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Wei Li, John Ball, Shan Chen; Cone Mitochondria Enhance Light Transmission. Invest. Ophthalmol. Vis. Sci. 2017;58(8):1037.
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© ARVO (1962-2015); The Authors (2016-present)
In the mammalian retina, before a photon can be detected at the photoreceptor outer segment, it must pass through each layer of the retina. The final obstacle in this path is the densely packed bundle of mitochondria present in each photoreceptor. We found that cone mitochondria in the ground squirrel retina possessed an intriguingly elongated, well-organized structure. We set out to test whether these mitochondria might serve to facilitate light transmission through the cone inner segment, and whether the less well organized mitochondria found in the cones of hibernating ground squirrels might be less favorable for such light focusing.
We investigated these questions using 3D reconstructions of cone mitochondria from serial block-face electron microscopy image stacks, electromagnetic simulations of light transmission through these reconstructed cones, and finally, confocal images of light transmission through whole-mount live retina samples. To assess the differential contributions of cones vs. the remainder of the retina for light transmission, we also obtained confocal images using horizontally sliced retina, in which part or all of the retina residing below cones was removed.
Simulations indicated that the spatial arrangement of mitochondria within cone inner segments enhance light focusing. Additionally, cones in non-hibernating squirrels have a modestly stronger light focusing capability than cones from hibernating animals. Images taken from horizontal retinal slices confirmed the simulation results and also showed that the remainder of the retina proximal to cone photoreceptors possesses minor focusing power.
We conclude from both simulations and imaging that, individual cones possess the ability to focus light upon the outer segments through the strategic location and spatial arrangement of the mitochondria bundle in the inner segment, potentially increasing visual sensitivity.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
Upper panel: Confocal live imaging of light passing through the inner segment of an isolated cone photoreceptor in a piece of ground squirrel retina (yellow: mitochondria labled by TMRE). Lower panel: Electromagnetic simulations of light transmission through the inner segment of a reconstructed cone photoreceptor (yellow: reconstructed mitochondria).
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