Scientists have long faced difficulties studying how human eyes develop, because so much of that growth happens in the womb. But innovations in the lab of Robert Johnston, a Pew biomedical scholar and a professor at Johns Hopkins University, have given researchers new techniques to overcome these obstacles and unravel more mysteries of sight.
In an Oct. 12, 2018, article in the journal Science, Johnston, Johns Hopkins doctoral candidate Kiara Eldred, and colleagues describe how they grew human retinas from stem cells over a nine-month period. Their observations during this process confirmed an important hypothesis: Varying thyroid hormone levels trigger creation of the retinal cells that people need to see red, green, and other colors on display in changing autumn leaves.
The methods that Johnston and his team used arose from their investigations into stem cells and the factors that influence what they become. The researchers began with human embryonic stem cells and supplied them with nutrients that mirrored those present at different stages of fetal development. It’s intensive work, Eldred told The Washington Post, with the growing yet miniscule clusters of cells needing weeks of regular feedings and cleanings.
Gradually, each cluster organizes itself into a functional 3D structure known as an organoid. As in a baby’s developing retina, cone cells that specialize in detecting light start to form, with those needed to perceive blue appearing first.
Johnston’s group found that the thyroid hormone was necessary for the retina to switch to building cones to perceive red or green light. Moreover, when his team grew organoids with cones that lacked receptors for this crucial hormone, the retinas made only cells that detected blue. The researchers also determined that the timing and level of the hormone are important to healthy retinal development. These results align with those from studies done in other animals.
Johnston’s group plans to use these techniques to study color vision further as well as eye diseases such as macular degeneration, one of the leading causes of blindness. “What’s exciting about this is our work establishes [lab-grown] human organoids as a model system to study mechanisms of human development,” Johnston said.
Kara Coleman directs The Pew Charitable Trusts’ biomedical programs, including the biomedical scholars, Pew-Stewart Scholars for Cancer Research, and Latin American fellows programs.