Vitamin A and Thyroid Hormones Work Together to Shape Human Sharp Vision Before Birth

In a Johns Hopkins laboratory, clusters of human retinal tissue no bigger than a speck have been growing for months, and watching them has overturned a theory that held for about 30 years. The clusters are organoids, grown from fetal cells, and they let researchers follow the development of the eye's sharpest-seeing region in a way that is nearly impossible during a real pregnancy.

The region in question is the foveola, a tiny patch at the center of the retina. The retina is the light-sensitive layer at the back of the eye that turns incoming light into signals the brain can read. The foveola handles the fine central vision that lets people read, recognize faces and make out small details. The researchers say it is involved in roughly 50% of human visual perception, despite occupying only a small fraction of the retina.

The puzzle concerns cone cells, which support daytime and color vision, unlike rods, which matter most in dim light. Humans have three kinds of cones, commonly called blue, green and red, each most sensitive to different wavelengths. The foveola contains red and green cones but no blue ones, and that exact arrangement is essential to its sharpness. How the pattern arises has resisted explanation, partly because mice and fish, the usual laboratory animals, do not share the human retinal layout.

The team, led by Robert J. Johnston Jr., an associate professor of biology at Johns Hopkins, found that two coordinated processes appear to build the pattern. Between weeks 10 and 12 of fetal development, a small number of blue cones are present in the foveola. By week 14, those cells appear to have become red and green cones. The findings were published in Proceedings of the National Academy of Sciences.

Two chemical signals are behind this. The first is retinoic acid, a molecule made from vitamin A that guides how cells specialize. In the developing retina, its breakdown limits how many blue cones form in the central region. The second is thyroid hormone, better known for regulating metabolism but also important for growth and development. Here it appears to push the remaining blue cones to convert to red and green. "First, retinoic acid helps set the pattern. Then, thyroid hormone plays a role in converting the leftover cells," Johnston said, adding that leftover blue cones in that spot would worsen sight.

The result challenges a model that has dominated the field for about 30 years. That model held that the few blue cones in the foveola simply move out of the way, and that cells fix their identity permanently. Johnston said the team cannot rule migration out yet, but its data supports a different picture in which the cells convert over time, which he called really surprising. The researchers stress that the work concerns eye development before birth. It does not show that vitamin A supplements improve sharp vision in adults.

The longer-term aim is treatment. The foveola's neighbor, the macula, is damaged in macular degeneration, a major cause of central vision loss in older adults that interferes with reading, driving and recognizing faces. Treatments can slow or manage some forms, but there is no cure. Johnston's group is now refining its organoids to better mimic the structure and function of the human eye, with the goal of producing precisely controlled populations of photoreceptors for possible transplantation.

Katarzyna Hussey, a study author and former doctoral student in Johnston's lab, now a molecular and cell biologist at CiRC Biosciences in Chicago, described the vision as a nearly made-to-order supply of photoreceptors for cell replacement therapy. Transplanted cells would have to survive, connect properly with the eye's existing circuitry and work safely. Hussey said the experiments are very long-term and need safety and efficacy studies before any clinic, but called it a viable journey.