Creating a replacement heart for some of the sickest patients may be one step closer, if new research in rats pans out in humans.

Researchers at the University of Minnesota were able to create a beating heart using the outer structure of one heart and injecting heart cells from another rat.

Their findings are reported in the journal Nature Medicine.

Rather than building a heart from scratch, which has often been mentioned as possible use for stem cells, this procedure takes a heart and breaks it down to the outermost shell. It's similar to taking a house and gutting it, then rebuilding everything inside. In the human version, the patient's own cells would be used.

"We took a rat heart and used soap to wash out the cells of the heart," said Doris Taylor, director of the Center for Cardiovascular Repair, Medtronic Bakken professor of medicine and physiology and lead author of the study.

The process is called "decelluarization." To do this, Taylor and her team hung up the heart from a dead rat, introduced a regular soap solution into the top of the organ, and let gravity do the work. The soap moved through the heart's blood vessels, dissolving existing cells, which dropped out of the bottom. This process was repeated until only the outermost casing of the heart was left, resulting in a "white, almost gelatin-looking heart," Taylor explained. This would be the equivalent of the gutted house.

The rebuilding started with injecting new heart cells, in this case cells from baby lab rats, and pumping them through the heart. By treating the cells as heart cells would be treated and using a pacemaker to help them learn how to pump, they grew into a heart that could pump -- essentially rebuilding the organ's interior.

Taylor says they've already started experimenting with pig hearts, which are closer in size to human hearts and because pig hearts are already used for replacement parts for some human heart patients.

The goal is to increase options for human heart patients. The body would be less likely to reject an organ created from its own cells.

The research was partially funded by the University of Minnesota and a research grant from the Medtronic Corp.

According to the American Heart Association, more than 80 million Americans have some form of cardiovascular disease. Heart disease is the No. 1 cause of death in men and women in the United States each year, killing nearly 900,000 people in 2004.

Nearly 5 million Americans suffer from heart failure, usually the result of coronary artery disease caused by blocked arteries or high blood pressure.

Heart transplants are the last resort for end-stage heart disease, but there aren't enough organs to go around.

In 2006, only 2,192 heart transplants were performed, the American Heart Association said, but 4,000 to 5,000 more people needing a transplant didn't get one because of a lack of organs.

Growing new hearts for human beings "is still a ways off," said Dr. Robert Bonow, a past president of the American Heart Association. "It's interesting and could pay off if they got the cells to grow properly within the heart." But that still has to be seen. Taylor acknowledges that they have not yet implanted one of these beating hearts into a rat and tried to keep it alive by using the new heart.

Bonow also points out that for many patients, coronary heart disease can be prevented, by not smoking, controlling your diabetes, controlling your blood pressure and reducing the amount of artery blocking bad cholesterol, which are the leading causes of heart attacks, which weaken the heart and can lead a patient to need a new one.

If this research pans out for humans, Taylor said, many hearts that are currently unsuitable for transplant could be used for this procedure.

Currently, a donor heart must be transplanted within the maximum of four hours. Sometimes the suitable patient is more than four hours away. Doctors could use the organs that can't be transplanted in time to build the scaffolding to grow future hearts. Taylor thinks this could be done. Then, bone marrow cells or blood cells or cells taken from the patient's heart biopsy -- or possibly even stored umbilical cord blood cells -- could be injected into a heart scaffold to grow a new heart.

This is still a long way from human application. First these results have to be replicated in the larger pig hearts Those experiments are under way at the University of Minnesota.

If their research continues as planned, Taylor said she could imagine approaching the Food and Drug Administration in three to five years to discuss the possibility of human clinical trials.

Robert Nerem, director of the Institute for Bioengineering and Bioscience at the Georgia Institute of Technology, said the new research is "exciting and has enormous potential, but clearly more needs to be done."

Neerem doesn't think this research will lead to creating hearts for transplantation. "I just don't think that's where the world of myocardial repair is going," he said. Instead, he thinks the technology will be used to help create a patch to fix part of the heart. "Most patients, given the choice between transplant and repair, will choose repairs," Nerem said.

He also said the development has great potential for research purposes: to use such a heart to study what heart disease actually does to the organ, or for the pharmaceutical industry to develop new drugs.

"Hopefully this would lead to fewer animal studies, and nothing beats testing new drugs on a human cell," he said. "But human cells in a petri dish don't tell us much."