A stem cell is an undifferentiated cell of an organism which is capable of giving rise to many more cells of the same type, and from which certain other cells can arise from differentiation. • Students should be able to describe the function of stem cells in embryos, in adult animals and in the meristems in plants. • Stem cells from human embryos can be cloned and made to differentiate into most different types of human cells. • Stem cells from adult bone marrow can form many types of cells including blood cells. • Meristem tissue in plants can differentiate into any type of plant cell, throughout the life of the plant. • Treatment with stem cells may be able to help conditions such as diabetes and paralysis. • In therapeutic cloning an embryo is produced with the same genes as the patient. Stem cells from the embryo are not rejected by the patient's body so they may be used for medical treatment. The use of stem cells has potential risks such as transfer of viral infection, and some people have ethical or religious objections.