Vitamin A and thyroid hormones in the retina shape fetal vision

Humans develop sharp vision during early fetal development through a complex interplay between a vitamin A derivative and thyroid hormones within the retina, according to scientists at Johns Hopkins University. This discovery, detailed in Proceedings of the National Academy of Sciences, could fundamentally change our understanding of how the eye develops and potentially lead to new treatments for vision disorders.

Unraveling the Mysteries of Vision Development

For decades, scientists have been puzzled by how the human eye develops its unique distribution of light-sensing cells. Unlike many animals used in research—such as mice and fish—humans possess three types of cones for color vision: blue, green, and red. This allows us to perceive a wider spectrum of colours. The research team at Johns Hopkins pioneered a method using lab-grown retinal tissue, called organoids, to study this process.

A Shift in Cellular Understanding

The study focused on the development of light-sensitive cone cells, which enable daytime vision. Initially, a limited number of blue cones are present in the foveola between weeks 10 and 12 of fetal development. However, by week 14, these blue cones begin to transform into red and green cones. This conversion is driven by two key processes. First, a molecule derived from vitamin A, called retinoic acid, is broken down, limiting the creation of new blue cones. Second, thyroid hormones actively encourage the existing blue cones to convert into red and green cones.

“First, retinoic acid helps set the pattern. Then, thyroid hormone plays a role in converting the leftover cells. That’s very important because if you have those blue cones in there, you don’t see as well,” explained Robert J. Johnston Jr., an associate professor of biology at Johns Hopkins who led the research.

Implications for Future Therapies

The Johns Hopkins team’s findings offer a new perspective on cone distribution in the foveola, suggesting a process of cell conversion rather than migration. Katarzyna Hussey, a former doctoral student from Johnston’s lab and now a molecular and cell biologist at CiRC Biosciences in Chicago, highlighted the potential for cell-based therapies. The goal is to eventually create “made-to-order” populations of photoreceptors that could be transplanted to restore vision lost to diseases like macular degeneration, which currently has no cure.

Researchers are now working to refine their organoid models to more accurately replicate human retina function. Further research and optimization for safety and efficacy will be necessary before these potential therapies can be tested in clinical trials.

Frequently Asked Questions

How do vitamin A and thyroid hormones contribute to vision development?

Vitamin A, in the form of retinoic acid, helps establish the initial pattern of cone cells, while thyroid hormones encourage the conversion of blue cones into red and green cones, ultimately shaping the foveola for optimal vision.

What is an organoid and how was it used in this study?

An organoid is a small, lab-grown cluster of tissue grown from fetal cells. Researchers used organoids to study eye development over several months, allowing them to observe the cellular mechanisms that shape the foveola.

Could this research lead to treatments for vision loss?

The findings could pave the way for new therapies, particularly cell-based treatments, for vision loss caused by diseases like macular degeneration. Researchers are working to refine organoid models to create photoreceptors for potential transplantation.

As scientists continue to unravel the intricacies of vision development, what role might personalized medicine play in addressing individual variations in retinal health?