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Enlarged image of the oganoid retinal tissue, showing photoreceptors allowing people to see the colors. (Kiara Eldred / Johns Hopkins University)
Kiara Eldred sometimes compares her nine-month scientific experiments, developing tiny human retinas in a lab dish, to the children's education.
Eldred, a graduate student at Johns Hopkins University, begins cultivating thousands of stem cells and provides them with nutrients and chemicals to guide them into the retina, the part of the eye that translates light into signals that drive to vision. After two weeks of laborious culture, these cells typically produce 20 to 60 small balls of cells, called retinal organelles. As they mature, these nascent retinas become dirty and detach from a large number of cells; they should also be washed when fed every other day – at least for the first month and a half.
After nine months of diligent caring, Eldred has a lot of miniature human retinas that react to light, are about two millimeters in diameter and have the shape of a tennis ball cut in half. But the growth of organoids is only the first step.
In a new study in the newspaper Science, Eldred and his colleagues described how to use this system to understand a fundamental question of vision that has remained astonishingly mysterious: how does color vision develop?
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The researchers found that blue cone cells, which detect blue light, develop first and that red and green photoreceptor cells develop later. They also found that the thyroid hormone appears to be the essential signal to determine which light-detecting cells are developing.
Eventually, researchers hope that this information could help develop treatments for diseases in which these light-sensing cells are depleted, such as macular degeneration. A better understanding of the process could lead to the development of treatments for vision defects that develop in premature infants.
"The ideal goal would be to take a person's cells, convert them into stem cells, and then reprogram them and reintroduce them into the person and treat the disease no matter what." said Robert Johnston Jr., neurobiologist of development at Johns Hopkins. runs the lab where Eldred works.
But organelles have limits. The human retina is about 15 times larger than organoids, which roughly corresponds to the size of the inner ring of a piece of Cheerios cereal, said Eldred. There are still many questions about their ability to reflect fetal eye development because they lack many other peripheral structures. But the opportunity to grow these organelles in a dish on a time scale reflecting human development opens a rare window onto issues that otherwise can not be easily probed.
"In the past, if we wanted to define the developmental mechanisms underlying a particular process, we turned to model organisms such as the mouse or the zebrafish," said Thomas Reh, a scientist at the University. from Washington, who studies the development of the eye. Reh, who did not participate in the study, described it as "really excellent basic biology".
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