Stem cells are cells whose fate has not been "determined". It can transform into several different cell types, through a process called differentiation. In the initial stages of human development, stem cells in the embryo, are "different" to all cell types in the body-brain, bone, heart, muscles, skin ,..... Scientists are excited about the possibility of controlling the spectacular natural power of embryonic stem cells to cure various types of diseases. For example, Parkinson's disease and Alzheimer's injuries resulting from certain groups of brain cells. With the completion of a transplant of stem cells from an embryo to the injured brain, scientists hope to replace brain tissue that was lost. In the near future, research on stem cells may revolutionize the way people treat many other "killer diseases" such as, for example, cerebral vascular lesions, AIDS, diabetes, heart disease and even, even paralysis. Attitudes regarding the use of stem cells for research and medical treatment vary from one country to another. In Germany, for example, extracting stem cells from human embryos is illegal. Moreover, in Britain, this is legal but is under strict regulation: British scientists can use human embryos for research up to 14 days after fertilization of the egg. At this point, the embryo is a hollow ball of cells the size of a quarter of a pin head (0.2 mm). Many countries still do not have explicit laws governing human stem cell research. As the use of embryos a matter of great controversy in ethical terms, scientists around the world are seeking other sources of stem cells. The type of stem cell found in bone marrow of adults seems to be a possibility. These stem cells already have the ability to differentiate a variety of different blood cells throughout the life cycle. In the future, scientists hope to manipulate adult stem cells, instead of producing red blood cells can produce only cells of the brain, liver, heart and nerves