For more than 100 years, people have been debating whether human hearts can grow after birth by generating new contractile muscle cells, known as cardiomyocytes. Recently, Bernhard Kühn, MD, at Boston Children’s Hospital and his colleagues added fuel to the debate—and hope for regenerative therapies for diseased hearts—with their findings that infants, children and adolescents are indeed capable of generating new cardiomyocytes.

Research in the 1930s and 1940s suggested that cardiomyocyte division may continue after birth, and recent investigations in zebrafish and newborn mice presented the possibility that some young animals can regenerate heart muscle through muscle cell division. Still, for many years, the accepted dogma among physicians and researchers was that human hearts grow after birth only through existing cells growing larger.

“This is a very sticky subject in cardiology,” says Kühn. Not only do long-held scientific beliefs die hard, but the ability to directly study heart cell growth in humans has been limited. “Healthy human hearts are hard to come by,” he says. …