Researchers from Johns Hopkins University have grown human retinal tissue from stem cells to try and understand how color vision develops in humans. This is one of the first vision studies conducted directly on human tissue grown in the lab, and the team hope it will lead to therapies to fight color blindness and macular degeneration.
The researchers were able to follow the development of the tissue like it was happening inside a human embryo. They discovered that of the three color light-sensitive cells, the ones that formed first were those that responded to blue light, followed by the red and green cells. The findings are reported in Science.
A crucial factor in the development of color cells, known as cones, is a thyroid hormone. By varying the amount of thyroid hormone these cells received, the team were able to alter which type of cones developed in the tissue.
"Trichromatic color vision delineates us from most other mammals," lead author Kiara Eldred, a Johns Hopkins graduate student, said in a statement. "Our research is really trying to figure out what pathways these cells take to give us that special color vision."
Previous research on retinal tissue focused on fish and mice in laboratories, which are only approximations of what our own bodies can do. But vision in humans is much different. None of the species tested have the dynamic daytime and color vision we humans sport, so the production of lab-grown human tissue was paramount.
"Everything we examine looks like a normal developing eye, just growing in a dish," added Robert Johnston, a developmental biologist at Johns Hopkins. "You have a model system that you can manipulate without studying humans directly."
The discovery of the thyroid hormone's role in the development of color vision provides further insight into why pre-term babies with lowered thyroid hormone levels have a higher incidence of vision disorders.
"If we can answer what leads a cell to its terminal fate, we are closer to being able to restore color vision for people who have damaged photoreceptors," Eldred explained. "This is a really beautiful question, both visually and intellectually – what is it that allows us to see color?"
The team now plans to use this approach to grow other regions of the retina, such as the macula. Macular degeneration is one of the leading causes of blindness in humans, so understanding its development could help millions worldwide.