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Fiber optics in mammals’ eyes separate colors

Müller cells shunt red and green light to cones and let blue light leak onto rods

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12:48pm, July 10, 2014
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Special cells in the retina split light into different colors to enable sharp vision during the day without harming night vision.

Those long and tubelike cells, called Müller cells, snake through the layers of the retina. Müller cells have long been known as support cells for light-gathering cone cells, which they pair with in the retina. Cones absorb red and green light and enable crisp daytime color vision. The retina also contains rod cells that absorb blue light for fuzzier monochromatic night vision.

In humans and many other animals, the retina sits at the back of the eye, instead of at the front, where cones and rods could absorb the most light. Evolutionarily, inverting the retina would seem to be a huge mistake, says Serguei Skatchkov, a biophysicist at the Central University of the Caribbean in Bayamón, Puerto Rico. Peering through several layers of eye tissue, he says, “is like looking through milk.” How light penetrates those tissues to reach the retina’s cones and rods was an enigma.

But then in 2007, researchers discovered that Müller cells act as fiber optic cables, helping light burrow through all of the layers of retinal tissue to land on the cones at the back of the retina (SN: 5/19/07, p. 317). But that discovery brought up another mystery: If Müller cells channel all available light to cone cells, how do rod cells get enough light to allow people to see in the dark?

Müller cells split light into component colors, streaming red and green wavelengths to cones while allowing blue and violet wavelengths to leak onto rods nearby, researchers report July 8 in Nature Communications. This separation of colors ensures that cones get the red and green wavelengths they need for clear sight during the day without harming night vision, says Ido Perlman, a neural physiologist at Technion-Israel Institute of Technology in Haifa, and his colleagues.

“It’s a very special problem nicely solved,” says Andreas Reichenbach, an anatomist and physiologist at the University of Leipzig in Germany. He and his colleagues made the 2007 discovery that Müller cells channel light, but he was not involved in the new study.

To solve the problem, Perlman and his colleagues first simulated Müller cells in a computer. The researchers’ mathematical manipulations suggested that the cells could take in long-wavelength light at the green to orange end of the color spectrum, but that the cells would scatter shorter-wavelength blue and violet light.

The researchers tested the hypothesis by shining light on a guinea pig retina and mapping where various wavelengths hit the back of the retina. The team found that red and green light shot down tubes, which corresponded with the location of Müller cells, and shone directly onto cone cells. Blue and purple light leaked out to the surrounding retina, where the rods could take in those wavelengths.

Skatchkov remains unconvinced of the evidence presented in the paper because the researchers don’t explain how Müller cells cleave light into different colors. He also notes that red light has wavelengths longer than the diameter of the cell, and the researchers don’t explain how the cells can possibly channel such long-wavelength light. “These light fibers work at the limit of optical physics,” he says.

But Reichenbach is satisfied with the Israeli team’s demonstration of Müller cells’ ability to channel certain wavelengths of light to the cones. He does have a slight problem, he admits: “I’m a bit unhappy that it wasn’t my idea.”

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