A surprising vitamin A process helps build sharp central vision, Johns Hopkins researchers found

Roughly half of human visual perception depends on a tiny patch of retina at its center. Johns Hopkins University biologist Robert J. Johnston Jr. and his team report that building that patch involves a vitamin A derivative, retinoic acid, working together with thyroid hormone before birth. Their account, published in the Proceedings of the National Academy of Sciences, also overturns a model that stood for about three decades.

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The finding concerns the developing fetal eye, not adult vision. Nothing in the study shows that taking vitamin A supplements sharpens eyesight in grown people.

The foveola and its missing blue cones

The foveola sits at the very center of the retina and handles the finest detail. Johnston’s group says this region is involved in about 50% of human visual perception. Its photoreceptors are cones that detect color, and the mature foveola contains red and green cones but no blue ones. According to the Johns Hopkins account of the work, that absence is what the study set out to explain.

For roughly 30 years the standard model held that blue cones start out in the central retina and then migrate outward, keeping their identity. Johnston’s data point to a different process, in which the cells change identity rather than leave. The team has not fully ruled out some migration, but the conversion is the main route they found.

Why the old model lasted is easy to see. Blue cones are rare in the center of an adult retina, and cells that seem to have left a region look, on a static tissue sample, much like cells that were never there. Only a system that can be watched over weeks, as the organoids allowed, separates migration from a change of identity. Neuroscience News describes the work as showing that cone cells in the developing fovea switch their type.

Two signals in sequence

The researchers worked with human retinal organoids, miniature tissue grown from fetal cells, because mice and fish do not share the human retinal layout. They tracked the organoids for several months, following changes that would be nearly impossible to observe during a pregnancy. The paper, “A cell fate specification and transition mechanism for human foveolar cone subtype patterning,” carries the DOI 10.1073/pnas.2510799123.

At weeks 10 to 12 of fetal development, a small number of blue cones show up in the foveola. Retinoic acid, a derivative of vitamin A, is broken down in the central region, which limits the creation of new blue cones. By week 14, thyroid hormone prompts the leftover blue cones to turn into red and green cones. Johnston summarized it as retinoic acid helping set the pattern, followed by thyroid hormone converting the leftover cells.

The sequence matters because the mature pattern, red and green cones packed in the center with no blue, is what supports the finest color and detail vision. If either signal were disrupted, the center of the retina could end up with the wrong mix of cone types. The study is a map of normal development, and it does not attribute any specific disease to this pathway, so the link to conditions of the human eye stays a direction for future work. Interesting Engineering notes that the central retina is the first area to fail in macular degeneration, which is why the group hopes organoids could eventually support cell-based therapies that replace damaged photoreceptors.

He said it was really surprising that these cells convert over time. The result also links two signaling systems, vitamin A and thyroid hormone, that were each known to matter for the eye but had not been tied together in this region.

From organoids toward transplants

Katarzyna Hussey, a study author who trained in Johnston’s lab and now works at the Chicago cell-therapy company CiRC Biosciences, described the longer goal in the ScienceDaily summary. She said the aim of the organoid technology is to eventually make an almost made-to-order population of photoreceptors. She was frank that the technology is far from routine clinical use, calling it a viable journey rather than an arrival.

The organoid approach itself is part of the finding. Cells taken from fetal tissue were grown into miniature retinas that recreate the layered structure of the human eye, and the researchers could watch them across the stretch of development that corresponds to the second trimester. That window is closed to direct observation in people, and animal models fall short, since mice and fish lack the same central retinal layout. The organoids gave the team a practical way to test what retinoic acid and thyroid hormone do to individual cone cells.

The work was supported by the National Institutes of Health, the Howard Hughes Medical Institute and the Maryland Technology Development Corporation. If the pattern can be controlled in a dish, grown tissue could eventually be transplanted to treat macular degeneration, glaucoma and other disorders that destroy vision, but safety and efficacy testing would come first. The unresolved piece is the migration question. Johnston’s group has not fully excluded some cell movement alongside the conversion, and how much each process contributes in a living fetus, as opposed to an organoid, is still unmeasured.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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