Until now, patients who suffer from one of the most common causes of vision loss have had little hope for treatment. Age-related macular degeneration, or AMD, typically strikes people older than 60 by thinning a layer of cells at the back of the eye known as the retinal pigment epithelium. This layer of cells eliminates waste from the eye and nourishes photoreceptors, the neurons that absorb and convert the light that creates the images we see. As the disease progresses, photoreceptors die, and patients lose central vision—the ability to see what is directly in front of them; peripheral vision is not affected.

Embryonic stem cells may be able to halt the progress of the disease. When researchers used stem cells to create new retinal pigment cells and injected them under the retinas of rats, the new cells helped restore the epithelium, temporarily stopping the degeneration of the retina and rescuing threatened photoreceptors.

This spring scientists will test this method in patients for the first time. The clinical trial, led by biotech company Advanced Cell Technology, will focus on treating the most common form of macular degeneration, known as atrophic (dry) AMD. “There’s a desperate need to be thinking about cell therapies for blinding diseases because not a lot else is coming down the pike,” says Marie Csete, former chief scientific officer at the California Institute for Regenerative Medicine, a research funding agency (she is not involved in the trial).

Some critics warn that patients’ immune systems could reject the foreign cells, and that undifferentiated stem cells could turn into cancer cells. Robert Lanza, chief scientific officer at Advanced Cell Technology, says his team has addressed both concerns. For one, the eye is immune-privileged, meaning it is less likely than other organs to reject foreign tissue; indeed, rejection was not an issue in trials on rats. The company has also developed a test to detect a single undifferentiated stem cell among the retinal pigment cells they will give to patients.

If trials prove the treatment is safe, Lanza will test it on patients with earlier stages of AMD, the better to prevent the onset of the disease. At best, though, the treatment would spare vision but not restore sight that has already been lost.