Bundles of mitochondria (yellow) inside gopher photoreceptor cones play an unexpected role in more precise focusing of diffuse light (glow from below) (blue beam). This optical behavior can improve vision by making the pigments in cone cells more efficient at capturing light.
A mosquito is watching you through a microlens array. You turn your head, hold the flyswatter in your hand, and look at the vampire with your humble, single-lensed eye. But it turns out that you can see each other – and the world – more than you think.
A study published last month in the journal Science Advances found that inside the mammalian eye, mitochondria, cell-nourishing organelles, can take on a second microlens role, helping to focus light on photopigments, these pigments convert light into nerve signals for the brain to interpret. The findings show striking similarities between mammalian eyes and the compound eyes of insects and other arthropods, suggesting that our own eyes have latent optical complexity and that evolution has made a very ancient part of our cellular anatomy found for new uses.
The lens at the front of the eye focuses light from the environment onto a thin layer of tissue at the back, called the retina. There, photoreceptor cells — the cones that color our world and the rods that help us navigate in low light — absorb light and convert it into neural signals that go to the brain. But photopigments are located at the very end of the photoreceptors, immediately behind the thick mitochondrial bundle. The strange arrangement of this bundle turns mitochondria into seemingly unnecessary light-scattering obstacles.
Mitochondria are the “last barrier” to light particles, said Wei Li, senior researcher at the National Eye Institute and lead author of the paper. For many years, vision scientists could not understand this strange arrangement of these organelles – after all, the mitochondria of most cells cling to their central organelle – the nucleus.
Some scientists have suggested that these beams may have evolved not far from where light signals are converted into neural signals, an energy-intensive process that allows energy to be easily pumped and delivered quickly. But then research began to show that photoreceptors don’t need as many mitochondria for energy—instead, they can get more energy in a process called glycolysis, which occurs in the cell’s gelatinous cytoplasm.
Lee and his team learned about the role of these mitochondrial tracts by analyzing the cone cells of a gopher, a small mammal that has excellent daytime vision but is actually blind at night because its cone photoreceptors are disproportionately large.
After computer simulations showed that mitochondrial bundles could have optical properties, Lee and his team began experiments on real objects. They used thin samples of squirrel retinas, and most of the cells were removed except for a few cones, so they “got just a bag of mitochondria” neatly packed inside a membrane, Lee said.
By illuminating this sample and carefully examining it under a special confocal microscope designed by John Ball, a scientist in Lee’s lab and lead author of the study, we found an unexpected result. Light passing through the mitochondrial beam appears as a bright, sharply focused beam. The researchers took photos and videos of light penetrating the darkness through these microlenses, where photopigments await in living animals.
The mitochondrial bundle plays a key role, not as an obstacle, but in delivering as much light as possible to the photoreceptors with minimal loss, Li says.
Using simulations, he and his colleagues confirmed that the lens effect is primarily caused by the mitochondrial bundle itself, and not by the membrane around it (although the membrane plays a role). A quirk of the gopher’s natural history also helped them demonstrate that the shape of the mitochondrial bundle is critical to its ability to focus: during the months the gopher hibernates, its mitochondrial bundles become disordered and shrink. When the researchers modeled what happens when light passes through the mitochondrial bundle of a sleeping ground squirrel, they found that it doesn’t concentrate light as much as when it is stretched out and highly ordered.
In the past, other scientists have suggested that mitochondrial bundles might help collect light in the retina, notes Janet Sparrow, professor of ophthalmology at Columbia University Medical Center. However, the idea seemed strange: “Some people like me laughed and said, ‘Come on, do you really have that many mitochondria to guide the light?’ – she said. “It’s really a document that proves it – and it’s very good.”
Lee and his colleagues believe that what they observed in gophers could also be happening in humans and other primates, which have a very similar pyramidal structure. They think it might even explain a phenomenon first described in 1933 called the Stiles-Crawford effect, in which light passing through the very center of the pupil is considered brighter than light passing at an angle. Because the central light can be more focused on the mitochondrial bundle, the researchers think it could be better focused on the cone pigment. They suggest that measuring the Stiles-Crawford effect could help in the early detection of retinal diseases, many of which lead to mitochondrial damage and changes. Lee’s team wanted to analyze how diseased mitochondria focus light differently.
It’s a “beautiful experimental model” and a very new discovery, said Yirong Peng, an assistant professor of ophthalmology at UCLA who was not involved in the study. It will be interesting to see if these mitochondrial bundles can also function inside rods to improve night vision, Peng added.
At least in cones, these mitochondria could have evolved into microlenses because their membranes are made up of lipids that naturally refract light, Lee said. “It’s simply the best material for the feature.”
Lipids also seem to find this function elsewhere in nature. In birds and reptiles, structures called oil droplets have developed in the retina that serve as color filters, but are also thought to function as microlenses, such as mitochondrial bundles. In a grand case of convergent evolution, birds circling overhead, mosquitoes buzzing around their delightful human prey, you read this with appropriate optical features that have evolved independently – adaptations that attract viewers. Here comes a clear and bright world.
Editor’s note: Yirong Peng received the support of the Klingenstein-Simons Fellowship, a project supported in part by the Simons Foundation, which also funds this independently edited magazine. The funding decision of the Simmons Foundation does not affect our reporting.
Correction: April 6, 2022 The title of the main image initially incorrectly identified the color of the mitochondrial bundles as purple instead of yellow. Purple staining is associated with the membrane surrounding the bundle.
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Post time: Aug-22-2022
