Where are we with stem cell therapy?
What is a stem cell? Embryonic stem cells are also called pluripotent cells, which means that they have the ability to potentially turn into any type of cell in the human body. These cells are present in human embryos, since the job of an embryo is to build a complete human body starting from just 2 cells, the egg and the sperm. After fertilization the cells first grow as “undifferentiated” cells, which means that they have not yet begun to turn into more specialized cells that make up the tissues of the human body, such as muscle cells, nerve cells, skin cells, blood vessels cells and retina cells etc. For most cells as they begin to change into specific types of cells, or differentiate, this is a one-way street. Once the cells have differentiated in to specialized cells they can’t go back and turn into other types of cells. This process of differentiation occurs as the cells in the body divide and the embryo grows.
During the growth of an embryo, as cells multiply, there is a very complex process by which the cells “know” where they are and what to turn into. This is governed by the genes encoded by the DNA in each cell. But the process of differentiation is triggered by chemical signals that tell the DNA in a cell which genes to turn on and which to turn off. For instance, a cell lying on the back wall of the eye may receive a signal telling it to turn into a retinal nerve cell.
When cells in the retina are damaged, why can’t we just grow new ones? There are actually some animals that can do this, but humans aren’t one of them and we don’t know the secret of how these other animals do it. We do know that there are no stem cells in the adult human retina. Once you are born and your retinal nerve cells are completely differentiated into retinal nerve cells, your body is no longer capable of replacing damaged nerve cells in the retina. Also, we don’t know for sure whether all of the chemical signals are gone that cause a stem cell to turn into a retinal nerve cell. Since humans can’t replace damaged retinal nerve cells, there would be no reason for those chemical signals to still be present in the retina, and as a matter of fact when scientists have tried injecting stem cells under a damaged retina, they don’t grow normally and repair the retina. They don’t even begin the process of changing into retinal nerve cells unless that are prompted to start differentiating in the test tube before they are injected.
So is there any hope that stem cells may someday restore vision to a patient with a damaged retina? Yes, but we aren’t close to that yet. There are some promising studies, but we aren’t able to regrow a new retina yet.
Where do stem cells come from? Most stem cells that have been traditionally used in research have come from human embryos. This, as most of us know, has been very controversial. It was opposed by Presidents Regan and Bush on moral and religious grounds, although Nancy Regan later petitioned George W Bush to relax these restrictions when her husband developed dementia and she felt that stem cell therapy might be a promising avenue of research. There have been other methods proposed for inducing fully differentiated cells to go the wrong way down the one-way street and turn back into pluripotent stems cells. These methods can produce stem cells from mature human cells, but the process still can’t produce enough pluripotent stem cells for use in regenerative medicine. Most tissues in the adult human body contain a small number of stem cells that can conceivably be used to regrow the tissues that they came from, but they are hard to identify and grow in culture and for this reason can’t be used for retinal research yet.
We hear a lot about stems cell therapies being offered at different clinics around the country. What is that all about? Stem cell therapy is a very attractive marketing term these days and it has been used by numerous medical facilities to attract patients and separate them from a significant sum of money. Many stem cells therapy clinics can cost many thousands of dollars out of pocket for therapy that has no basis in medical science. At this point in time there is no stem cell therapy that has been shown to be effective for any retinal condition. There are some early clinical trials going on, but these trial do not charge patients for the treatment and make no claims of effectiveness.
Why not just inject stem cells into a vein and let them do their work? This is what a lot of the stem cell therapy clinics are doing and there are good reasons to question the safety and effectiveness. For one thing, adult stem cells taken from your own body are usually labeled “stem cells” but they may not have the ability to differentiate into the type of cell that is damaged in your body. For instance, there is no reason why a stem cell from blood forming tissue would turn into a retinal nerve cell. In order for a cell that is injected into the blood stream to repair the retina it would have to find its way to the damaged area of the retina and somehow embed itself in the tissues and then differentiate into a retinal nerve cell and form all the right connections with the other nerve cells in the area. Expecting this to happen by injecting stem cells into a vein is sort of like opening up your broken computer, throwing a handful of electrical components in, closing the panel and expecting all of the components to find their way to the proper place and connecting up to the correct circuits. Even when scientists have surgically implanted stem cells directly under the retina in the area of retinal damage they haven’t been able to get the stem cells to do this yet. Scientists are making progress and I believe there will be break throughs in the future, but at this point in time there are only a limited number of experimental studies that are being conducted to determine if this process is safe to try in humans. No one has yet had their sight restored by stem cell therapy.
Of course when we have a condition that traditional medicine has little to offer, as with so many of our advanced retinal conditions, we want to believe that there must be something out there that can be done. We would caution our patients not to be taken in by the stem cell therapy hype at this stage of the game when there could be more to lose than to gain. There is an excellent web site that goes into more detail about stem cell therapy that I would encourage you to visit at http://www.closerlookatstemcells.org/