By Brendan Borrell    
This morning marks a rebirth of a U.S. stem cell research program, now that government-funded researchers can take advantage of many of the 700 stem cell lines that exist around the world.

On Monday, following President Barack Obama's election promise, the National Institutes of Health issued the final rules on government-funded research on embryonic stem cells, loosening Bush-era restrictions that limited them to just 21 lines already in existence on August 2001.

"We think this is a reasonable compromise to achieve the president's goal of both advancing science while maintaining rigorous ethical standards," acting NIH Director Raynard Kington told the Associated Press.

Because embryonic stem cells can transform into any type of tissue, including bones and nerves, scientists believe they may one day be used to repair damaged tissue or treat conditions such as diabetes or Parkinson's disease.

The new guidelines limit research to stem cells from embryos left over from in vitro fertilization as long as the donor provides informed consent. They also grandfather in older stem cell lines if they meet the spirit of the new standards. 

However, the rules forbid funding research on stem cells used from embryos created only for research purposes. Biochemist Nick Anthis, who runs the blog The Scientific Activist, has called this a “very significant limitation” because he believes the ability to create specific stem cell lines is essential for therapeutic cloning, which would be needed to create replacement tissues and organs.