Patients with age-related macular degeneration (AMD) often ask whether stem cell therapy might improve their vision now, or at some point in the future.
The concept is that stem cells are “undifferentiated,” meaning that they have the potential to develop into many different cell types, including retinal cells, which can then replace cells that have died due to AMD. The two major cell types that die in AMD are retinal pigment epithelial (RPE) cells and photoreceptors.
It is much easier to replace RPE cells than photoreceptors. RPE cells normally sit on a membrane in a single layer, where they support the photoreceptors. Without RPE cells, photoreceptors die. This is the sequence of events in at least some patients with the “geographic atrophy” form of advanced, dry AMD. Stem cells can be induced to become RPE cells relatively easily, and then injected into their proper location. The challenge is that often these cells form clumps rather than a single layer.
Last year, researchers published preliminary results from a small clinical trial of injected stem cell-derived RPE cells for AMD and the related disease, Stargardt’s. The results show evidence that these cells injected into a small area of the macula can survive for three months, the procedure was not associated with any adverse events, and there is a hint that the vision may have improved slightly. While these results represent a promising start, more clinical trials and fine-tuning of techniques are needed.
It is more difficult to transplant photoreceptors, however. While stem cells can be induced to form photoreceptors, it has harder to “convince” them to become photoreceptors than RPE cells. Also, after photoreceptors are transplanted into the retina, they must connect with other retinal neurons and form synapses in order to conduct their signals. While this normally occurs in the developing retina, it is much harder to make this happen in an adult retina. Experiments in mice suggest that it can be accomplished, but so far this happens mainly when the donor cells and recipient retinas are at early developmental stages. Combinations of RPE cells with photoreceptors can also be generated from stem cells, and could also be considered for transplantation.
In summary, there is much excitement about the potential for stem cell transplantation in AMD, and this approach has a reasonable chance of helping patients with wet AMD or advanced dry (geographic atrophy) AMD at some point in the future. In the meantime, studies in mice as well as additional human clinical trials will continue to pave the way.
Joshua L. Dunaief, MD, PhD Associate Professor of Ophthalmology Scheie Eye Institute University of Pennsylvania
Posted: December 2013
Retinal Pigmented Epithelium (RPE) - a layer of cells that protects and nourishes the retina, removes waste products, prevents new blood vessel growth into the retinal layer and absorbs light not absorbed by the photoreceptor cells; these actions prevent the scattering of the light and enhance clarity of vision.
Photoreceptors - the light sensing nerve cells (rods and cones) located in the retina.
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