Eyes have been called the window to the soul, but they are also proving to be a potential wellspring for regenerative medicine.
Researchers say they have discovered a new source of stem cells at the back of the eye, which they hope may one day provide a way to repair the damage from age-related macular degeneration, or AMD, the leading cause of vision loss in people over 60.
"We're just thrilled to have found these cells because we know that age-related macular degeneration affects so many people, something like 10 million people in North America," Sally Temple, a developmental neuroscientist at the Neural Stem Cell Institute, said in an interview from Albany, N.Y.
Temple's team, whose finding is described in the January issue of the journal Cell Stem Cell, identified the central nervous system stem cells in a single layer called the retinal pigment epithelium, or RPE, which lies behind the retina.
The retina, which contains millions of photoreceptor cells, converts light into electrical signals that are sent to the brain via the optic nerve. The brain then translates those signals into the images we see.
The researchers salvaged the stem cells from the RPE layer in the eyes of more than 100 deceased donors, who ranged in age from 22 to 99. But the cells can also be isolated from the fluid surrounding the retina at the back of the eye, meaning they're also accessible in living people.
"You can literally go in and poke a needle in the eye and get these cells from the sub-retinal space. It sounds awful, but retinal surgeons do it every day," said Temple, adding that other neural stem cell populations would require major surgery deep within the brain.
"These cells are laid down in the embryo and can remain dormant for 100 years. Yet you can pull them out and put them in culture and they begin dividing. It is kind of mind-boggling."
In culture dishes in the lab, the researchers were able to coax about 10 per cent of the RPE-derived stem cells to grow in the lab. Further prodding caused the cells to differentiate into, or give rise to, a variety of cell types — those that make bone, fat or cartilage.
Temple said her team also generated a progenitor cell that carries some characteristics of one type of nervous system cell, although it was not fully differentiated. "But the fact that we could make these cells that were part-way, that were immature, indicates to us that if we keep on manipulating them, going forward in the future, we should be able to find ways to create other types of central nervous system cells," she said.
One goal would be to produce neurons, the electrical-signalling cells in the brain and other parts of the central nervous system. That would mark a major step towards the holy grail of regenerative medicine: the ability to repair spinal cord injuries and brain damage caused by such diseases as Alzheimer's or Parkinson's.
"And a really important cell type that we'd love to see if we can make would be the retinal cells, the neural retinal cells like the photoreceptors that are in the eye," said Temple. "So if we could help make new photoreceptors as well as the RPE — which we've already shown we can make — then we would be making two really valuable cell types for age-related macular degeneration."
Stem cells might fight AMD
AMD is caused by damage to the central part of the retina, called the macula, which contains the photoreceptors needed for straight-ahead vision and for seeing fine details and colour. Over time, people with AMD lose the central part of their vision.
Temple said cell types generated from the stem cells could be used for testing drugs for AMD and other eye diseases, but the ultimate goal would be to regenerate eye tissue destroyed by AMD so sight could be restored.
Tests in the lab and in animals suggest the cells are stable and do not appear to cause tumours, a potential danger inherent in some types of stem cells. More animal studies are needed to determine if the cells can actually rescue lost vision, she said.
If all goes well, the first trial looking strictly at the safety of an AMD treatment using the new source of stem cells could begin in humans in about five years, Temple predicted.
These stem cells aren't the first to be discovered in the eye. In 2000, Derek van der Kooy of the University of Toronto and colleagues identified stem cells in a part of the retina closer to the front of the eye. In late 2010, his team published a paper describing how they had used human retinal stem cells to restore sight in blind mice.
Several research groups have begun preliminary trials of stem cells in people with AMD in the hope of restoring their vision. Valerie Wallace, associate director of vision research at the Ottawa Hospital Research Institute, said Temple's group appears to have found a stem cell type that differs from the one isolated by the Toronto researchers.
"The advance here is showing that you can grow these cells," Wallace said Thursday from Ottawa. "So this kind of thing could maybe help them expand those cells in culture, so maybe improve the possibility of using these RPE cells for transplantation."