Light-induced smooth endoplasmic reticulum rearrangement in a unique interlaced compartmental pattern in Macaca mulatta retinal pigment epithelium

Purpose: To investigate light-induced modifications of smooth endoplasmic reticulum (SER) of retinal pigment epithelium (RPE) in primates. Methods: Eyes of three terminally anesthetized Rhesus monkeys were exposed to 5000 lux for 10 min or kept in the dark. Transmission electron microscopy (TEM) and electron tomography were conducted on small fragments of retina sampled from different regions of the retina. Results: RPE cells SER shows a previously unknown arrangement characterized by an interlaced compartmental pattern (ICP). Electron tomograms and 3D-modelling demonstrated that the SER with an ICP (ICPSER) consisted of four parallel, independent and interwoven networks of tubules arranged as interconnected coiled coils. Its architecture realized a compact labyrinthine structure of tightly packed tubules stabilized by intertubular filamentous tethers. On average, the ICPSER is present in about 14.6% of RPE cells. Though ICPSER was preferentially found in cells located in the peripheral and in the para/perifoveal retina, ICPSER cells significantly increased in number upon light exposure in the para/perifovea and in the fovea. Conclusions: An ICPSER is apparently a unique feature to primate RPE. Its rapid appearance in the area centralis of the retina upon light exposure suggests a function related to the foveate structure of primate retina or to the diurnal habits of animals that may require additional protection from photo-oxidation or enhanced requests of visual pigments regeneration.