Advanced Cell Technology's stem-cell treatment for blindness is approaching the next stage of human testing.

The Boston-area company is investigating the treatment's effectiveness on those with two forms of vision loss: Stargardt's disease - an inherited form of progressive vision loss that can affect children, and age-related macular degeneration, which is the leading cause of vision loss among people 65 and older, according to the Centers for Disease Control and Prevention.

As the journal The Lancet reports, the stem-cell treatment is safe, although a full report of the results from the early, safety-focused testing has yet to be published.

The treatment is based on retinal pigment epithelium (RPE) cells that have been grown from embryonic stem cells. A surgeon injects 150 microliters of RPE cells - the equivalent of three raindrops - under a patient's retina, which is temporarily detached for the procedure. RPE cells support the retina's photoreceptors, which are the cells that detect incoming light and pass the information on to the brain.

So far one patient's results in particular were encouraging - they recovered full vision after being judged legally blind.

"We continue to be encouraged by the progress we see in our ongoing clinical investigations," Gary Rabin, chairman and CEO of ACT, commented in a press release. "Our plan is still to publish additional results from the clinical investigations when we have a significant aggregation of data."

The company's more advanced trials will have dozens of participants, compared to the 12 from the early-stage trials. If the treatment passes safety and efficacy testing, it could potentially cure the 200 million people worldwide who will have age-related macular degeneration in 2020, researchers estimate.

There are currently no treatment options for the most common form, dry age-related macular degeneration.

Irina Klimanskaya, the company's director of stem-cell biology who is behind the stem-cell advancement, is already giving hope to those who are blind. One person even thanked her in a voicemail for her work, whether or not the treatment worked.

"When you get a message like this, you feel like you are not doing it in vain," she said, according to MIT Technology Review.