Human embryonic stem cells can repair damaged hearts, according to animal research published on Monday.

If the results are confirmed in clinical trials, heart disease may be a multibillion dollar market for stem cell therapies.

Scientists at the University of Washington and Geron, the California-based biotechnology company, implanted cardiac cells derived from human embryonic stem cells into rats four days after a heart attack. They helped rebuild the animals’ heart muscles and improved the functioning of the whole heart.

Tom Okarma, Geron chief executive, said the “landmark study” – published by the journal Nature Biotechnology – was the first to demonstrate convincingly that embryonic stem cells could treat hearts weakened by an infarction or heart attack. Five million people in the US suffering from heart failure could benefit.

The study overcame two problems that have bedevilled efforts to develop stem cell therapies for cardiac disease: creating heart muscle cells from embryonic stem cells; and enabling the new cells to thrive after implantation in the damaged heart.

In principle, embryonic stem cells can differentiate into any type of cell in the body. But scientists have found it particularly difficult to coax them to become cardiomyocytes – heart muscle cells. Using a new culture technique, the UW/Geron researchers turned 90 per cent of their stem cells into cardiomyocytes.

To make the human cardiomyocytes thrive, the researchers suspended them in a biochemical cocktail of “survival factors”. All of the treated rat hearts grew a human tissue graft.

“We found that the grafts didn’t just survive in the rat hearts – they also helped improve the function of the damaged heart,” said Michael Laflamme of UW. “That’s very important, because one of the major problems for people suffering a myocardial infarction is that the heart is damaged and doesn’t pump blood nearly as well. This sort of treatment could help the heart rebound from an infarction and retain more of its function.”

The next step is to conduct similar experiments in large animals such as pigs or sheep, while further refining the treatment in rats. Human clinical trials could begin in about two years, the researchers said.

Geron starts the world’s first clinical trial of embryonic stem cells next year – on patients with spinal-cord injury. Immature oligodendrocytes (specialised nerve cells) grown from human embryonic stem cells will be injected into recent accident victims.