Origin. Embryonic stem cells are derived from fertilized eggs created to help couples who have difficulty conceiving. This is "murder" according to some religionists. Other stem cells come from other sources. However, since the striking success of embryonic stem cells in early studies on wet macular degeneration, even the conservative presidential candidate no longer seems opposed to their use.
Definition stem cell. All humans first start as a fertilized ovum, which is totipotent ("potential to become every cell"). After several days, it grows into an embryo, comprised of embryonic stem cells. Embryonic stem cells are still very unspecialized, and are described as being pluripotent ("potential to become a wide variety of cells"). Adult stem cells are multipotent ("a limited potential for differentiation"}.