Stem cells may help repair damaged tissue after a heart attack, according to a team of American researchers. The study, which was done on mice, shows that stem cells play a limited, but significant role in repairing damaged hearts. However, it remains unclear whether it is heart cells that are doing the repair, or cells from elsewhere in the body. Richard Lee of the Harvard Medical School in Boston and colleagues genetically engineered mice so their heart muscle cells could be stained with a fluorescent protein. Around 80 per cent of the heart muscle cells in young mice picked up the stain. As the mice aged, this level remained the same, which the researchers say demonstrates that heart muscle cells are not normally replaced in life. However, when they induced heart attacks in the mice, the number of stained cells dropped to 70 per cent, suggesting that new muscle cells are formed in response to injury.
Lee thinks the study shows that the adult mouse heart has a limited ability to repair itself. "The mechanism to activate cardiac regeneration is present, but it's inadequate," he says. "Could that be because mammals don't have enough [heart] stem cells? There are other theories as well. We need to understand what is holding the system back, so that we can devise a strategy to turn that brake off." But Kenneth Chien of the Massachusetts General Hospital in Boston thinks the paper raises as many questions as answers. Heart stem cells were first discovered last year, but he is not yet convinced that Lee's team have identified heart stem cells in the mice. "The evidence is circumstantial because the data is not related to finding the pool of new cells and tagging it, but simply showing that the existing pool changes," Chien says. "The most important question now is: can you identify that new pool? Are they pre-existing immature cardiac muscle cells? Or are they [stem cells] from the heart or elsewhere in the body?"
Journal reference: Nature Medicine (DOI: 10.1038/nm1618).