- Researchers grew human retinas from scratch to find out how cells that enable us to see in color are developed.
- The study explains why some people suffer from vision disorders.
- It could lead to the development of treatments for diseases like macular degeneration and color blindness.
The color and high-acuity vision comes from cone photoreceptors in the human retina. These cone cells work best in relatively bright light and are densely packed in the fovea centralis. There are up to 7 million cones in a human eye that are most concentrated towards the oval-shaped pigmented area near the center of the retina, macula.
Despite tons of biological advances made in the last couple of decade, it’s still not well-understood what allows people to see in color. So far, most vision studies have been conducted on fish and mice, but they don’t have the dynamic color vision similar to humans.
Now, researches at Johns Hopkins University have grown artificial human retinas from scratch to find out how cells that enable us to see in color are developed. It could lead to the development of treatments for a variety of eye diseases including macular degeneration and color blindness. Also, it establishes a model of lab-generated ‘organoids’ to examine human development on a cellular scale.
Growing Retina In A Dish
The aim is to create a replica of a normal developing eye, but this time it should be growing in a dish rather than inside a living organism. Scientists can manipulate this model the way they want without investigating human eyes directly.
We don’t exactly know what happens in the womb that triggers the development of a particular type of cells that allow us to see in color. To explore this phenomenon, researchers focused on the 3 subtypes of cone cells in the human eye:
- Blue-opsin (short wavelength)
- Green-opsin (medium wavelength)
- Red-opsin (long wavelength)
This trichromatic color vision makes humans different from most other species. To determine what route these cells take to provide us with special color vision, the team grew a full-blown retina for over months.
They discovered that blue-opsin cells materialized first, followed by the red and green ones. This temporal switch from specification of blue-opsin to generation of red- and green-opsin is controlled by thyroid hormone signaling The level of this hormone was not steered by the thyroid gland (not present in the dish), but completely by the eye itself.
The team analyzed what amount of thyroid hormone triggers the cells to become either blue or red and green. Then they used this data to tweak the outcomes of lab-generated retinas. They made eyes that could only see red and green color, and ones that would only see blue.
The study explains why pre-term babies with lower levels of thyroid hormones have higher chances of developing vision disorders. However, we can’t treat damaged photoreceptors: we are not there yet. To do this, we need to figure out what drives a cells to its terminal fate.
Researchers plan to use organoids — like a human baby, organoids take 9 months to develop — to further investigate color vision and the process involved in the development of other parts of the retina like macula. Eventually, this could lead to clinical treatments of macular degeneration, the leading cause of blindness.