Abstract
Purpose :
The vertebrate retina features an inverted structure with multiple neural layers through which photons must pass–risking premature absorption or scattering—prior to detection by light-sensitive opsin molecules located in photoreceptor outer segments (OS). Intriguingly, mammalian photoceptors hold numerous mitochondria in their ellipsoid region immediately before light reaching the OS. These mitochondria, likely supporting the high metabolic needs of phototransduction, however, could potentially impair light delivery to the OS due to their high membrane contents that might cause excessive light scattering. Conversely, they might enhance the delivery of light to the OS, taking up a potential optic role, which is not unprecedented for retinal structures (e.g. rod nuclei, Müller glia, and cone oil droplets). We thus set out to investigate the potential optic role of mitochondria in photoreceptor ellipsoid region.
Methods :
Here, using a horizontal slice preparation from the ground squirrel retina, in which plentiful cones contain mitochondria in a bundled arrangement closely resembling those in primate, we directly imaged light transmitted through the mitochondria bundle (MtB). In addition, we performed electromagnetic simulations of light transmission based on the MtB structures translated from reconstructions of Blockface Scanning EM images.
Results :
We directly demonstrated that such MtB concentrates light several fold onto the OS for detection. In addition, this “microlens”-like feature of cone mitochondria produces an angular dependence of light intensity quantitively consistent with the Stiles-Crawford effect, a psychophysical phenomenon believed to improve visual resolution.
Conclusions :
Thus, in addition to their function as a necessary powerhouse, cone mitochondria play a critical optical role. These findings provide needed insights into their role in interpreting results from noninvasive optical tools in ophthalmology.
This is a 2021 ARVO Annual Meeting abstract.