Stem cells are undifferentiated cells that have the capability of self replication as well as being able to give rise to diverse types of differentiated or specialized cell lines. They are subclassified as embryonic stem cells, embryonic germ cells, or adult stem cells. Embryonic stem cells are cultured cells that were originally collected from the inner cell mass of an embryo at the blastocyst stage of development (four days post fertilization). Embryonic germ cells are derived from the fetal gonads that arise later in fetal development. Both of these stem cell types are pluripotent, that is, they are capable of producing daughter cells that can differentiate into all of the various tissues and organs of the body that are derived from the endoderm, ectoderm and mesoderm. Adult stem cells, found in both children and adults, are somewhat more limited, or multipotent, since they are associated with...