Vulnerability of the Retina to Reactive Oxygen Species
Experimental data have shown that retinal photoreceptors degenerate when exposed to oxidative challenges such as hyperbaric oxygen, iron overload, or injection of lipid peroxide into the vitreous humor. In addition, the retina degenerates when antioxidant defenses are reduced, which presumably increases lipid peroxidation even in the absence of unusual oxidative stress. The retina has several distinctive characteristics that make it vulnerable to damage from lipid peroxidation; 4 of them are considered here:
Vertebrate rod outer segments are susceptible to damage by oxygen because of their high levels of PUFAs. Their phospholipids contain docosahexaenoic acid, the most highly polyunsaturated fatty acid occurring in nature. It is well established that PUFAs are sensitive to peroxidation in proportion to their number of double bonds.
Rod inner segments are very rich in mitochondria. The majority of endogenous ROS are produced by the mitochondrial electron transport chain, which may leak activated oxygen species.
The abundant oxygen supply through the choroid and the retinal vessels elevates the risk of oxidative damage. Vertebrate retinas maintained in vitro showed at least a sevenfold-higher rate of oxygen consumption per milligram of protein than all other tissues tested (except the adrenal gland). The oxygen tension is highest at the choroid and decreases toward the inner segments of the retina.
There are many chromophores in the outer retina. Light exposure may trigger photo-oxidative processes mediated by 1O2.
Intense light at levels that may be encountered in daily life is phototoxic to the retina. Even though the cornea absorbs UV radiation, the retinas of young people are exposed to UV light in the range of 350–400 nm (young lenses transmit these wavelengths). As the lens yellows with age, it can block wavelengths of up to approximately 430 nm. Because the adult lens absorbs nearly 100% of light below 400 nm, little or no UV light reaches the retina in older people.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.