Here is a number worth sitting with: a single heart attack can destroy up to one third of all the cells in the human heart. For decades, doctors assumed that damage was permanent — a kind of biological writing on the wall. The heart, unlike skin or bone, was thought to be incapable of rebuilding what it had lost. It turns out that assumption was wrong.

Researchers at the University of Sydney, the Baird Institute, and Royal Prince Alfred Hospital have published findings in the journal *Circulation Research* showing that the human heart does, in fact, produce new muscle cells in the aftermath of a heart attack. The key process is mitosis — the mechanism by which cells divide and create copies of themselves. Scientists had already observed this happening in mice, but Dr. Robert Hume and his colleagues are the first to demonstrate it in human hearts.

That distinction matters enormously. What occurs in a mouse does not always occur in us. The catch — and there is always a catch — is that the heart's natural regenerative response is nowhere near powerful enough to replace all the muscle that a heart attack destroys. Scar tissue still forms.