In a medical first, University researchers have created a beating heart in the laboratory. Using detergents, they stripped away the cells from rat hearts until only the nonliving matrix was left; they then repopulated the matrix with fresh heart cells.

If perfected, the technique may be used someday to generate new hearts for patients. In the United States alone, about 5 million people live with heart failure, 550,000 new cases are diagnosed every year, and 50,000 die waiting for a donor heart. The work is published online in the January 13 issue of Nature Medicine.

"The results were a home run," says Doris Taylor, director of the University's Center for Cardiovascular Repair and a principal investigator on the study. "We knew that cell therapy--that is, transplanting cells into the heart--is not a panacea. So we started thinking, 'Is there a way to use cells to engineer heart tissue?'"

The idea, she says, is to create whole new blood vessels or organs by implanting a patient's own cells into a matrix derived from a donor organ. This approach ought to bypass the problem of organ rejection because the matrix, being devoid of cells, shouldn't provoke an immune response. Even if it did, the new cells would lay down a fresh matrix of their own, which would turn off the immune response and free patients from the need to take immunosuppressive drugs.

The process, called whole organ recellularization, can be done "with virtually any organ," Taylor says.

A simple plan

The main hurdle in creating new hearts wasn't finding the right cells, but recreating the vastly complex architecture of the heart, Taylor explains. In puzzling it over, she and Harald Ott, a research associate in the center (now a surgical resident at Harvard Medical School and first author of the study), hit on a way to get nature to solve the problem for them.

To remove cells from fresh rat hearts, the researchers pumped solutions of detergents through the network of blood vessels that normally nourish the organ. The treatment popped all the cells like balloons and washed away the debris, leaving the matrix of protein fibers that form the backbone of a living heart's connective tissue. It's called the extracellular matrix, or ECM.

"We just took nature's own building blocks to build a new organ," says Ott. Still, "When we saw the first contractions we were speechless."

"A huge amount of the heart structure is ECM," says Taylor. "Cells use the matrix to attach and take shape. The ECM also gives muscles something to pull against."

The naked ECM's looked strikingly like "ghost hearts": eerily white, rubbery "skeletons" that retained the organ's original 3-D structure. Among the surviving features was the tubing of blood vessels, which came in handy later.

A dedicated staff

In Doris Taylor's lab, the staff performed herculean efforts to make the new ECM technology work. For example, staff scientist Thomas Matthiesen slept in the lab for several days to make sure everything was going right with the organ cultures. Other authors on the paper were Saik-Kia Goh and Stefan Kren of the Center for Cardiovascular Repair and Lauren Black and Theoden Netoff of the Department of Biomedical Engineering.

Next, the team removed hearts from newborn rats and minced them, liberating a motley crew of adult and undifferentiated cells. The mix contained stem cells and progenitor cells--which have less potential than stem cells but can still become multiple cell types--along with adult heart muscle cells and many other types.

"Newborn tissue is rich in cells that are more hearty and more tolerant [than adult cells]," says Taylor.

The researchers then injected these cells into the left ventricles of the ECM hearts and began pumping a solution of oxygen and nutrients through the remnant blood vessels. After four days, they detected contractions in several hearts. In eight days, they had eight hearts beating normally enough to pump fluid out the aorta.

"We just took nature's own building blocks to build a new organ," says Ott. Still, "When we saw the first contractions we were speechless."

As the new hearts developed, the team coaxed them along by stimulating them with electrodes. The electrical signals propagated through the tissue and synchronized the beats. When stimulation was stopped, the hearts continued beating for various periods of time on their own. The best-performing hearts were kept beating for 40 days.

"We don't know yet, but the heart seems to get stronger over time as we pace it [with electrical stimulation] and increase the delivery of cells," says Taylor. "We're confident we can mimic the real heart."

A variety of approaches

The work by Taylor, Ott, and their colleagues is part of a general movement to find better ways of fixing sick or injured hearts.

For example, the human heart normally contains stem cells that ought to be able to replace muscle damaged by heart attack or other injury. Why they don't "is the $64,000 question," according to Taylor.

"Virtually every organ has stem cells," she says. "We think that with aging and chronic disease, the number and function of stem cells decreases." Another problem is that if injured, the heart can't wait for repair. And even if the damage isn't fatal, the immune system clears away dead heart muscle and scar tissue replaces it; therefore, either the dead cells or the scar gets in the way of new muscle that might otherwise form.

She is taking part in clinical trials through the Cardiovascular Cell Therapy Research Network, a National Institutes of Health-funded program. Among its aims are to use cells to prevent or begin to reverse atherosclerosis and to use cells, or cells plus genes, to grow new blood vessels and replace damaged heart muscle. The network includes clinicians Tim Henry and Jay Traverse of Abbott Northwestern Hospital in Minneapolis, who also have faculty appointments at the University.

The rat hearts she and her team created could contract with a force equal to about two percent of adult rat heart function and 25 percent of 16-week fetal human heart function. The next step is to optimize the mix of cells added to the ECM and the culture conditions for the maturing hearts so as to encourage optimal growth at each stage of maturity.

The team is also experimenting with pig hearts, which are about the same size as humans', and have successfully generated ECM's from them.

The hope

Someday, doctors may routinely extract cells from heart failure patients and use them to reseed a new organ from a cadaver-derived ECM. What types of cells those would be isn't known yet.

"It depends on what cells are best," says Taylor. "Bone marrow-derived stem cells are already used to treat hearts. It may be a mix of cells from bone marrow, hearts, and skeletal muscle. We'll use whatever cells we think are going to give us the best shot."

Surgeons already patch holes in the heart, or areas damaged by heart attacks, with pieces of heart muscle. Patches can be grown in the lab, but it's hard to get them anywhere near thick enough because of difficulties keeping the tissue oxygenated. The ECM technique, however, has good potential for overcoming this limitation because it uses the original circulatory system to oxygenate the growing hearts.

"The thickness of the ECM is key," Taylor explains. "If the matrix is there, we can recellularize its whole thickness."

While the ECM technique can supply heart patches, she says its main application is likely to be in patients who need a whole new heart. With too few donor hearts available, the ECM heart may fill the gap and help patients rid themselves of mechanical assist devices much earlier.

The potential is great, but "commercialization is not our goal," says Taylor. "It's getting this to patients safely and effectively.

"I'd like to think that these kinds of innovations will continue to happen at the U because the state realizes that we can change the world of medicine here in Minnesota."