The retina is the light-sensitive area at the back of our eye. It contains photosensitive cells called rods and cones that translate an image into electrical neural impulses through the optic nerve to our brain, creating the image you’re seeing now.
Now normally, retinas develop and grow within the confines of our eyeballs – which shouldn’t come as a surprise. But biologists at Johns Hopkins University have recently grown human retinas from scratch in petri dishes, helping them determine how the cells that allow us to see in color are made.
The study, published in the journal Science, focuses on the cells that allow people to see blue, red, and green – the three cone photoreceptors in the human eye. Most vision research is done with mice or fish, but neither of these species have dynamic daytime and color vision like us humans. And since they obviously couldn’t use human models, the research team created the human eyes they needed using stem cells.
“Everything we examine looks like a normal developing eye, just growing in a dish,” said Robert Johnston, a developmental biologist at Johns Hopkins. “You have a model system that you can manipulate without studying humans directly.”
It took months for the cells to grow and become complete retinas. During this time, the researchers observed that blue-detecting cells materialized first, followed by the red- and green-detecting cells. They also determined that the key to the molecular switch was the ebb and flow of thyroid hormone. But the level of this hormone was not controlled by the thyroid gland – as it obviously wasn’t in the dish with the retina – rather, the retina controlled its own levels of thyroid hormone.
Johnston’s team was then able to manipulate whether the cells became blue, or red and green by adjusting the amounts of thyroid hormone. Through this, they could make retinas that – were they a part of a complete human eye – would only see blue or only see green and red.
This finding regarding thyroid hormone makes sense, as pre-term babies who have lowered thyroid hormone levels also have a higher incidence of vision disorders.
“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,” says Kiara Eldred, a graduate student at Johns Hopkins. “This is a really beautiful question, both visually and intellectually – what is it that allows us to see color?”
Their work could be the foundation for developing new therapies for eye diseases such as color blindness and macular degeneration. Furthermore, the research team has established lab-generated “organoids” as a model that can be used to study human development on a cellular level.
Johnston goes even further when he says, “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.”