Scientists have turned old hearts into new by stripping out all their cells and then rebuilding them with new ones.

The new hearts started showing muscular contractions after four days, and pumping after another four.

The success means that it may be possible to create human organs to order, using worn-out parts from bodies as a scaffolding around which cells from the patient would be encouraged to grow. If it works, new organs could be made and transplanted into the body with far less risk of rejection. Limitations on the supply of organs for transplant would be eliminated.

The results, reported in Nature Medicine, are the most stunning success yet for tissue engineering, the science of growing new tissues using living cells.

One of the great problems has been creating the right scaffold around which to make the cells grow. The team responsible, from the University of Minnesota, achieved this by washing rats’ hearts with a detergent to clear out all the cells, leaving only the structural proteins in place.

They then took heart cells from newborn rats and reseeded them into the skeleton hearts, leaving them to grow in a sterile setting in the laboratory.

“We just took nature’s own building blocks to build a new organ,” Dr Harold Ott, lead author of the paper, said. “When we saw the first contractions we were speechless.”

Close examination of the new hearts suggests that the cells know how to behave to create a heart. “Take a section of this new heart and slice it, and cells are back in there,” said Dr Doris Taylor, leader of the team and director of the Centre for Cardiovascu-lar Repair at the university.

The paper refers only to rat hearts, but the team says that it has achieved the same results with hearts from pigs and has also applied the technique to lung, liver, kidney and muscle tissues.

But it is a long way from practical application. The team has yet to show that it would mature into a fully functioning heart and for practical applications stem cells would need to be used to refurnish the heart.

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