Scientists from Johns Hopkins University have uncovered a mechanism for the development of sharp central vision in humans before birth, after identifying a precisely timed interaction between a molecule derived from vitamin A and thyroid hormones within the retina. This discovery challenges a long-standing scientific explanation for how the main light-sensing cells form, and may pave the way for future treatments for vision-threatening diseases such as macular degeneration, glaucoma, and other retinal disorders.

Robert Johnston, associate professor of biology at Johns Hopkins University and lead researcher, described the mechanism as "a fundamental step toward understanding the inner workings of the retinal center, a vital part of the eye that is first affected in people with macular degeneration." By better understanding this region and developing organoids that mimic its function, scientists hope to one day grow and transplant these tissues to restore vision.

To investigate how the human eye develops, researchers used organoids—small clusters of tissue grown from embryonic cells that accurately mimic parts of the retina. After observing these lab-grown retinas over several months, the team identified the cellular processes that form the fovea centralis, the small region at the center of the retina responsible for the sharpest vision, according to a report published by Science Daily citing the journal Proceedings of the National Academy of Sciences.

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Cone cells

Furthermore, the study focused on cone photoreceptor cells, which are light-sensitive cells that provide daytime and color vision. These cells eventually become blue, green, or red cones, each responding to different wavelengths of light. Although the fovea centralis constitutes only a small part of the retina, it is responsible for about half of human visual perception. Unlike the rest of the retina, where all three cone types are present, the fovea contains only red and green cones.

Essentially, humans have three different types of cone cells in the retina, which together provide a wide range of color vision. However, how this specialized pattern develops has been a mystery for decades. According to Johnston, scientists faced difficulty studying this process because common laboratory animals, such as mice and fish, do not develop the same arrangement of photoreceptor cells.

Scientists unravel one of the biggest mysteries of human eye development (illustrative - iStock)

Red and green cells

The new findings indicate that the pattern of cone cells in the fovea centralis is formed through a coordinated series of events during early embryonic development. Between weeks 10 and 12, a few blue cone cells appear in the developing fovea, but by week 14, these cells transform into red and green cone cells.

The researchers found that this occurs through two separate mechanisms: first, retinoic acid, a molecule derived from vitamin A, degrades, reducing the formation of new blue cone cells; then, thyroid hormones stimulate the remaining blue cone cells to transform into red and green cone cells.

Johnston said: "First, retinoic acid helps establish the pattern. Then thyroid hormones play a role in converting the remaining cells. This is crucial because if those blue cones were present, you wouldn't see clearly."

Challenging a long-standing theory

The findings offer a new explanation for a question that has puzzled vision researchers for decades. The prevailing theory suggested that blue cones form in the center of the retina and then migrate outward. But the new evidence indicates that these cells stay in place but change their identity to red and green cones, producing the specialized arrangement necessary for sharp vision.

The researchers believe these discoveries could ultimately support new approaches to treating vision loss. Johnston's team continues to refine their retinal organoids to more closely resemble the function of the human retina. Improved models could help scientists generate healthier photoreceptor cells for future cell replacement therapies targeting diseases such as macular degeneration, for which there is currently no cure.

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