The eye is often compared to a camera, but although its basic design is as simple as an old-fashioned box Brownie, its detailed structure is more complex than the most advanced electronics. This means that, unlike simpler organs, studies of retinal disease rely heavily on animal studies, and treating such illnesses is extremely difficult. One ray of hope in the field comes from researchers at Johns Hopkins, who have constructed a functioning segment of a human retina out of stem cells that is able to respond to light.
The retina is the complex lining of the human eye that acts like the the film (or the imaging sensor, for the younger crowd) in a camera. It’s made of some 10 layers of tissue, including structural membranes, nerve ganglia, and photoreceptor cells; the rods that detect black and white images and work best in low light, and the cones, which detect color. If scientists could recreate this structure in the laboratory, it would be a major breakthrough in treating eye diseases.
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The Johns Hopkins researchers’ approach was to use human-induced pluripotent stem cells (iPS). In other words, adult cells were induced to revert to stem cells, from which any of the 200 specialized cells in the human body can be derived. The Johns Hopkins team programmed the stem cells to grow into retinal progenitor cells in a culture dish.
These cells developed into retina cells, much in the same way and at the same rate as in a human embryo. As they did so, the cells differentiated into the some of the seven different kinds of cells that make up the retina and organized themselves into the three-dimensional outer segment structures necessary for the photoreceptors to work.
"We knew that a 3D cellular structure was necessary if we wanted to reproduce functional characteristics of the retina," says M Valeria Canto-Soler, an assistant professor of ophthalmology at the Johns Hopkins University School of Medicine, "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."
Growing retina segments has been achieved before, but where Johns Hopkins’ work stands out is that these mini-retinas actually function. When the mini-retinas reached the equivalent of 28-weeks of development, the researchers hooked the photoreceptor cells up to electrodes and flashed pulses of light at them. According to the scientists, the cells displayed the same photochemical reactions as in a normal retina – especially in regard to the rods that make up the majority of the photoreceptors.
The result of the Johns Hopkins research is a mini-retina that responds to light, but doesn't have the ability to form images. The structure is nowhere near complete and there’s no way to connect the artificial retina to the brain’s visual cortex. However, being able to produce retinal structures of such complexity holds the promise of developing new ways of studying eye diseases and developing new ways to treat them.
According to Cano-Soler, the technique will allow doctors to grow hundreds of mini-retina from a patient’s own cells, allowing for advanced testing and individualized drug treatments. In addition, it will allow scientists to study eye diseases without animal tests. Eventually, it may even lead to a means of restoring vision in patients with retinal diseases with transplants of lab-grown retinas.
The team’s results were published in the journal Nature Communications.
Source: Johns Hopkins