In A Breakthrough Study, Johns Hopkins University Researchers Discovered How An “Offshoot” Of Vitamin A Helps In Forming The Specific Cells That Make It Possible For Humans To See A Rainbow Of Colors
Humans have the advantage of seeing a broad range of colors– a mix of red, blue, and green– that many other mammals simply cannot. Dogs, for instance, are actually only able to see blue and yellow.
But, through a breakthrough study conducted by researchers at Johns Hopkins University, we may finally understand why. They discovered how a derivative, or “offshoot,” of vitamin A helps in forming the specific cells responsible for our ability to see various colors.
The team credited their success to using human retinas cultivated in petri dishes, a key element in their research.
“These retinal organoids allowed us for the first time to study this very human-specific trait,” said Robert Johnston, one of the study’s authors.
“It’s a huge question about what makes us human, what makes us different.”
The study’s findings broaden our understanding of age-related vision deterioration, color blindness, and other diseases associated with photoreceptor cells. The research also revealed how genes guide the human retina in creating cells that detect specific colors. Previously, it was believed that thyroid hormones were responsible for this process.
After adjusting the cellular characteristics of the organoids, the research team found that retinoic acid plays a crucial role in deciding whether a “cone,” or a type of photoreceptor cell in the retina, will be specialized in detecting red or green light. The development of the red sensor is exclusive to humans with normal vision and some primates closely related to us.
Scientists assumed that the formation of red cones was a random process for decades. It was thought to be similar to a “coin toss,” where cells randomly chose to sense either green or red light.
Recent studies also hinted that this might be governed by the levels of thyroid hormone. But, this new research suggests that the creation of red cones follows a specific series of events, all directed by retinoic acid inside our eyes.
Gorodenkoff – stock.adobe.com – illustrative purposes only, not the actual people
The researchers found elevated levels of retinoic acid in the initial stages of the organoids’ development, which corresponds to a greater proportion of green cones. Conversely, reduced levels of the acid altered the genetic directives of the retina, leading to the production of red cones in the later stages of development.
“There still might be some randomness to it, but our big finding is that you make retinoic acid early in development. This timing really matters for learning and understanding how these cone cells are made,” explained Johnston.
The key difference between green and red cone cells is a light-detecting protein called opsin, which informs the brain about perceived colors. This protein determines whether a cone cell will specialize in sensing green or red light. Still, genes in each type of sensor are 96% identical.
Utilizing a novel method that identified these minor genetic variations in the organoids, the team was able to monitor changes in the ratio of cone cells over a period of 200 days.
“Because we can control in organoids the population of green and red cells, we can kind of push the pool to be more green or more red. That has implications for figuring out exactly how retinoic acid is acting on genes,” detailed study co-author Sarah Hadyniak.
The researchers also meticulously charted the diverse ratios of these cells in the retinas of 700 adults. Hadyniak noted that observing the variations in the proportions of green and red cones among human subjects was one of their more unexpected discoveries.
What still remains a mystery, though, is how such significant variations in the ratio of green and red cones don’t seem to impact vision. According to one of Johnston’s analogies, if these cells were responsible for determining the length of a human arm, the varying ratios would result in “amazingly different” lengths.
Now, the team plans to continue their research in collaboration with other labs at Johns Hopkins University to deepen their understanding of conditions like macular degeneration, which can cause the loss of light-sensing cells in the central part of the retina.
“The future hope is to help people with these vision problems. It’s going to be a little while before that happens, but just knowing that we can make these different cell types is very, very promising,” Johnston concluded.
To read the study’s complete findings, which have since been published in PLoS Biology, visit the link here.
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