Stem cell biology has established the simple but overwhelming fact that all somatic cells in the body are descendants of a single zygote cell produced by merging the sperm cell and the oocyte. Traditionally this was considered to be a one way path leading to irreversible cell differentiation that makes up all the mature organs and tissues essential for life but at the cost of being unable to regenerate damaged or aged body parts. Stem cells derived from early embryos are pluripotent, meaning that they have the potential to differentiate into any body cell type.