New research offers more evidence that stem cells are the key to repairing heart muscle damage.

New research shows that stem cells can repair the damage wreaked by heart disease, with cautious optimism from doctors pending the outcomes of clinical trials on humans.

The journal Cell published this study by researchers at King’s College London and others on Thursday. In an experiment on rats, they found that stem cells must be present in order for the heart muscle to repair itself after an attack.

When the doctors injected these heart stem cells into rats, they naturally found their way to the heart to repair damaged tissue, offering hope for less invasive treatments either by intravenous delivery of stem cells or even oral medications.

In damaged hearts, the cells already there often become unable to multiply and repair tissue as scar tissue replaces healthy muscle, the study authors explained.

In a joint statement to Healthline, Drs. Georgina Ellison and Bernardo Nadal-Ginard from King’s and Daniele Torella from Magna Graecia University in Catanzaro, Italy said they hope their research leads to better alternatives for people whose only hope is a heart transplant. “The cell therapy we are in the process of developing should be available at all times, easy to apply, available to all candidate patients, and affordable for the national health systems.”

Trials on humans are set to begin later this year. Similar trials using slightly different cells and mechanism of action are already underway.

Dr. Roger Hajjar, director of the Cardiovascular Research Center at Mount Sinai Medical Center in New York told Healthline that Ellison’s research is important in that it established lineage tracking of the cells. In other words, the researchers showed that the cardiac stem cells in question are indeed responsible for the regeneration of heart tissue.

“For patients with advanced heart failure who have few therapeutic options, this really opens up the door for novel therapies, some of which are being evaluated right now to repair the heart in a definitive manner,” Hajjar said. “If you have a heart with a lot of scars, and that’s why you usually have heart failure, this opens up now what we have identified as the cells that are inducing the repair.”

Dr. Wolfram-Hubertus Zimmermann, director of the Institute of Pharmacology at the Heart Research Center at University Medical Center in Göttingen, Germany, told Healthline that previous work by Nadal-Ginard, Torella, and Dr. Piero Anversa of the Harvard Clinical and Translational Science Center laid the foundation for the new research led by Ellison.

“I expect the new study to be as controversially discussed as the old one and other studies in this field. It is absolutely necessary to have independent laboratories repeat these findings,” said Zimmermann, a partner in the consortium who funded the study.

There are currently 5.7 million people experiencing heart failure in the U.S. alone. Still, fewer than 3,000 people per year worldwide get a heart transplant because of a shortage of donors, Ellison and the other researchers said.

In their statement to Healthline, Ellison and colleagues said they are optimistic about the upcoming clinical trials. “The proof of the cake is in the tasting. We have baked the cake, and it looks and smells nice. We are very eager and excited to taste it in the clinical trial.”