A mosquito is watching you. You look back, fly swatter in hand, tracking the bloodsucker with your single-lens eyes. It turns out that the way you see each other and the world are very similar.
A study published last month in Science Advances found that the power cells in the eye's mitochondria may serve a second role, helping to focus light on the photoreceptors that convert the light into neural signals. The findings suggest that our own eyes have hidden levels of optical complexity and that evolution has found new uses for parts of our cellular anatomy.
The back of the eye has a thin layer of tissue called the retina. There, the cones that paint our world in color and rods that help us navigate in low light, absorb the light and translate it into nerve signals that travel into the brain. There are light-sensitive pigments at the end of the photoreceptors. TheMitochondria are turned into seemingly unnecessary, light-scattering obstacles by the placement of this bundle.
The National Eye Institute's senior author on the paper said that the final hurdle for light particles was the mitochondria. Vision scientists couldn't make sense of the placement of the organelles in the nucleus, after all, most cells have their mitochondria hugging their center organelle.
The bundles might have evolved to sit close to the place where light signals are converted into nerve signals, a highly energy-demanding process, to easily pump out energy and deliver it. Studies suggest that the photoreceptors don't need as many mitochondria for energy, but instead receive more of their energy from a process called glycolysis.
Li and his team studied the cones of a ground squirrel, a small mammal that has amazing vision during the day but is almost blind at night because of its disproportionately cones.
Li said that the mitochondrial bundles seem to play a critical role in helping to funnel as much light as possible to the photoreceptors with minimal loss.
Li and his team began experimenting with the real thing after computer simulations suggested that the mitochondrial bundles might have optical properties. They used a small sample of the squirrel's retina, which they mostly stripped of its cells except for parts of its cones, so that they had just a bag of mitochondria.
A striking result was revealed when John Ball, a staff scientist in Li's lab and the lead author of the study, looked at the sample under a special confocal microscope. Light passing through the bundle was bright and focused. The researchers captured photos and videos of light that was beamed through the microlenses into darkness.
Li said that the mitochondrial bundles seem to play a critical role in helping to funnel as much light as possible to the photoreceptors with minimal loss.
He and his colleagues confirmed that the lens effect was caused by the bundle itself, not the surrounding area. The ground squirrel's natural history helped them prove that the shape of the mitochondrial bundle was important to its focusing abilities. The researchers found that when light passes through a squirrel's mitochondria, it doesn't concentrate the light as well as when it's ordered.
Janet Sparrow, a professor in the department of ophthalmology at the Columbia University Medical Center who was not involved in Li's study, said that other scientists have speculated that the mitochondrial bundles might be assisting with light collection in the retina. Some people like me laughed and said, "Come on, are you going to really have that many mitochondria just to guide light?"
Li and his colleagues believe that what they saw in ground squirrels is likely to happen in humans and other primate with similar cone structures. They suggested that it could explain a phenomenon that was first reported in 1933 and called the Stiles-Crawford effect, in which light entering at an angle is perceived as brighter than light passing through the center of the pupils. The researchers think that the central light may be better focused on the cone. The early detection of diseases that cause damage to the cells of the eye might be helped by measuring the Stiles-Crawford effect. Li's team hopes to find out how the mitochondria focus light.
Yi-Rong Peng is an assistant professor in the department of ophthalmology at the University of California, Los Angeles who was not involved in the study. It would be interesting to see if these bundles could be used inside rods to enhance night vision.
The best material to achieve this function is the mitochondria, which have a natural ability to bend light.
This function has been found elsewhere in nature. Birds and reptiles have evolved a structure in their retinas called oil droplets that serve as a color filter but are also believed to act as microlenses. In a grand case of convergent evolution, birds circling high overhead, mosquitoes buzzing around their delicious human victims, and you reading this article have all independently evolved related optical functions that bring a sharp and vibrant world to the eye of the beholder.
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