Many diseases cause the degeneration of the retina, including retinitis pigmentosa (RP), Leber's Congenital Amaurosis (LCA), and macular degeneration. The retina is made up of photoreceptor cells that capture light and turn it into electrical signals which are then sent back to the brain through the optic nerve. As these photoreceptors die off, vision is lost by the patient. In some diseases, like macular degeneration, this vision loss occurs slowly over time, while in others, like LCA, the vision loss is extensive from birth.
But what if we could transplant the retina or create a new retina?
Here's where the emerging science of stem cell research takes center stage. The basic idea behind stem cell therapy is that embryonic stem cells are blue print cells that, with a little poking and prodding, can be turned into any kind of cell in the human body, including retinal cells. Each cell has it's own "recipe," so to speak, and scientists just need to unlock that mysterious combination of ingredients that signal a stem cell to grow into any other given cell. Some cells are harder to map than others, though, and the retina is a very complicated specimen.
Despite federal limitations on stem cell research, many private organizations around the country (and around the globe, for that matter) continue to work with stem cell lines funded with non-American-taxpayer monies. Researchers at the University of Washington in Seattle, for example, have announced they can now reliably create retinal cells using embryonic stem cell technology.
Regrowing damaged retina was once considered too difficult, mainly because there wasn't a large store of retinal cells with which to experiment, but now that scientists can create their own batch of retinal cells, the possibility seems near-at-hand. Right now, scientists are injecting their newly created retinal cells into the eyes of blind animals in the hopes of restoring vision. Once they've perfected the technique in animal models, humans will be next in line.
But how will the human retina react to an injection of new retinal cells? One advantage of stem cell therapy, says Raymond D. Lund, Ph.D., Professor at the Moran Eye Center, University of Utah Health Science Center, Salt Lake City, is that "a range of lines can be derived allowing the opportunity to 'tissue match' donor cells with recipient, a real advantage given that [retinal] cells are highly immunogenic and susceptible to rejection without some form of immunosuppression."
Another important point: the stem cell therapy seems to be completely safe. Already under way in experiments with rats, the therapy has caused no unwanted responses in the animals, according to researchers at Advanced Cell Technology, Inc., a private biotechnology company located in Worcester, Massachusetts.
This new stem cell therapy seems very promising, but how will it benefit patients with genetic diseases like LCA? If LCA is basically caused by a genetic mutation, then is simply replacing or regrowing the retina going to be enough? In other words, if your genetic material is telling your retina to stop creating a certain protein or to shut down in some other way, why would your body react any differently to a "new" retina? I think we may be looking at a combination of gene therapy and stem cell therapy at some point in the not-too-distant future where gene therapy fixes the genetic mutation then stem cell therapy comes in and patches up the "broken" retina. And, with the advances we're seeing in research and technology, this SciFi scenario may be achieved within our lifetimes!
Nobody ever said the retina was simple. There are many different types of cells in the retina, so researchers need to be able to produce many different stem cell recipes. However, scientists have found that retinal progenitor cells can go on to become any type of retinal cell the eye needs. In other words, once the retina is injected with retinal progenitor cells, the progenitor cells move about the retina and begin to act like different cells, such as photoreceptors, retinal ganglion cells, or other cells. Retinal progenitor cells are sort of like wild cards that fill any role needed of them.