Scientists at Johns Hopkins University replicated a human retina in a dish to explore how the eye’s color-detecting cells develop. Their findings could help premature babies better develop eyesight, and the elderly keep theirs for longer.

If you’re imagining a whole eyeball lolling around in a wet dish, it wasn’t quite that gory; without a microscope, it looked like a tiny translucent lump of goo.

This is an example of an “organoid,” a small piece of tissue developed from stem cells to replicate part of a larger organ. For this study, the organoid was made up of the cells that respond to light and communicate that information back to the brain.

“The big [retina organoids] are the size of the inside of a Cheerio,” Kiara Eldred, Johns Hopkins graduate student and lead author on the study, told me over the phone. “They’re really small, but they have all of the cells that are in the retina that are necessary for receiving light and signaling that to the brain.”

Led by Robert Johnston, developmental biologist at Johns Hopkins, the research will be published in the journal Science on Thursday.

It takes nine months to grow a retina from scratch in the lab, but the researchers kept some organoids growing for more than a year, diligently feeding them glucose and other nonessential amino acids every day at first, and then every other day for the rest of their “lives.”

While they grew, the researchers studied how particular cone cells developed. They already knew from studying model organisms (such as mice and zebrafish) that thyroid hormones are important to early development of the eye, but hadn’t tested them on human tissue before.

When they added thyroid hormone to the organoid in the early stages of development, they saw red-green cones develop in the dish.

“This was really exciting, because it was the first time we were able to test this hypothesis and prove it in human tissue,” Eldred said.

They realized that this correlates with how babies develop in infancy: Premature babies are at a higher risk of visual defects, possibly because they’re removed from the thyroid-rich environment of their mothers’ womb too soon. Much more research is needed, of course, but giving preemies thyroid hormone treatments could someday help them see better as they grow.

Stem-cell therapies have also shown promise for treating diseases such as macular degeneration—a leading cause of blindness in the elderly.