Washington University LogoEver wonder how your eyes adjust during a blackout? When we go from light to near total darkness, cells in the retina have to rapidly adjust. Vision scientists at Washington University School of Medicine in St. Louis seem to have identified a complicated process which allows the human eye to adapt to darkness very quickly. In addition, the same process is believed to permit the eye to function in bright light.

This discovery could possibly add to better understanding of human diseases which affect the retina. These diseases include namely age-related macular degeneration which is the foremost cause of blindness in Americans over 50 years of age.

The researchers have recognized that it is due to the disease and the pathway which involve cells called cone cells. The retina’s main light-sensing cells are known as rods and cones. Apparently, both use similar mechanisms in order to convert light into vision. However they appear to function in a different manner.

Rods are believed to be extremely sensitive and work well in dim light, but they can rapidly become saturated with light and stop responding. Moreover, they fail to sense color either, which is why we seldom see colors in dim light. Cones, in contrast, allow us to see colors and may perhaps adapt quickly in order to stark changes in light intensity.

Lead investigator, Vladimir J. Kefalov, Ph.D. and assistant professor of ophthalmology and visual sciences stated that, “Age-related macular degeneration may be modulated, perhaps, through this pathway we’ve identified in the retina. Deficiencies in this pathway affect cone cells, and so does macular degeneration, so it’s possible that if we could enhance activity in this pathway, we could prevent or reverse some of that damage to cone cells.”

The researchers were observed to have begun with investigations of salamanders because their cone cells seem to be plentiful and easy to identify. Additionally, cones appear to rely on light-sensing molecules which bind collectively in order to create visual pigments.

They discovered that the pigments could possibly be destroyed when they absorb light. Also, they may be rebuilt, or recycled, for the cone cells to carry on sensing light. After exposure to light, chief components of pigments called chromophores seem to have left the cells and travel to the close by pigment epithelium near the retina. There the chromophore appears to be restored and returned to the photoreceptor cells.

Previously this year, the research team was noted to have separated the pigment epithelium layer in salamander retinas, so that pigment molecules could not be recycled that way.

Allegedly, they then exposed retinal cells both to bright light and to darkness. They found that the rods no longer seemed to function, but the cones continued to work appropriately, even without the eye’s pigment epithelium.

“Exposure to bright light destroyed visual pigments in rods, and those cells could not recycle chromophores. Pigments in cones, by contrast, quickly regenerated and continued to detect light even without the pigment epithelium, so it was clear a second pathway was involved,” explains Kefalov.

In the latest study, Kefalov was observed to have performed the same experiments in cells from mice, primates and humans with the same result. For the purpose of finding out how cones were able to recycle pigments without pigment epithelium, Kefalov’s team seems to have focused on a particular type of cell in the retina. These cells inside the retina are known as Muller cells and they support and interact with rods and cones. The researchers treated mouse retinas with a chemical which may have destroyed the Muller cells and they then exposed the retina to bright light, followed by darkness.

Kefalov claimed that, “When we blocked the function of Muller cells, the retinal visual pathway could not function because cones ran out of photo pigment and could not adapt to dark.”

The new study suggests that Muller cells seem to be essential to this pathway in mammals, including humans. Kefalov stated that when those cells work properly, cones in the mouse, primate and human retinas appear to be functioning in bright light and adapt to darkness, independently of the pigment epithelium.

He further claimed that this discovery may one day be able to influence this pathway in the retina thereby improving vision when the other pathway, involving pigment epithelium seems to have been interrupted by injury or disease, such as age-related macular degeneration.

The findings of the research have been published in the journal, Current Biology.