Age-related macular degeneration is one of the most common causes of vision loss in developed countries, affecting between 30 million and 50 million people worldwide.
As the name suggests, age-related macular degeneration (AMD) is most common in people over 60, and more people are suffering from AMD as the population ages. About 90 percent of AMD cases are dry AMD, for which there is no good treatment today.
In this section:
What is macular degeneration?
The retina is the nerve tissue lining the inside of the eye that starts the conversion of light into vision. At the center, in the back of the retina, is the macula, a small area about 5.5 millimeters in diameter that is responsible for central vision, which is essential for tasks such as reading, driving and facial recognition. The macula is densely packed with photoreceptor cells called rods and cones that react to light and send electrical nerve impulses to the optic nerve and into the brain. Behind the photoreceptors is another layer of cells called retinal pigment epithelium (RPE), which support the rods and cones by delivering nutrients from the bloodstream and removing waste that the rods and cones generate.
In AMD, the RPE cells stop performing their support functions and the rods and cones die, resulting in a loss of central vision. Dry AMD typically progresses over several years. In the less common wet AMD, something (scientists aren’t sure what) spurs abnormal blood vessel growth, and central vision can be lost in a matter of weeks or days.
How is macular degeneration treated?
Doctors have drugs and surgical techniques that are effective in treating wet AMD, but there is currently no good treatment for the dry form of the disease. There is also some evidence that the drugs used to treat wet AMD may make the underlying dry AMD worse. People with a new diagnosis of dry AMD can take vitamin supplements, but dry AMD is progressive and, over time, central vision worsens.
How are we using stem cells to understand macular degeneration?
Stem cell research is helping scientists understand how the different cell types in the retina function together, which has led to exploring ways to replace both rods and cones and the supporting RPE cells.
Replacing rods and cones is challenging, because these cells have to establish connections with nerve fibers that feed signals into the optic nerve, which sends those signals to the brain to interpret. Researchers are actively working on this approach, but ensuring new rods and cones integrate properly with nerve fibers alongside the patient’s existing rods and cones is extremely complex.
Special Attributes of the Eye.
The eye is a good target for stem cell treatments. It is relatively self-contained, with many barriers that keep cells from migrating to other parts of the body. It is easy to assess the effectiveness of treatments in the eye because researchers have tools for viewing the eye’s interior and for measuring its visual function. They are also able to compare results from a treated eye to an untreated eye in the same patient.
RPE cells don’t need to connect with nerve fibers, so getting them to integrate with existing retinal cells may be easier. New RPE cells could replace diseased RPE cells and take on some of their supportive functions. If the transplant is done before rods and cones have been lost, new RPE cells may be able to prevent them from dying, thus improving central vision, or at least stopping the progression of the disease. RPE cells are also easier to make as a uniform cell type from stem cells, reducing problems associated with the generation of a uniform population of cells for transplantation.
Stem cells are also being used in drug discovery, the process by which new treatments are discovered. Healthy RPE cells can be stressed to produce abnormal cells that display features of AMD. These cells can be grown in the lab and studied to better understand how AMD progresses, which may lead to earlier detection and better diagnosis. The cells can also be used to screen potential drugs for safety and effectiveness.
What is the potential for stem cells to treat macular degeneration?
Stem cell researchers are making great progress in their efforts to replace the RPE layer, which they believe will halt or even reverse the vision loss associated with AMD.
Some researchers are using induced pluripotent stem (iPS) cells — tissue-specific cells (usually skin cells, but sometimes other tissue cells) that are reprogrammed in the lab to behave like embryonic stem cells – to grow rods and cones or RPE cells. Other groups are using human embryonic stem cells, and others are exploring RPE-specific stem cells that can be grown from the adult RPE, for example, from eyes donated to eye banks.
Researchers are working to determine the optimal maturation for these cells. The more mature (that is, more differentiated) the cells are when transplanted, the less likely they are to over-grow (to generate too many RPE cells, which can lead to scar tissue) or to migrate away from their intended place in the body. On the other hand, less mature cells have more self-renewal properties and possibly more potential to integrate and repair the eye’s rods and cones.
Researchers are also exploring different methods to deliver stem cells to the eye, including creating patches of RPE cells in the lab. In one approach, a one-cell-thick layer of RPE cells derived from human embryonic stem cells or adult RPE stem cells is placed on a material that allows nutrients and waste materials to pass through and is implanted in the eye. In tests in animals, the patch has shown promise; the RPE cells appear to be stable and don’t migrate to other areas of the eye.
Another delivery showing promise is a suspension of cells, which is injected into the eye under the retina. The cells, derived from iPS cells, RPE stem cells, or human embryonic stem cells, are grown and differentiated in the lab, then placed in a harmless fluid to be injected.
For both approaches, a critical question is whether these cells will integrate well with the patient’s own RPE cells and do their job of supporting the rods and cones over the long term.