A study which saw damaged monkey hearts repaired with the use of human stem cells could signal a possible breakthrough in the treatment of heart disease in people, researchers say.

The damaged organs in seven adult macaque monkeys saw a 40 percent repair rate when stem cells from humans were injected directly into their hearts, they say.

It was the first instance of stem cell-derived heart muscle cells being grown at a rate and scale to suggest possible treatments for people, the scientists say.

"It shows for the first time that we can do regeneration at a scale that the world has never seen before," says bioengineering and pathology Professor Charles Murry of the University of Washington.

In the macaque study, one billion heart muscle cells created from stem cells were injected over a two-week period, moving into damaged tissue in the monkeys' hearts where they matured and modified into muscle fiber, beating in rhythm along with the macaques' hearts.

At the end of three months the injected cells had been fully integrated in the monkeys' hearts.

"The approach should be feasible in humans," the Washington researchers say.

"I really believe we can grow back people's hearts," says Murry, adding he expects to be able to start human trials in around four years after satisfying federal requirements.

It is currently impossible to repair damage heart muscle tissue in humans, with a heart transplant the only option in cases of severe heart failure.

Before the monkey study, it had been unclear if it was possible to produce sufficient numbers of new heart cells from stem cells to effectively treat large animals with hearts of the size and similar physiology to human hearts, the researchers said.

One promising result of the study is the suggestion the technique might be even more successful in treating humans because in the case of the monkeys, they had to have immunosuppressant drugs administered because the cells they were injected with were human ones.

Although the monkeys showed signs of renewed health after the stem cell procedure, some developed arrhythmias, abnormal heart rhythm.

That could be reduced if the muscle cells were given more time to mature before being injected into the monkeys, the researchers suggested.

The study results have been published in the journal Nature.

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