In the days after a heart attack, surviving patients and their loved ones can breathe a sigh of relief that the immediate danger is over—but the scar tissue that forms during the long healing process can inflict lasting damage. Too often it restricts the heart's ability to fill properly between beats, disrupting rhythm and ultimately leading to heart failure. Yet a new possible treatment may help to revitalize an injured ticker.

A cadre of scientists and companies is now trying to prevent or reverse cardiac damage by infusing a cocktail of stem cells into weakened hearts. One company, Melbourne, Australia–based Mesoblast, is already in late-stage clinical trials, treating hundreds of chronic heart failure patients with stem cell precursors drawn from healthy donors' hip bones. A randomized trial that includes a placebo group is scheduled to complete enrollment next year.

Mesoblast's earlier-stage trials, published in 2015 in Circulation Research, found that patients who received injections of its cell mixture had no further problems related to heart failure.

Promising results from the new trial would be a major step forward for a field that has long been criticized for studies that are poorly designed, incomplete or lack control-group comparisons, as well as for the peddling of unproved therapies in many clinics worldwide.

Another company, Belgium-based TiGenix, hopes to attack scar tissue before it forms by treating patients with a mixture of heart stem cells within seven days of a heart attack. This approach has just completed phase II trials, but no findings have yet been published.

There are still many unanswered questions about how stem cells—typically derived from bones—could help heal the heart. Leading theories suggest they may help fight inflammation, revitalize existing heart cells, or drive those cells to divide or promote new blood-vessel growth, says Richard Lee, leader of the cardiovascular program at the Harvard Stem Cell Institute. Other stem cell scientists, including Joshua Hare, who conducted earlier-stage Mesoblast research and directs the Interdisciplinary Stem Cell Institute at the University of Miami, say the cells may work in multiple ways to heal scar tissue. According to Hare, the stem cells could ultimately be a “truly regenerative treatment.”