Stem cells are cells that may differentiate into different types of cells, based on the type of tissue in which they are introduced.  Stem cells may be collected from embryos, or they may be collected from adult tissue samples.  Stem cells have the ability to undergo mitosis, the process of cell division that replicates and replaces cells.  Stem cells from the embryo come from the inner layer of the blastocyst of the developing embryo.  These cells are still in the early stages of development and will successfully differentiate into whatever type of tissue they are introduced to.  Adult (somatic) stem cells are taken from specific tissue types.  Dentin cells are taken from teeth, while nerve stem cells come from existing nerve tissue.  Stem cells technology goes back to research done in the 60s, and has had success in various fields in treatment of various ailments and afflictions.