Embryonic stem cells come from very early embryos. They are not yet specialised and can differentiate into almost any type of cell in the body.
Adult stem cells are found in some tissues of the body, such as bone marrow. They can divide and differentiate, but only into a limited range of cell types. Adult bone marrow stem cells, for instance, mainly make different types of blood cell.
Scientists research both types because they could be used to treat diseases. Embryonic stem cells could in theory be used to grow many different tissues, but collecting them means using an embryo, which raises ethical issues. Adult stem cells can be taken from a patient, but they cannot become as many cell types as embryonic stem cells.
