Retinal glial cells act as optical fibers

By any reasonable standard, the vertebrate eye is laid out pretty stupidly. Because of the way the eye branches off the central nervous system, the photoreceptors that are key to sensing light form at the deepest layer of the eye. To get to them, light must travel through a number of other cell types, potentially blurring and distorting images. Squid, which form their eyes through a different mechanism, don't have this problem—it appears to be a vertebrate-specific issue.

But that doesn't mean that vertebrates haven't made the best of a bad situation. Research that will appear in the next issue of PNAS reveals that we've evolved a clever hack. Glial cells within the retina act as optical fibers, directing light to the back of the eye where it can be perceived.

A European research team noticed that when light is shone through a retina with the photoreceptors removed, it exits the other side in a grid of bright spots and dark areas, suggesting that light can take favored paths through the retina. Looking at it from the other side, they saw that most of the retinal surface that normally faces the light is reflective, with a grid dark patches where the light was channeled through.

The pattern and size of these light channels matched the appearance of a specific cell type in the retina, the Müller cells, which are glia that support the nerves of the eye. So they isolated some Müller cells, and determined that they had an unusually high refractive index, a feature shared with material used for optical fibers. They showed that, despite their complex shape, these cells could propagate visible light along their entire lengths.

For their next trick, the research team used an optical trap to line up individual Müller cells floating in a solution, and used a microscopic fiber optic device to shine light into them. They were able to show that when the cell was lined up straight between this light source and a meter, the amount of transmitted light jumped. Either changing the alignment or removing the cell eliminated the effect.

The authors note that in mammals, most Müller cells are associated with a single cone cell, which is responsible for color perception in high-light environments. Because the Müller cells cells bring light directly to the cells that sense it, the otherwise clumsy arrangement of cells in front of the photoreceptors may actually limit the scattered or extraneous light that's sensed.