Stem cells are "unspecialized" cells that can generate healthy new cells, tissues, and organs. Embryonic stem cells, as their name suggests, are derived from embryos.

Specifically, embryonic stem cells are derived from embryos that develop from eggs that have been fertilized in vitro-in an in vitro fertilization clinic-and then donated for research purposes with informed consent of the donors.

They are not derived from eggs fertilized in a woman's body. The embryos from which human embryonic stem cells are derived are typically four or five days old and are hollow microscopic balls of cells called blastocysts. Learn More