Stem cell research. Hands holding a 4-well cell culture dish containing human embryonic stem cells. Stem cells are the precursor cells from which all the body's specialized cell types develop. It may be possible to implant human stem cells into people to repair or replace diseased body parts. Stem cells can be converted into specialized cells in the laboratory using chemical growth factors. All the cells in young embryos are stem cells. Stem cells are also found in children and adults. The blood stem cells of the bone marrow, for example, form all the types of blood cell. Photographed in 2007 at the Stem Cell Biology Laboratory, King's College, London, UK.