For the first time researchers have shown that transplanted stem cells can preserve and improve vision in eyes damaged by retinal disease.
In the cover article in the November 2004 Investigative Ophthalmology and Visual Science, scientists from Harvard's Schepens Eye Research Institute describe results of a mouse study in which transplanted stem cells develop into retinal cells, prevent the death of "at risk" retina cells in the recipient mice and improve the vision of those treated mice.
"These findings hold great promise for potential treatments for people suffering from macular degeneration, diabetic retinopathy and other retinal diseases," says Michael Young, PhD, an assistant scientist at Schepens Eye Research Institute and the lead author of the study.
The retina is a tissue-thin membrane at the back of the eye responsible for sending light and images from the outside world through the optic nerve to the brain, which interprets them. The retina contains light sensitive cells, known as rods, which make it possible for us to see in black and white and in low light, and cones, which are responsible for color and high-acuity vision. In diseases such as macular degeneration, it is these cells that are being destroyed.
Believing that stem cells -- cells that have the capacity to change into other kinds of cells -- could potentially save vision, Young and his team decided to test their theory in mice. They transplanted retinal stem cells from young "green" mice into the eyes of normal-colored mice that had retinal disease. Green mice are genetically raised so that all their tissues are fluorescent green. The green color makes it possible to detect where the transplanted cells are and how they are growing and changing.
After several weeks the team evaluated the eyes of the treated mice and found that the green cells had migrated to where they were needed in the damaged retina and had changed into what looked like normal retinal cells. The scientists also found that many of the cone cells that were on the verge of dying before the transplant appeared to regain or retain their function. The researchers speculated that the transplanted cells were secreting a factor or substance that saved these fragile cells. (There is growing evidence that rod cells keep cone cells alive by secreting a special factor.)
To test whether the mice with transplanted stem cells could see better, the team then placed them and the control mice (without the transplants or with non-stem cell transplants) in dark cages and flashed a series of increasingly lower level lights at both groups over a period of time. Mice are photophobic and stop their normal activity when they detect light. The researchers took advantage of this natural response and found that the mice with the transplanted tissue continued to respond to the light as it reached the lowest levels. The control mice did not.
"These are the first steps toward the use of stem cells for saving existing vision and then -- down the road -- restoring vision that has already been lost," says Young, who believes that stems cells will have many roles to play in the fight against blinding diseases.
Young and his team are now investigating the same phenomenon in pigs, whose eyes are larger and more like human eyes.