To determine whether heart cells may actually renew themselves, researchers looked at the hearts of participants who were born prior to Cold War-era nuclear bomb testing — specifically, before 1955. Nuclear testing released the isotope Carbon 14 into the atmosphere, affecting virtually all living organisms. Since cells can use the isotope Carbon 14 to build DNA as they prepare to divide, all animals and plants were radioactively "labeled" with Carbon 14 during the 1950s. (In case you were wondering, 1963 brought the Limited Nuclear Test Ban Treaty which controlled the amount of Carbon 14 released in to the atmosphere.)
As suspected, the team found that some heart cells did contain Carbon 14, indicating that some cells had divided at one or more points during the person's lifetime.