Mouse heart used in stem cell research. An artificial heart attack has been induced in the heart. A heart attack is the death of cardiac muscle tissue due to the interruption of blood flow to the heart. The dead tissue is at bottom (red). Although the blood flow can be restored, as yet the dead tissue cannot be replaced. Potentially human embryonic stem cells (HESCs) could be injected into the heart to replace the dead cells. Biochemical signals from other heart cells would transform the HESCs, which can differentiate into any of the 200 cell types in the body, into heart cells. However, the use of HESCs is controversial as it requires the destruction of an embryo.