The researchers don't yet know if the cells can actually integrate into the complex circuitry of the eye to restore vision, but early results are promising. The transplanted cells do express many of the proteins needed to respond to light, and they make neural connections when grown in a dish with other retinal cells. However, the true test will come with Reh's current experiments: transplanting the cells into blind animals. "We should know within the next year if the cells can restore vision," says Reh.

Other groups are also developing stem cell therapies for the retina. Advanced Cell Technology (ACT), a stem cell biotechnology company based in Alameda, CA, has developed a way to turn embryonic stem cells into pigment epithelial cells, another cell type lost in macular degeneration.

When implanted into the eyes of animal models, the cells protect against further degeneration of the photoreceptors and improve vision, says Robert Lanza, vice president of research and scientific development at ACT. The company plans to file for permission from the Food and Drug Administration to start human trials of the therapy by the end of next year, he says.

Experts say that work such as Reh's and Lanza's is finally bringing hope to an area of research that has struggled for years. Scientists have had some success in transplanting retinal cells from fetuses in animal models, and a small clinical trial of this type of therapy is currently underway. "But the logistics of using fetal tissue has a lot of uncertainties and ethical issues, and there is always the danger of transmitting infective agents to the host," says Lund.

Stem cell-derived retinal cells provide a much larger and more reliable source of cells. "I think cell therapies for eye disease are really going to take off in the next few years," says Lund.

Reh and others still have a lot to work out before stem cell therapies for retinal degeneration become a reality. For example, it's not yet clear whether it's better to implant cells when they are still in a somewhat undifferentiated state, such as the progenitor cells, or whether it is better to turn the cells into photoreceptors and then transplant. Reh plans to try both.

Restoring vision could be one of the most promising early uses of stem cell therapies because scientists know exactly what cells they need to replace. "We're always reading that embryonic cells are going to cure every disease," says Lund. "But in this case, we're clearly working with the idea that embryonic stem cells will have very specific functions in eye disease."