Potential role of myeloid bodies in protection against photo-oxidative damage of the retinal pigment epithelium

Myeloid bodies are organized forms of the smooth endoplasmic reticulum that occur in the retinal pigment epithelium of vertebrates. The latter is a monolayer of cells, situated between the neural retina and the chroroid vasculature, that is essential for photoreceptor homeostasis and function. Myeloid bodies operate in an environment of high oxygen tension and chronic light exposure where phagocytosis of photoreceptor outer segments takes place, resulting in photo-oxidative stress. Over time, such damage can result in pigment epithelium cell death, retinal degeneration, and blindness. The purpose of myeloid bodies is unknown. Here, unusually large myeloid bodies are described with cisternae perpendicular to the basal epithelium (Bruch's) membrane that occur throughout the differentiated retina of the sablefish, Anoplopoma fimbria, a marine fish with early life history exposure to the full spectrum of light. Immunogold labelling of thin sections demonstrated exclusive cisternal binding of short-wavelength (blue) light absorbing opsins and a zeaxanthin carotenoid associated enzyme. Because blue light is highly efficient at inducing photo-oxidation, the sequestration of these compounds within myeloid bodies suggests a novel, optical role for the smooth endoplasmic reticulum in filtering light to prevent photo-oxidative damage.