structural changes. We examined retinas from 33 donors (56 eyes; age span 13-94 years) by electron microscopy to see morphological changes in the cones with ageing. We show mitochondrial alterations and occurrence of electron-dense globules in the cone inner segments from the fifth decade of life. The globules are more prevalent in the macular cones than those in the mid-peripheral or nasal retinas (p<0.05) and absent in peripheral retinal cones and rods. They peak in the sixth decade and then decline in the seventh decade (p<0.05), from seventh to ninth decade, however, there was no significant change in their occurrence in the cones. We also show a type of inclusion, made up of bundled microtubules, which occur exclusively in the macular cones at the eighth decade of life. Evidence suggests that altered cone mitochondria with cristae remnants and dense matrix participate in globule formation in the ageing retina. Such mitochondrial changes may cause energy depletion, and bundling of microtubules (to form filamentous inclusions) could result in decreasing intracellular transport, in which case cones may die in the long run. These factors may be responsible for reported cone loss in the human retina with ageing.