New Study Reveals Diverse Vision Among Humans, Linked to Red and Green Cones in Retinas
Have you ever wondered what the world looks like through someone else’s eyes? Well, a groundbreaking new study has shed light on the incredible diversity of human vision. Using lab-grown human retinas, researchers have discovered that our vision varies greatly even among different individuals. And it turns out that this diversity may be connected to the formation of red and green cones in our retinas.
Cones are specialized cells in the eyes of vertebrates that are responsible for sensing light. By responding to different wavelengths, cones enable us to perceive colors. Humans and some closely related primates are among the few mammals capable of seeing the colors red, green, and blue. Interestingly, other animals such as birds and certain insects also possess the ability to see red. This ability is believed to have evolved alongside plants that produce fruits and flowers, allowing animals to spot ripe red fruits amidst a sea of green foliage.
One mammal that stands out in terms of its ability to see red is the honey possum, a unique Australian marsupial pollinator. With its bird-like capability to extract nectar from blushing banksia flowers, the honey possum showcases a fascinating example of convergent evolution.
The red and green cones in our retinas are remarkably similar, differing only slightly in their chemistry to determine which color they detect. These cones contain a protein called opsin, which comes in two variations: red-sensitive and green-sensitive. The genetic instructions for these variations are located side by side on the X chromosome, making it easy for them to get mixed up during recombination. This can result in variations of congenital red-green color blindness.
However, recent research has brought some clarity to the factors that determine cone development. While it was previously believed that cone determination was random, studies have suggested that thyroid levels may play a role. Now, a team of scientists from Johns Hopkins University and the University of Washington has made a groundbreaking discovery. They found that levels of a molecule derived from vitamin A, called retinoic acid, play a crucial role in determining the ratio of red to green cones in lab-grown retinas.
Using retinal organoids, researchers exposed developing retinas to varying levels of retinoic acid. They discovered that higher levels of the acid during early development resulted in a higher ratio of green cones in the organoids after 200 days. Conversely, immature cones exposed to low levels of the acid developed into red cones later on. The timing of exposure also proved to be important, as introducing retinoic acid after 130 days had no effect on cone development. This suggests that the acid determines cone type early on and cannot cause red cones to transform into already matured green cones.
To understand the potential impact of these findings on human vision, the researchers examined the retinas of 738 adult males with no signs of color vision deficiency. They were astonished by the natural variation in red-green cone ratios observed among this group. According to developmental biologist Sarah Hadyniak, co-author of the study, “Seeing how the green and red cone proportions changed in humans was one of the most surprising findings of the new research.”
The implications of this research are far-reaching. Understanding how retinoic acid influences genes could provide valuable insights into what makes us human and what sets us apart from other species. The researchers speculate that the significant variation in red-green cone ratios could have a profound impact on human vision. As Robert Johnston, a developmental biologist from Johns Hopkins University, explains, “If these types of cells determined the length of a human arm, the different ratios would produce amazingly different arm lengths.”
This groundbreaking study, published in PLOS Biology, has opened up new avenues for exploring the intricacies of human vision. By unraveling the role of retinoic acid in determining cone ratios, researchers have taken a significant step towards understanding the fascinating diversity of our visual experiences.
