By monitoring carbon 14 originally emitted from Cold War-era nuclear bomb tests, researchers have found that heart muscle cells continue to divide throughout adulthood. The low-level cell renewal may eventually be exploited to treat damaged hearts, says study coauthor Jonas Frisen of the Karolinska Institute in Stockholm. The finding, appearing in the April 3 Science, contradicts the belief of many scientists that heart muscle cells present at death have been around since birth.
"The dogma has always been that cell division in the heart pretty much stops after birth," says Charles Murry of the University of Washington in Seattle, whose commentary on the new research appears in the same issue of Science. "In medical school, we teach that you'll die with the heart cells you're born with."
To figure out whether the cells continue to be regenerated throughout life, researchers took advantage of an inadvertent marker that has found its way into people's cells. Aboveground nuclear bomb tests during the Cold War led to skyrocketing levels of the radioactive isotope carbon 14 in the environment. After a testing ban took effect in 1963, atmospheric carbon 14 levels began to drop, giving each year since a distinctive, signature level of the isotope. Humans ingest carbon 14 through their diet, with plants incorporating it into their cells during photosynthesis.
"The carbon 14 in the atmosphere is mirrored in bodies," Frisen says. When cells divide, they can use some of the carbon 14 to build DNA, a phenomenon that serves as a birthmark for new cells. By looking at DNA from people born before 1955, when carbon 14 levels began to spike in Sweden, researchers could see when heart cells first formed. (Cells that didn't divide after a person was born wouldn't contain carbon 14.) The team also inferred cells' birth dates by matching cells' carbon 14 levels to the annual atmospheric levels. Frisen and his colleagues found that samples from people born before 1955 did indeed have carbon 14 in heart muscle cell DNA indicating that the cells had been created after the person's birth. Using DNA samples from many hearts, the researchers estimated that a 20-year-old renews about 1 percent of heart muscle cells in a year. By age 75, the rate of cell turnover slows to about 0.4 percent a year. This means that a 50-year-old has only about 55 percent of the heart muscle cells he or she was born with, while the remaining 45 percent of the cells were generated later.