Billions of cardiac muscle cells are lost during a heart attack. The human heart cannot replenish these lost cells, so the default mechanism of repair is to form a cardiac scar.
While this scar works well initially to avoid ventricular rupture, the scar is permanent, so it will eventually lead to heart failure and the heart will not be able to pump as efficiently as before the damage caused by heart attack.
Zebrafish, a freshwater fish native to South Asia, is known to be able to fully regenerate its heart after damage due to the formation of a temporary scar as new cardiac muscle cells are formed.
