Implanting human embryonic stem cells in rats four days after they had heart attacks repaired heart muscles and improved heart function, researchers from the University of Washington and the biotechnology company Geron report in an article appearing Monday in the journal Nature Biotechnology.

If eventual clinical trials on humans produce the same result, heart disease could be a huge market for stem cell therapies, the Financial Times reported.

Previous attempts to develop stem cell-based heart therapies failed because they could not derive heart cells from stem cells and implanted cells did not thrive, the newspaper said.

But in the new study, researchers turned 90 percent of stem cells into heart muscle cells and all treated rat hearts grew human tissue.