A Harvard-led research team released a study this month demonstrating that transplanted stem cells may hold the key to fighting eye-related problems, including macular degeneration and other retinal diseases.

Researchers at the Schepens Eye Research Institute, an affiliate of Harvard Medical School (HMS), found that transplanting stem cells into the eyes of visually impaired mice preserved and even improved vision.

The experiment involved only tissue-specific stem cells—in this case, neural progenitor cells—which cannot become any kind of cell other than that of their source tissue. The controversial embryonic stem cells, which can potentially change into any kind of cell, were not part of the experiment.

Michael J. Young, an assistant professor of ophthalmology at HMS, said yesterday he was optimistic about the study’s findings, estimating that the research may soon make way for clinical purposes.

“I would say, between four and five years, there will be a therapy of some kind,” Young said.

“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,” Young announced in a press release last week.

But Co-Director of the Harvard Stem Cell Institute Douglas Melton remained cautious about the study’s implications, saying that the type of stem cells in the article may not make for the most exact science.

“There is no evidence presented as to whether the neural progenitor cells represent a single type of cell or are, instead, a population of several different progenitor pools,” Melton wrote in an e-mail.

Melton, a leading stem cell researcher who will chair the Faculty of Arts and Sciences Life Sciences Council beginning this spring, said further studies would be necessary to “explore their clinical potential.”

The retina is a thin, multi-layered membrane at the back of the eye that converts light and images into neural signals that reach the brain through the optic nerve. Macular degeneration, a disturbance of the center of the retina, is the leading cause of blindness in people over 55 years old.

The retinal stem cells used in the experiment came from mice which had been raised so that their tissues are fluorescent green. After being transplanted, these green cells can be readily distinguished from the host’s normal-colored tissues.

Most notably, Young’s study found that the stem cells were able to repair the retinae by differentiating into the types of retinal cells that had been damaged. In effect, the cells were able to “sense” and repair the problem areas within weeks.

The team also observed that some retinal cells that had appeared to be dying before the procedure became functional after the transplant. Young speculated that this finding might point to broader regenerative capacity of stem cells.

Young said yesterday he and his researchers have already begun similar tests in pigs, whose eyes resemble human eyes more closely than those of mice do.

The use of embryonic stem cells has aroused controversy because their cultivation can lead to the destruction of human embryos. President Bush has prohibited the use of federal funding for embryonic stem cell lines created after Aug. 2001.

A spokesman for the National Eye Institute, an agency of the U.S. Department of Health and Human Services that specializes in ophthalmology, declined to comment on the report.

Margaret DeAngelis, an ophthalmology researcher at the Massachusetts Eye and Ear Infirmary, said that more work still needs to be done, but she recognizes the potential benefits of further research. “Though this work is still preliminary,” she said, “this finding is exciting and certainly holds promise for future study and treatment of retinal diseases.”

In conducting this study, Young led six other researchers from Harvard, the University of Sheffield and the Children’s Hospital of Orange County.

The study, titled “Multipotent Retinal Progenitors Express Developmental Markers, Differentiate into Retinal Neurons, and Preserve Light-Mediated Behavior,” was published in this month’s issue of Investigative Ophthalmology and Visual Science.