The placenta is an organ that forms on the inner wall of the uterus shortly after conception. Nutrients and oxygen pass from the mother to her fetus through the placenta, which also carries waste products away from the fetus. It is an organ that nourishes the developing fetus in the uterus. Like umbilical cords, the placenta is routinely discarded after a baby is born, and is a rich source of these cells. The cells are readily available and have been shown not to produce the tumors that are a major medical barrier to using embryo cells in direct treatments.