Mitochondria Double as Tiny Lenses in the Eye

A mosquito is watching you. You look back, fly swatter in hand, tracking the bloodsucker with your single-lens eyes. The way you see each other and the world may have more in common than you think.

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 are cones that help us navigate in low light by absorbing the light and sending it 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. Most cells have their mitochondria hugging their center organelle, so vision scientists couldn't make sense of this odd placement.

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 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 micro-lenses 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.