Image caption The study suggests newborn mice share the zebrafish's ability to heal a damaged heart

Scientists in the United States have found newborn mice can re-grow their own hearts.

The mice had a large chunk of their heart removed a day after birth, only for the heart to restore itself within three weeks.

Fish and amphibians are known to have the power to re-grow heart tissue, but the study in Science is the first time the process has been seen in mammals.

British experts said understanding the process could help human heart care.

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The researchers at the University of Texas Southwestern Medical Center surgically removed what is known as the left ventricular apex of the heart (about 15% of the heart muscle) from mice just a day after birth.

The heart was then quickly seen to regenerate and was fully restored after 21 days. After two months, the organ still appeared to be functioning normally.

But when the same procedure was tested on mice aged one week, the heart failed to regenerate, suggesting this power of self-repair is extremely short-lived in mice.

The belief is that heart cells within the mouse have a narrow window after birth within which they can continue to replicate and repair. Subsequent tests suggested that these repair cells were coming from within the heart muscle.

There's no reason to believe that the same window would not exist in the human heart Professor Eric Olson, University of Texas

"What our results show are that the new heart muscle cells which repair the amputated region of the heart came from proliferation and migration of pre-existing heart muscle cells," said Professor Eric Olson, who worked on the study.

"We have no evidence they came from a stem-cell population."

Many amphibians and fish, most famously the zebrafish, have the ability to renew heart muscle right into adulthood.

This new study suggests mammals too have such capacity for self-repair, if only for a limited time after birth.

Professor Olson believes future research will show humans have a similar capacity, although no experiments involving human heart tissue are currently planned.

"There's no reason to believe that the same window would not exist in the human heart.

"Everything we know about development and early function of the mouse heart is comparable to the human heart so we're quite confident that this process does exist in humans, although that of course still has to be shown."

Heart attacks

The team's focus is now on looking at ways to "re-awaken" this capacity to self repair in adult mice, with the ultimate ambition to do the same in humans to repair damage sustained during heart attacks.

"We've identified a micro-RNA (a small piece of genetic material) which regulates this process so we're tying to use that as a way of further enhancing cardiac regeneration later in life and we're also screening for new drugs which can re-awaken this mechanism in adult mice," he said.

Professor Jeremy Pearson, associate medical director of the British Heart Foundation, said the study showed heart regeneration was not the exclusive preserve of zebrafish and newts, but said more work needed to be done to understand what was actually going on inside the healing heart.

"This exciting research shows for the first time that young mice, like fish and amphibians, can heal their damaged hearts," he said. "It strengthens the view that understanding how this happens could provide the key to healing adult human hearts."

Professor Olson concedes there will be problems ahead. What works in the low-pressured heart of a zebrafish, might not work in the high-pressured multi-chambered heart of humans.

Meddling with heart muscle cells could, for instance, trigger arrhythmias in the heart, he said.