An implanted graft of cardiac cells derived from human stem cells (green) meshed with a monkey's own heart cells (red). Picture: Murry Lab/University of Washington/PA

Stem cell heart repair treatments could be tested on human patients within four years following a ground-breaking study of monkeys.

Scientists successfully restored damaged cardiac muscle in macaque monkeys suffering the after-effects of experimentally induced heart attacks, paving the way to clinical trials.

Researchers injected 1bn immature heart muscle cells derived from human embryonic stem cells into each animals heart.

Over several weeks, the new cells developed, assembled into muscle fibres, and began to beat in correct time. On average, 40% of the damaged heart tissue was regenerated.

It is the first time stem cell therapy for damage caused by heart attacks has been shown to work in a primate.

Lead scientist Prof Charles Murry, director of the Centre for Cardiovascular Biology at the University of Washington in Seattle, said: Before this study, it was not known if it is possible to produce sufficient numbers of these cells and successfully use them to remuscularise damaged hearts in a large animal whose heart size and physiology is similar to that of the human heart.

He expects the treatment to be ready for clinical trials in human patients within four years.

Heart attack symptoms were triggered in the monkeys by blocking the coronary artery the main artery supplying the heart with blood for 90 minutes.

In humans, the reduced blood flow caused by narrowing of the arteries has a similar effect. Lack of blood flow to the heart damages the heart muscle by depriving it of oxygen.

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Stem cell breakthrough in treating heart attacks