Retinoic acid, a metabolite of vitamin A, is responsible for sensitizing cone cells developing in the human eye to red or green light. These cells allow humans, unlike dogs, cats and other mammals, to see millions of colors. The results of the research were published in the journal PLOS Biology.
People can distinguish many colors. This ability is not present in other species, even if they have sharper vision. For example, only humans and closely related primates with normal vision develop a high sensitivity to the color red: most animals see it as gray. Cone cells in the retina are responsible for color perception. For decades, scientists believed that red cones were formed by cells randomly adjusting to the green or red regions of the spectrum.
Green and red cones are quite similar (they share 96% of their genes), but they contain different forms of the opsin protein responsible for sensitivity to different colors. In the new study, researchers created a model of the human retina and then monitored changes in cone cell ratio for 200 days. They found that the higher the level of retinoic acid during early retinal development, the more green cones and fewer red cones are produced. This means that cone specialization does not occur entirely by chance, but rather through a series of events triggered by retinoic acid.
The researchers also analyzed cone ratios in the retinas of 700 adults. The proportions of green and red cones were surprisingly different. Scientists don’t know how the ratio of green cones to red cones can vary so much without affecting someone’s vision. If these cell types determined the length of a human arm, the different ratios would produce “stunningly different” lengths, the authors noted.
The results will help better understand diseases such as macular degeneration, which causes the loss of light-sensitive cells near the center of the retina. Additionally, scientists now know how to obtain red or green cones that could be useful in treating vision problems.
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Source: Gazeta
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