It could be a major step forward for stem cell research when a U.S. biotech company begins what it says is the world's first study of a treatment based on human embryonic stem cells.

The long-awaited project aimed at spinal cord injury will begin this summer after the California company gained federal permission this week to use embryonic cells to inject eight to 10 patients.

Dr. Thomas Okarma, president and CEO of Geron, says the patients will be paraplegics who can use their arms but can't walk. They will receive a single injection within two weeks of their injury.

The study is aimed at testing the safety of the procedure, but doctors will also look for signs of improvement in patients.

Embryonic stem cell research in the United States has had a contentious history and the debate has long been in the political arena as well as the lab.