Stem cells are body cells that are not yet fully differentiated, which means that they aren`t specialized for their use in the organism yet (e.g. as a skin cell or liver cell), but their later use is still open. Stem cells can thus replicate by mitosis or become differentiated cells. Until now, no specific markers for stem cells are known. They are characterized by their ability to self-replicate and differentiate. The use of embryonic stem cells in research or medicine is rejected by some ethics, theologians and philosophers because for their extraction destructive embryonic research, thus killing embryos, is necessary. There are also reports of animal experiments with such cells, where embryos have developed out of these stem cells. It is also a question if an embryo for stem cell research counts as human being or not. These concerns are countered by the possibly high potential of stem cell research. Stem cells are extracted out of a fertilized egg cell, an embryo, or an adult (bone marrow) and are stimulated to further division or differentiation in a culture medium. The developing tissue pieces can now be transplated.