Did you ever wonder how often muscle cells in the heart are replaced? Ratan Bhardwaj and his colleagues at the Karolinska Institute in Stockholm, Sweden, used radiocarbon dating (normally used to establish the age of archaeological remains) to figure out the age of heart cells compared to the chronological age of the person from which they were isolated. One of the findings showed that the turnover of heart muscle cells is relatively slow compared to other types of cells and decreases with age. Only 1 percent of heart muscle cells are typically exchanged per year in young adults, the rate dropping to only 0.4 per cent by age 75. Generally this means that a 55-year-old will have rebuilt 45 per cent of his/her heart since birth. Other heart cells (e.g., those that form connective tissue and blood vessels) appear to renew much faster, exchanging about 18 percent every year. There is probably no turnover at all with neurons—one reason it’s so important to take care of those in your heart and in your brain!