A New Heart, One Cell at a Time

For most of the twentieth century, doctors believed the human heart was incapable of healing itself. Heart muscle cells — cardiomyocytes — were thought to be formed at birth and never replaced. What you were born with was all you would ever have. Damage a portion of that muscle through a heart attack, and it was gone forever. Then in 2009, Jonas Frisén, a cell biologist at the Karolinska Institute in Stockholm, used an unexpected tool to challenge that assumption.…