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Eyes exploit fiber optics to see in dark: Muller cells shunt red and green light to cones; blue leaks to rods.

Special cells in the retina split light into different colors to enable sharp daytime vision without harming night vision.

Those tubelike cells, called Muller cells, snake through the retina, where they pair with and support light-gathering cone cells. Cones absorb red and green light, enabling daytime color vision. The retina's rod cells 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. In addition, human retinas are inverted so that the light-gathering cells form the last layer. Evolutionarily, inverting the retina seems to be a mistake, says Serguei Skatchkov, a biophysicist at the Central University of the Caribbean in Bayamon, 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 has been an enigma.

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

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

"It's a very special problem nicely solved," says Andreas Reichenbach, a physiologist at the University of Leipzig in Germany who was not involved in the new study. He and colleagues made the discovery that Muller cells channel light.

Perlman's team simulated Muller cells in a computer. The simulations suggested that the cells could take in long-wavelength light at the green to orange end of the spectrum but 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 it. The team found that red and green light shot down tubes, which corresponded with the location of Muller 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.

Reichenbach is satisfied with the demonstration of Muller cells' ability to channel certain wavelengths of light to cones. Skatchkov remains unconvinced, though. He says the researchers don't explain how Muller cells cleave light into different colors and don't explain how the cells can channel red light having wavelengths longer than the diameter of the cell.

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Title Annotation:GENES & CELLS
Author:Saey, Tina Hesman
Publication:Science News
Date:Aug 23, 2014
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