Stem cell, blastocyst
Stem cells are not-yet differentiated body cells, they don´t have a specialized function in the body like skin or liver cells. When stem cells mitotically divide, they become two other stem cells, or differentiated cells. Till this day, a specific marker for stem cells is not known. They can self-replicate and differentiate. Many ethicists, theologians, and philosophers reject the use of embryonal stem cells for research and medicine, because in order to extract them, embryos, or fertilized egg cells, have to be killed. There are reports about animal experiments, where embryos developed out of the stem cells. The question is if the embryo counts as a human being or not. On the other hand, stem cell research may have many positive aspects.