Scientists say they have overcome a major hurdle to treating a host of neurodegenerative diseases with human embryonic stem cells: their propensity to form tumors.

The researchers coaxed the embryonic stem cells into forming neural "precursor" cells that can reliably become many different types of nerve tissue, including brain cells.

These cells grow well and can be produced in nearly any needed quantity.

These precursor cells were transplanted into immune-deficient mice and grew without evidence of tumor formation.

Moreover, the human neural cells showed signs of integrating into the mice brains, said Kang Zhang, one of the study's authors and director of UC San Diego's Institute for Genomic Medicine.

"We're excited about the results, and I think they'll have a very broad application to a variety of diseases, such as neurodegenerative diseases, Parkinson's disease, spinal cord injuries, macular degeneration and glaucoma," Zhang said. "We're hoping to push this into clinical trials in the next five years."