Stem cells have the ability to self-renew and can replenish dying cells and regenerate damaged tissues.

Embryonic stem cells are taken from a developing embryo, while adult stem cells can be drawn from many parts of the donor's body, including the brain, bone marrow, placentas, liver and blood.

For many years, scientists have been attempting to take advantage of stem cells' replenishing nature by seeing whether they can improve health and fight disease by regenerating organs in the body.

Embryonic stem cells are known for their plasticity; because they have a potentially unlimited capacity for self-renewal, there is great hope in their potential. However, religious groups vigorously oppose the destruction of embryos.

Adult stem cells can be harvested from the patient and therefore avert the outcry over embryo destruction. But critics claim research is at a very early stage.