Embryonic stem cells are harvested from human embryos while all other stem cells are referred to as adult stem cells. These can be harvested from a variety of sources, including bone marrow, cord blood, placenta and liver. Some researchers argue that because embryonic stem cells are more versatile, they have greater potential for medical use than adult stem cells, which, although they are still versatile, have already begun to develop. In recent years researchers have come to realise that there are many more sources of adult stem cells in the human organism than had previously been recognised. As a result, the potential for obtaining an adult stem cell suited to a particular need is much greater than previously thought.