In order to ascertain how cells that enable people to see in color are made, scientists at the Johns Hopkins University have fabricated human retinas from scratch in the lab. The work is expected to establishes the framework to create treatments for eye infections, for example, visual weakness and macular degeneration.
Robert Johnston, a developmental biologist at Johns Hopkins said, “Everything we examine looks like a normal developing eye, just growing in a dish. You have a model system that you can manipulate without studying humans directly.”
Scientists examined how a cell’s fate is determined—ohuman retinas, Johns Hopkins University, human eye tissue what happens in the womb to turn a developing cell into a specific type of cell, an aspect of human biology that is largely unknown. They specifically focused on the cells that allow people to see blue, red and green.
Scientists then created human eye tissue using stem cells. Over months, as the cells grew in the lab and became full-blown retina tissue, the team found the blue-detecting cells materialized first, followed by the red- and green-detecting ones. In both cases, they found the key to the molecular switch was the ebb and flow of thyroid hormone. Importantly, the level of this hormone wasn’t controlled by the thyroid gland, which of course isn’t in the dish, but entirely by the eye itself.
Understanding how the amount of thyroid hormone dictated whether the cells became blue or red and green receptors, the team was able to manipulate the outcome, creating retinas that if they were part of a complete human eye would only see blue, and ones that could only detect green and red.
Lead author Kiara Eldred, a Johns Hopkins graduate student said, “Trichromatic color vision differentiates us from most other mammals. Our research is really trying to figure out what pathways these cells take to give us that special color vision.”
The study suggests that thyroid hormone is essential for creating red-green cones provides insight into why pre-term babies, who have lowered thyroid hormone levels as they are lacking the maternal supply, have a higher incidence of vision disorders.
Eldred said, “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. This is a really beautiful question, both visually and intellectually—what is it that allows us to see color?”
Johnston said, “What’s exciting about this is our work establishes human organoids as a model system to study mechanisms of human development. What’s really pushing the limit here is that these organoids take nine months to develop just like a human baby. So what we’re really studying is fetal development.”
The work is set for publication in the journal Science.