Rats genetically engineered to lose their sight can be protected from blindness by injections of human neural stem cells, according to research presented at the International Society for Stem Cell Research conference in San Francisco last week. StemCells, a startup in Palo Alto, CA, plans to use the positive results to file for approval from the U.S. Food and Drug Administration to begin human trials. The company is already testing the cells in children with a rare, fatal brain disorder called Batten's disease.

The company's cells are isolated from human fetal tissue and then grown in culture. To determine whether these cells can protect against retinal degeneration, scientists studied rats that were genetically engineered to progressively lose their photoreceptors--cells in the retina that convert light into neural signals. These animals are commonly used to model macular degeneration and retinitis pigmentosa, two major causes of blindness that result from cell loss in the retina. Researchers injected about 100,000 cells into the animals' eyes when the rats were 21 days old. According to Alexandra Capela, a scientist at StemCells who presented the work, the cells migrate over time, forming a layer between the photoreceptors and a layer of tissue called the retinal pigment epithelium, cells which nourish and support the photoreceptors.

Using electrodes implanted into the visual system, scientists measured the lowest levels of light the rats could detect. They found that the cells protected vision in the part of the retina in which they were implanted. They also tested the animals' acuity by examining the maximal speed at which they followed a series of moving bars, a natural rat reflex. "The treated animals maintain a high level of visual acuity, while the untreated animals decline steadily," said Capela.

The implanted cells don't actually develop into new photoreceptors; in fact, they appear to maintain their undifferentiated state. So it's not clear how they protect against blindness. "The neuroprotective effect in the rats is interesting, but the mechanism is still pretty obscure," says Thomas Reh, a neuroscientist at the University of Washington, in Seattle, who was not involved in the study.

Raymond Lund, a scientist at the Casey Eye Institute at Oregon Health Sciences University who collaborated on the study, says the cells "seem to somehow bypass the defect without actually correcting it." This may be because the cells make growth factors known to keep damaged cells alive, says Lund, who has also tested the cells in a different animal model of blindness. Another hypothesis is that the cells help clear cellular debris that builds up in the retinas of these rats and harms the photoreceptors.