Optical Properties of Mitochondria in the Retina

Article: Vision scientists discover new angle on path of light through photoreceptors
Source: National Eye Institute / National Institutes of Health
Published: March 2, 2022

Isolated ellipsoids in the ground squirrel retina exhibit
angular dependence similar to the Stiles-Crawford effect

Scientists studying mitochondria in the retina recently discovered that these energy-producing organelles also have an optical function to channel light from the inner segment of photoreceptors, where the mitochondria are concentrated in what is called the ellipsoid zone, to the photoreceptors' outer segment where the light is converted to electrical signals to be relayed to the brain. Müller glia, which axially span the retina from its inner surface to the base of the photoreceptor, have been observed to have fiber-optic-like properties to guide light to the photoreceptors. However, the photoreceptors' mitochondria are the last structure that light must pass through from the inner segment to the outer segment. Using a ground squirrel model, which have retinas comprised mostly of cone photoreceptors, and modified confocal microscopy, the researchers observed that instead of scattering or absorbing light (as would be expected of an obstructing medium), the bundle of tightly packed mitochondria concentrated the light into a thin pencil-like trajectory. This lens-like property was similar to the Stiles-Crawford effect, in which the direction of incident light determines its intensity; for example, light entering the pupil along the optical axis is perceived as more intense than light entering from other angles. That the photoreceptors of the retina exhibit the Stiles-Crawford effect could be clinically useful to detect diseases of retinal degeneration that are thought to involve mitochondrial dysfunction, such as retinitis pigmentosa. Finally, the researchers note that the "microlens" properties of the photoreceptors' mitochondria are reminiscent of the compound eyes of insects and other arthropods, providing insights into the convergent evolution between the compound eyes of arthropods and the camera-like eyes of vertebrates.

My rating of this study: ⭐⭐⭐

Ball JM, Chen S and Li W. "Mitochondria in cone photoreceptors act as microlenses to enhance photon delivery and confer directional sensitivity to light." Science Advances.  8(9). 2 March 2022. https://doi.org/10.1126/sciadv.abn2070