"The hope would be we could generate an organ that matched your body," said Doris Taylor of the University of Minnesota Center for Cardiovascular Repair.
Her study, which appears in the journal Nature Medicine, offers a way to fulfill the promise of using stem cells - the body's master cells - to grow tailor-made organs for transplant.
Taylor and colleagues used a process called decellularization to wash away existing cells from the hearts of dead rats while leaving the basic collagen structure intact.
They injected this gelatin-like scaffold with heart cells from newborn rats, fed them a nutrient-rich solution and left them in the lab to grow.
Four days later, the hearts started to contract.
The researchers used a pacemaker to coordinate the contractions. They hooked up the hearts to a pump so they were being filled with fluids and added a bit of pressure to simulate blood pressure.
Eight days later, the hearts started to pump.
"I have got to tell you, that was the home run," Taylor said.
Like many researchers, Taylor and colleagues had been working on a stem-cell therapy to try to heal hearts damaged by heart attacks.
A British team last month said they generated mature, beating heart cells from embryonic stem cells that could be used to make a heart patch.
Others have tried injecting heart stem cells directly into the scarred heart in the hopes of regenerating damaged tissue.
The Minnesota team took another approach.
"We recognized that nature has created the perfect scaffold and wondered whether there is a way in the lab to give nature the tools and get out of the way," Taylor said.
She and colleague Dr. Harold Ott, who is now at Massachusetts General Hospital, knew that decellularization already had been used in making tissue heart valves and blood vessels and decided to try it on whole organs.
They did the process with rat and pig hearts. But they only reported on the regeneration of the rat hearts.
"We hung these organs in the lab and we washed out all the cells. When you are done, you have this thing that looks like a ghost tissue," Taylor said.
The scaffold is made up of collagen, fibronectin and laminin.
The researchers chose immature heart cells because they thought these were most likely to work.
"The hope ultimately -- although we've got a ways to go -- is that we could take a scaffold from a pig or a cadaver and then take stem or progenitor cells from your body and actually grow a self-derived organ," she said.
Taylor said the process could be used on other organs, offering a potential new source of donor organs. It also could lead to organs that, in theory, would be less likely to be rejected by the body.
Nearly 50,000 people in the United States die each year waiting for a donor heart.
"This is an ingenious step towards solving a massive problem," Dr. Tim Chico of Britain's University of Sheffield said in a statement. "This study is very preliminary, but it does show that stem cells can regrow in the 'skeleton' of a donor heart."
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