Scientists from Johns Hopkins University have uncovered an unexpected mechanism by which humans develop sharp central vision even before birth. The study, published in the Proceedings of the National Academy of Sciences, overturns previous ideas about eye development.
The researchers used miniature human retinas grown in the laboratory — organoids. They found that a derivative of vitamin A — retinoic acid — and thyroid hormones together form the foveola, a tiny zone in the center of the retina responsible for the sharpest image.
It was previously thought that blue cones simply migrate from the center. The new data show otherwise: at 10–12 weeks of intrauterine development, a few blue cones appear in the foveola, and by 14 weeks they are converted into red and green ones.
The process takes place in two stages. First, retinoic acid is broken down, limiting the emergence of new blue cones. Then thyroid hormones trigger the conversion of the remaining cells. “First retinoic acid helps set the pattern. Then the thyroid hormone plays a role in the conversion of the remaining cells,” explained the head of the study, Robert Johnston.
If blue cones remain in this zone, vision deteriorates. The discovery concerns specifically vitamin A signaling in eye development before birth and may influence the understanding of congenital vision disorders.
How exactly do these molecular signals interact in a real human embryo?
The results provide a new basis for studying age-related retinal diseases and potential cell therapies. They emphasize the importance of a precise balance of vitamin A and hormones during the critical weeks of development.
