Researchers at Johns Hopkins University in the United States have grown human eye retinas from scratch in the laboratory to determine how cells allow people to see in color.
The new study lays the foundation to develop therapies for eye diseases such as color blindness and macular degeneration. It also establishes lab-created model ‘organoids’ that enable the study and manipulation of human development on a cellular level, without involving human subjects.
In their latest study, the team created human eyes from stem cells in order to focus on the specific development of cells that allow people to see blue, red and green — the three cone photoreceptors found in the retina of human eyes.
Over nine months, as the cells grew in the lab and became full-blown retinas, the team found the blue-detecting cells materialized first, followed by the red- and green-detecting ones. In both cases, they found the key to the molecular switch was the ebb and flow of thyroid hormone. Importantly, the level of this hormone was controlled entirely by the eye, and not produced from the thyroid gland that was absent in the laboratory model.
Understanding how the amount of thyroid hormone dictated whether the cells became blue or red and green, the team was able to manipulate the outcome, creating retinas that if they were part of a complete human eye, would only see blue, and ones that could only see green and red.
The finding that thyroid hormone is essential for creating photoreceptor cones in the retina provides insight into why pre-term babies, who have lower thyroid hormone levels as they are lacking the maternal supply, have a higher incidence of vision disorders.
Finding answers to what leads a cell to its eventual color could help restore color vision to people who have damaged photoreceptors. In future the researchers would like to use organoids to learn even more about color vision and the mechanisms involved in the creation of other regions of the retina, such as the macula. Since macular degeneration is one of the leading causes of blindness in people, understanding how to grow a new macula could lead to clinical treatments.