The RPE (retinal pigment epithelium) is a tight membrane that lies between the retina and choroid (a blood-rich tissue between the white of the eye and the retina) layers of the eye. This layer, long known to absorb excess light, is now known to have many other roles. It supplies the retina with glucose (an energy source), fatty acids (which help construct the membrane of the receptor, and retinal (part of the visual pigment. Likewise, it removes wastes and water (which must pass through the cells). Receptors, in this case, are the “rod” and “cone” cells of the retina. The retina proper and the RPE co-differentiate starting in the embryo.
Both light received by the receptors and the oxygen in the choroid bear heavy oxidative loads. The disk-like membranes of the rod and cone tips are shed after their destruction, which are later absorbed by RPE cells. Free radicals are also generated. This noxious environment warrants antioxidants, heat absorption by the choroid, and intrinsic repair methods. Despite this defense mechanism that can be sustained for decades, a by-product of shedding receptors can cause decline thereof, starting at a rather young age.
This absorption by the RPE, which takes place in the morning, is promoted by light and the turnover for the entire length of the receptor’s outer segment (the region where light is absorbed) usually takes about 11 days. Together with circadian rhythms, receptor coordination is required for the full effect. Both nocturnal and diurnal animals have the same daily rhythm.
So, what does this mean for us? Well, since I’m not a medical doctor (and certainly not an ophthalmologist), I can’t convey the whole truth or give relevant medical advice. But these phenomena continue to show the universal wear-and-tear decline of the body (and the eyes are certainly no exception), no matter how nourished it is.
(Source: Strauss, Olaf, Webvision, webvision.med.utah.edu)