Biologists at Johns Hopkins University (JHU) have grown human retinas from scratch using stem cell technology. The purpose of the on-going study was to determine how cells that allow people to see in color are made. The research could later be used to develop therapies for eye diseases such as color blindness and macular degeneration.
“Everything we examine looks like a normal developing eye, just growing in a dish,” said Robert Johnston, a developmental biologist at JHU. “You have a model system that you can manipulate without studying humans directly.”
Johnston’s lab explores how a cell’s fate is determined in the womb, an aspect of human biology that remains unknown so far. He and his team mainly focus on the cells that allow people to see blue, red and green — the three photoreceptors in the human eye.
Most vision-related research relies on using mice and fish as test subjects. However, neither of these species has the dynamic daytime and color vision of humans. This is why Johnston’s team used stem cells to create the human eyes they needed.
Elaborating on the nature of the research, JHU graduate student and the study’s lead author Kiara Eldred stated, “Trichromatic color vision delineates 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.”
As the cells grew over the months and became fully-formed retinas, Johnston’s team found that the cells which detect the color blue developed first, followed by those that detect red and green respectively. The researchers found that the ebb and flow of the thyroid hormone was the key to this molecular switch. It is pertinent to note that the levels of this hormone were not regulated by the thyroid gland (which obviously was not present in the dish) but the eye itself.
With this finding, the team was able to create retinas that if they were part of a human eye, would only see the colors blue or green and red. This brings biological science closer to being able to restore color vision to individuals with damaged photoreceptors. Furthermore, since macular degeneration is the leading cause of blindness, learning how to grow new macula could lead to new treatments to restore vision in blind people.
“Our work establishes human organoids as a model system to study mechanisms of human development,” Johnston said. “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.”