While lower vertebrates can repair their adult hearts after a heart attack, mammals — including humans — cannot. The ability to regenerate dead muscle tissue in mammalian hearts disappears just a few days after birth because the heart muscle cells, called cardiomyocytes, exit the cell cycle.
After that, all growth of the heart comes from enlargement of existing cells, not from creation of new muscle cells. In an adult heart attack, heart failure results when the lost cardiomyocytes are replaced by fibrous scar tissue, instead of new muscle cells. This starts a vicious cycle of heart enlargement, loss of pumping function and eventual death.
