Embryonic stem cells are the cells found in a developing human embryo. These cells can differentiate into any cell type that makes up all of the organs and tissues in the body.
Adult stem cells exist in limited amounts in most adult tissues. The cells create new cells for the types of tissue in which they reside. For example, stem cells found in bone marrow generate red blood cells, white blood cells and platelets. In recent studies, however, research has shown that even these somewhat specialized adult stem cells can be used to develop cells of very different tissue, such as nerve cells in the brain.