A study in ground squirrels shows that not only do mitochondria produce bioenergy in the cone-shaped photoreceptors of the retina of the eye, but they also act as micro-lenses that redirect light to the outer parts tapering of these cells where light is converted into electrical energy. signals.
The finding, published on March 2, 2022, in the journal Scientists progress (“Mitochondria in conical photoreceptors act as microlenses to enhance photon delivery and confer directional sensitivity to light“) by scientists from the National Eye Institute (NEI) provides a clearer picture of the evolution and physiology of vision and the illusory optical properties of the retina. The results of the study could also help in the early detection of eye diseases.
The study’s lead author, Wei Li, PhD, a senior researcher in the NEI Retinal Neurophysiology Section, said, “We were surprised by this fascinating phenomenon that mitochondria seem to serve a dual purpose: their metabolic role established in the production of energy, as well as this optical effect.
Once light reaches the retina, it must pass through several neural layers to reach the outer segment of photoreceptors, where the physical energy of light is converted into neural signals through a process called phototransduction. Between the inner and outer segments of the photoreceptor cones is a dense bundle of mitochondria that light must pass through to be transduced. Although it may appear that these mitochondria hinder the vision process by scattering or absorbing light, the current study shows that they perform a unique function to facilitate vision.
Li’s team studied the role of mitochondria in cone photoreceptors in the 13-lined ground squirrel. The retina of the 13-lined ground squirrel comprises mostly cones, which detect color, unlike rod-shaped photoreceptors which aid in seeing in low light conditions.
Using a modified confocal microscope to observe the optical properties of live cone mitochondria exposed to light, researchers observed that instead of scattering light, tightly packed mitochondria focused light along a pencil-like trajectory on the outer light-sensitive segment. High-resolution mitochondrial reconstructions corroborated the results of live imaging. Moreover, the authors show that the remodeling of mitochondrial architecture affects this light concentration.
“Mitochondria’s lensing function may also explain the phenomenon known as the Stiles Crawford effect,” said John Ball, PhD, researcher in the Section of Retinal Neurophysiology and first author of the paper. The Stiles Crawford effect that improves visual resolution describes a basic phenomenon where light entering the eye through the periphery of the pupil does not appear as bright as light passing through its center.
Li’s team found that mitochondria lensing follows a directional light intensity profile similar to that of the Stiles Crawford effect. Depending on the location of the light source, the mitochondria in the cones focused the light into the outer segment of the cell reflecting the Stiles Crawford effect.
Linking the optical role of mitochondria to the Stiles-Crawford effect suggests. the established effect can be used for the non-invasive detection of retinal diseases, many of which are thought to involve mitochondrial dysfunction. For example, patients with retinitis pigmentosa have been reported to have an abnormal Stiles-Crawford effect even though their visual acuity was unaffected.
The study also sheds new light on how our eyes may have evolved. In bird and reptile photoreceptors, tiny oil droplets at the junction of the inner and outer segments that may play an optical role are reminiscent of the lipid-rich cone mitochondria in the current ground squirrel study. Moreover, the mitochondrial “microlens” in mammalian cone photoreceptors is functionally similar to the biological effect achieved by the compound eye in insects.
Li said, “This idea conceptually links arthropod compound eyes to vertebrate camera eyes, two independently evolved imaging systems, demonstrating the power of convergent evolution.”
In future studies, the team intends to explore structural and functional changes in cone mitochondria and how these affect the optics of the eye.