American researchers tracked the peculiarities of human color vision. For this purpose, human retinas were grown in the laboratory, and they also discovered some interesting things, about which little is known so far.

Human vision is a real miracle: it can claim some of the “best” functions compared to the animal world. It is no coincidence that humans have become such a successful species. the ZME SciencesH In this regard, it is noteworthy that a study conducted in 2018 to compare the visual acuity of nearly 600 species of insects, birds, mammals, fish and other animals found that human vision is an anomaly. A human's vision is about seven times sharper than that of a cat, 40 to 60 times sharper than that of a rat or goldfish, and hundreds of times sharper than that of a fly or mosquito.

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And that's not all. Humans are also ahead of most mammals in color vision. The human eye sees millions of colors, a much wider color palette than dogs, for example. According to a new study, this advantage is due to special eye cells whose growth is controlled by retinoic acid, a metabolite of vitamin A. This finding contradicts the long-standing belief that thyroid hormones control this process.

Color vision results from a gene duplication event that gave rise to three different genes, each of which encodes a photoreceptor that is sensitive to different wavelengths of light: blue (short wavelength), green (medium wavelength), and red (long wavelength). The genetic code that tells the retina to produce these color-sensitive cells is mediated by certain hormones produced in the thyroid gland. At least that's what they thought so far, but according to an associate professor of biology at Johns Hopkins University, that's not the case.

He and his colleagues cultured human retinas in petri dishes and discovered that the level of retinoic acid during early development is critical for cone cell development and differentiation. By modifying the properties of organelle cells, they found that retinoic acid determines whether the cone is specialized for sensing red or green light. Only humans with normal vision and closely related primates develop the red sensor.

Organoid of retina. Blue cones are cyan, green/red cones are green. Cells called rods, which help the eye see in dim light or darkness, appear purple

For decades, scientists have thought that red cones are formed through a so-called coin-tossing mechanism, where cells randomly commit to sensing green or red wavelengths. In addition, many people believe that this process can be regulated by the level of thyroid hormones. However, current research suggests that red cones occur through a specific sequence of events controlled by retinoic acid in the eye.

The team found that high levels of retinoic acid in early organelle development were associated with a higher proportion of green cones, and that lower levels of the acid changed the genetic instructions of the retina and produced red cones during later development. A high level leads to a predominance of green cones, while a lower level leads to a predominance of red cones. “For the first time, these network organoids have made it possible to study this human characteristic,” the lead researcher said in a press release.

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The researchers also mapped the ratio of red to green cones in the retinas of 700 adults. They found surprising differences that do not affect visual acuity: people with normal vision see slightly different colors. If the same ratios were applied to the cells that determine human arm length, we would see “surprisingly different” arm lengths, and the researchers have explained what this means.

The results of the current research that PLoS Biology Published in the journal, it may improve understanding of color blindness, age-related vision loss, and other diseases associated with photoreceptor cells.

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