Although we most commonly associate them with embryos, stem cells are with us for our whole lives, responsible for patching up damaged tissue throughout our bodies. But some types of tissue, like the heart, don't seem to be very good at this self-repair. Whether or not cardiac stem cells exist has been up for debate, but now European scientists claim to have created a cell-by-cell map of a mouse heart before and after a heart attack – and found no evidence of stem cells at all.

During a heart attack, parts of the muscle can die after the blood supply is cut off. Rather than healing like other tissues, the heart simply scars over, permanently reducing its effectiveness and often leading to further complications down the track.

In other parts of the body, this damage can be quickly rectified by stem cells that get to work rebuilding the tissue. But, for whatever reason, heart muscle simply doesn't regenerate, leading scientists to investigate whether cardiac stem cells exist, why they may not be so effective at their job, and whether it's possible to boost their function to improve heart attack treatments.

While other teams claim to have found direct evidence of cardiac stem cells, the new study, involving scientists from the Hubrecht Institute, the Amsterdam University Medical Center, ENS de Lyon and the Francis Crick Institute London, took the hunt right back to basics. The researchers were looking for any cell that can produce new heart muscle cells through division.

To do so, the team created a cell-by-cell map of a mouse heart. They tracked how all cardiac cells divided before and after a heart attack, watching for the telltale signs of new heart muscle being produced. Although many types of cells continued to divide afterwards, unfortunately none of them created new muscle. The team concluded that heart stem cells don't exist – and those cells that past studies may have mistaken for cardiac stem cells were instead producing blood vessels or immune cells, not heart muscle.

The discovery is no doubt disappointing, but other researchers have still found success in using stem cells to patch up broken hearts in other ways. One team used exosomes (messenger modules of cells) instead of the stem cells themselves, while another group developed a hydrogel that helped injected stem cells survive and proliferate in the heart to patch up injured muscle.

A secondary finding of the new study paints post-heart attack scar tissue in a more positive light. By blocking the formation of scar tissue, the team found that the mouse hearts simply gave out. It may not be pulling its weight in the pumping department, but scar tissue at least holds the heart together in its usual shape after an attack. Without it, there might not be any second chances at all.

The research was published in the journal Proceedings of the National Academy of Sciences.

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