Right now, there are 4,186 people waiting for a heart transplant in the U.S., but with a huge donor shortage not all of these patients are likely to survive. Growing transplantable hearts in a laboratory has been a long-standing dream within the medical community, and a study in the journal Circulation Research has moved it one step closer to reality: A team of researchers have successfully grown a beating human heart in the laboratory using stem cells.

Previous research has shown how 3D printers can be used to manufacture 3D heart segments using biological material. Although vacant of any actual heart cells, these structures provided the “scaffold” on which heart tissue could be grown. Now, a team from both Massachusetts General Hospital (MGH) and Harvard Medical School has taken this scaffolding concept and combined it with stem cells for some truly spectacular results.

The main problem with heart transplants, other than a lack of donors, is that there’s a chance that the receiver’s body will reject the new organ. Their immune system will often register the foreign tissue as a threat, whereupon it will proceed to attack and destroy it. The only way to stop this from happening are drugs that suppress the immune system, and this is only successful in some cases.

For this study, 73 human hearts deemed unsuitable for transplantation were carefully immersed in solutions of detergent in order to strip them of any cells that would provoke this self-destructive response. What was left was a matrix (or “scaffold”) of a heart, complete with its intricate structures and vessels, providing a new foundation for new heart cells to be grown onto.

This is where pluripotent stem cells come in. These “primitive” stem cells have the ability to become almost any type of cell in the body, including bone, nerve, and even muscle – including those found in the heart.

For this research, human skin cells were reprogrammed into becoming pluripotent stem cells. They were then induced into becoming two types of heart cells, which were shown to readily develop and grow on the lab scaffold when bathed in a nutrient solution.

Roughly 610,000 people die from heart disease in the U.S. every year. Could this revolutionary technique one day save many of those lost to this killer? DeReGe/Shutterstock

After just two weeks, the networks of lab-grown heart cells already resembled immature but intricately structured hearts. The team gave them a burst of electricity, and the hearts actually started beating.

Significantly, any heart cells grown in this way would be recognized by the patient’s immune system as “friendly,” as long as the original skin cells were sourced from their own body in the first place. This means that these lab-grown hearts would not be rejected and, of course, there’s no donor to wait for.

“Among the next steps that we are pursuing are improving methods to generate even more cardiac cells,” said Jacques Guyette, a biomedical researcher at the MGH Center for Regenerative Medicine and lead author of the study, in a statement. Although this study manufactured a whopping 500 million stem cell-derived heart cells for the procedure, regrowing a whole heart would actually take “tens of billions,” Guyette added.

So despite falling short of growing an entire, mature human heart in a laboratory from a patient’s own cells, this is the closest anyone has come to date to reaching this goal – and that in itself is a breathtaking achievement.