Vitamin A & Thyroid Hormones Key to Sharp Human Vision Development

Humans develop sharp vision during early fetal development through a carefully orchestrated interplay between a derivative of vitamin A and thyroid hormones within the retina, according to scientists at Johns Hopkins University. This discovery, published today in Proceedings of the National Academy of Sciences, challenges long-held beliefs about how the eye develops and could open new avenues for treating age-related vision disorders.

For decades, the prevailing theory suggested that the precise arrangement of light-sensing cells in the retina – crucial for sharp daytime vision – was achieved through the migration of these cells into specific positions. However, research led by Robert J. Johnston Jr., an associate professor of biology at Johns Hopkins, reveals a different mechanism: cellular conversion. So that cells don’t simply move into place, but actually change their function.

The research team pioneered a method of studying eye development using lab-grown retinal organoids – small, three-dimensional tissue clusters grown from fetal cells. By observing these miniature retinas over several months, they were able to unravel the cellular processes that shape the foveola, the central region of the retina responsible for high-acuity vision. The foveola, though a tiny fraction of the retina, is responsible for approximately 50% of human visual perception.

Human vision is unique among animals in its capacity for full color perception, relying on three types of cone cells – blue, green, and red – each sensitive to different wavelengths of light. The foveola is densely populated with red and green cones, while blue cones are more widely distributed across the rest of the retina. Understanding how this specific distribution develops has been a longstanding puzzle for scientists, particularly because commonly used research organisms like mice and fish don’t exhibit the same pattern.

The study reveals that the development of this cone distribution occurs in two distinct phases. Initially, between weeks 10 and 12 of fetal development, a limited number of blue cones are present in the foveola. 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, known as retinoic acid, breaks down, limiting the production of new blue cones. Second, thyroid hormones actively encourage the existing blue cones to transition 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 Johnston.

This finding refutes the previous assumption that blue cones simply migrate out of the foveola during development. Instead, the data strongly suggest that these cells undergo a fundamental change in their identity to achieve the optimal cone distribution necessary for sharp vision. “The main model in the field from about 30 years ago was that somehow the few blue cones you get in that region just move out of the way, that these cells decide what they’re going to be, and they remain this type of cell forever,” Johnston said. “One can’t really rule that out yet, but our data supports a different model. These cells actually convert over time, which is really surprising.”

The implications of this research extend beyond a deeper understanding of basic eye development. The findings offer a potential blueprint for developing new therapies for vision loss, particularly age-related macular degeneration, and glaucoma. Johnston and his team are now focused on refining their organoid models to more accurately replicate the complex functions of the human retina.

Katarzyna Hussey, a former doctoral student in Johnston’s lab and now a molecular and cell biologist at CiRC Biosciences in Chicago, highlighted the potential for cell-based therapies. “The goal with using this organoid tech is to eventually make an almost made-to-order population of photoreceptors. A big avenue of potential is cell replacement therapy to introduce healthy cells that can reintegrate into the eye and potentially restore that lost vision,” Hussey said. She cautioned that significant research and safety testing are still required before such therapies could be implemented clinically, but emphasized the viability of this approach.

The research team acknowledges that further investigation is needed to fully elucidate the mechanisms governing this cellular conversion process. However, this study represents a significant step forward in understanding the intricacies of human vision development and offers a promising new direction for the treatment of debilitating eye diseases. The insights gained from studying these lab-grown retinal tissues could ultimately lead to innovative strategies for restoring vision in individuals affected by macular degeneration, glaucoma, and other conditions that currently have limited treatment options.