New Research Reveals the Explanation Behind Humans' Ability to See Colors Dogs Cannot

Recent studies reveal that the development of color-sensitive cells crucial to human vision, grown in laboratory conditions, is influenced by retinoic acid instead of thyroid hormones. As a result, our understanding of color blindness, vision loss, and the genetics of color perception stands to develop significantly, improving the potential for future vision disorder treatments.

Scientists have been able to grow human retinas in a lab, an accomplishment that led them to discover that vitamin A derivatives are responsible for cell creation that allows humans to comprehend a broad spectrum of colors. It is noteworthy to add that this color comprehension ability is not present in dogs, cats, or several other mammals.

Author Robert Johnston, an associate professor of biology conveyed, “The retinal organoids we developed granted us the ability to understand this peculiarly human trait for the first time.” He also emphasized the gravity of the question about what sets humans apart, and who we are as individuals.

The PLOS Biology-published research deepens our understanding of color blindness, age-related vision loss, and other diseases relating to photoreceptor cells. It also reveals how the human retina derives instructions from genetics to form particular color-detecting cells. Scientists initially believed that thyroid hormone control this process.

The researchers were able to manipulate the organoid's cellular properties and concluded that retinoic acid determines whether a cone will differentiate into red or green light detectors. Such red light detecting cones only occur in humans with normal vision and primates closely related to them.

For years, researchers believed the development of red cones was randomly determined, either detecting red or green wavelengths, with recent studies suggesting thyroid hormones could control this process. However, it appears that red cones originate from a specific method orchestrated by retinoic acid within the eye.

The research team discovered that increased levels of retinoic acid during the early development stages of the organoids resulted in a higher count of green cones. Conversely, a reduction in retinoic acid levels altered the retina's genetic instructions resulting in the formation of red cones at a later development stage.

Johnston implied that retinoic acid is crucially produced early in development and its production timing is vital for understanding how cone cells develop. Although there may still be an element of randomness, the fundamental finding is the importance of retinoic acid in this process.

Red and green cone cells are largely identical barring an opsin protein that is responsible for detecting light and conveying color data to the brain. An innovative technique that detects subtle genetic differences in the organoids helped the research team observe cone ratio changes over 200 days.

Sarah Hadyniak, a co-researcher who performed the study as a doctoral student in Johnston's lab and is now associated with Duke University, explains, “Since we can regulate the ratio of green and red cells in organoids, we have the capacity to influence the pool to be predominantly green or red”. She stressed its potential implications on better comprehending how retinoic acid modulates genes.

The research also entailed mapping the remarkably varying ratio of green and red cone cells in the retinas of 700 adults, one of the study's surprising finds according to Hadyniak.

Johnston expressed his astonishment at such varying proportions of green and red cones, wondering how this doesn’t seem to affect an individual’s vision. Drawing a parallel, he observed that if these cells controlled human arm length the resulting variations would be nothing short of astonishing.

The team plans to liaise with other Johns Hopkins labs for further studies like macular degeneration, which leads to loss of light-detecting vision cells at the retina's center. The ultimate aim is to enhance their understanding of how cones connect to the nervous system.

Johnston concluded with optimism, “While it may take some time, our goal is to be able to assist people suffering from vision problems. The fact that we can develop these diverse cell types is a very positive sign.”