US biologists have grown human retina tissue in the laboratory to determine how cells that allow people to see in colour are made, an advance that can lead to the development of therapies for diseases such as colour blindness and macular degeneration.
The study explored how a cell's fate is determined or 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.
The team from the Johns Hopkins University in Maryland, US, focused on the cells that allow people to see blue, red and green -- the three cone photoreceptors in the human eye.
"Trichromatic colour vision delineates us from most other mammals," said lead author Kiara Eldred, a graduate student at the varsity.
"Our research is really trying to figure out what pathways these cells take to give us that special colour vision," he added, in the paper to be published in the journal Science.
Using stem cells, the team created human eyes in the lab. When these cells grew and became full-blown retinas, the blue-detecting cells were found to materialise first, followed by the red and green-detecting ones.
In both cases, the key to the molecular switch was the ebb and flow of thyroid hormone. Importantly, the level of this hormone was not controlled by the thyroid gland, but entirely by the eye itself.
As a result, the team was then able to create retinas that would only see blue, and ones that could only see green and red.
The finding that thyroid hormone is essential for creating red-green cones provides an 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, the researchers noted.
"If we can answer what leads a cell to its terminal fate, we are closer to being able to restore colour vision for people who have damaged photoreceptors," Eldred said. "This is a really beautiful question, both visually and intellectually -- what is it that allows us to see colour?"