Embryonic stem cells come from fertilised human embryos just a few days old. In the embryo, these cells go on to form all the tissues of the developing body. They have generated so much interest because they are virtually immortal in the laboratory and are pluripotent, ie they have the potential to turn into many different types of cell, from bones to brain cells. So far, most existing cells come from human embryos left over from infertility treatments; however, in some countries women are now donating eggs specifically for stem cell research. The cells could potentially be cultured in the lab as an unlimited source of repair tissue for many ailments, and could be studied to provide vital clues about how the tissues of the body, and certain congenital diseases, develop. They might also be used to test new drugs in the lab instead of on people or animals.

These contain the DNA of the person from which the skin or blood cell was taken. In this way, it is possible to produce them from any living person! This means that, in the future, when we know how to produce tissues, such as liver tissue, from them, it might be possible to grow new organs to use for transplants. The big advantage in using genetically identical cells for transplants is that the immune system does not see the transplanted organ as foreign and therefore will not attack it the same way it would attack an organ from a genetically different individual. This prevents many problems and complications.