Researchers from the Johns Hopkins University School of Medicine in Baltimore, MD, have created a miniature human retina in a dish from human stem cells, which they say can sense light. They are hopeful their creation could eventually lead to technologies one day that restore vision.
The study, led by assistant professor M. Valeria Canto-Soler of Johns Hopkins, is published in the journal Nature Communications.
She and her team explain that they created a 3D complement of human retinal tissue in their lab, and that it includes photoreceptor cells that are able to respond to light - the first step in the process of converting it into images.
They arrived at their creation after experimenting with human induced pluripotent stem cells (iPS), which are adult cells that have been genetically reprogrammed back to their most primitive state.
Such cells are capable of developing into most of the 200 cell types in the human body, the team explains, and for their latest study, they were able to turn them into retinal progenitor cells that form light-sensitive retinal tissue lining the back of the eye.
In their petri dishes, the researchers observed that the growth corresponded in both timing and duration with a human fetus' retinal development in the womb. Additionally, the photoreceptors were able to develop outer segments, which is necessary for photoreceptors to properly work.
This is quite an achievement, as retinal tissue comprises seven major cell types with six kinds of neurons that correspond with specific cell layers that absorb and process light, and transmit those signals to the brain to interpret.
Lab-grown photoreceptors responded same way retinal rods do
When the retinal tissue had reached the equivalent of 28 weeks' development in the womb, the researchers tested the mini-retinas to see if the photoreceptors could sense and transform light into visual signals.
Researchers derived a "mini retina" from human iPS cells in the lab. The rod photoreceptors are in green.Image credit: Johns Hopkins Medicine
"We knew that a 3D cellular structure was necessary if we wanted to reproduce functional characteristics of the retina," says Canto-Soler, "but when we began this work, we didn't think stem cells would be able to build up a retina almost on their own. In our system, somehow the cells knew what to do."
The team tested the retinas by putting an electrode into a single photoreceptor cell and sending a pulse of light to the cell. This reacted in a biochemical pattern similar to the behavior of photoreceptors in humans exposed to light, the team reports.
In detail, Canto-Soler explains that their lab-grown photoreceptors responded to light the same way retinal rods do, adding that the majority of photoreceptors in humans are rods - rather than cones - which permit vision in low light.
What this ultimately means, she says, is that their system allows them to create hundreds of mini-retinas at a time from a person affected by a retinal disease.
In addition, their findings have provided a unique system to study the cause of human retinal disease in human tissue, rather than using animal models.
'A good start,' but more work to do
Canto-Soler adds that their system opens up the potential for personalized medicine, including testing drugs to treat retinal diseases in a patient-specific way. And they may be able to eventually replace dead retinal tissue with lab-grown material to restore vision.
However, she cautions that photoreceptors are just part of the complex precess of vision. Their lab has not yet recreated all the human eye functions or its links to the visual cortex in the brain.
"Is our lab retina capable of producing a visual signal that the brain can interpret into an image?" she says. "Probably not, but this is a good start."
A study funded by the National Institutes of Health recently revealed that scientists were able to use electrical stimulation of retinal cells to produce the same activity patterns our retinas produce when they see a moving object. They say this could be a step toward restoring vision in blind people.
Written by Marie Ellis