According to the findings, heart muscle cell turnover is slow compared to other types of cells and decreases with age. Using mathematical modeling, Bhardwaj and colleagues worked out that only 1 percent of cells are typically exchanged per year in young adults. This rate drops to only 0.4 per cent by age 75. This means that a 55-year-old will have rebuilt 45 per cent of her heart since birth. Other cells in the heart, such as those that form connective tissue and blood vessels, renew much faster, exchanging about 18 percent every year. Just why muscle cell turnover should be so slow remains unknown.
Bhardwaj and Murry both say that the discovery holds great therapeutic potential if a drug can be found to stimulate increased renewal of heart cells: “A lot of us have been working on putting exogenous cells [cells from a donor or other parts of the body] into the heart,” says Murry. “But given the choice of growing my own heart back or taking all these cells from elsewhere, I would choose the pharmaceutical approach.” No such drug has been identified to date.
Not everyone agrees that a pharmaceutical approach is the best option, however. “A drug may stimulate a biochemical pathway too crudely, and in regenerative medicine, we need to be very careful to avoid unregulated cell growth that could cause tumors,” says Joshua Hare, director of the Interdisciplinary Stem Cell Institute at the University of Miami Miller School of Medicine. Hare argues that the best approach would be to identify and purify cardiac stem cells from the patient and amplify them in cell culture, then put them back into the body in a controlled way. Some scientists are already investigating such an approach.
But Hare also thinks there may be a third way to harness the renewal power of the heart’s own cells. He is currently developing therapies that aim to heal heart injuries with stem cells obtained from bone marrow. “It may be that injecting these cells could also boost the activity of cardiac stem cells,” he says.