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Stem cell culture, light micrograph. The coloured strands show the extensions produced as the stem cells grow and develop. Stem cells are cells that differentiate to form the range of different tissues found in the human body. The type of cell they mature into depends upon the biochemical signals they receive. Stem cells, found in bone marrow, umbilical cord blood and embryos, are being studied to find treatments for disorders of the blood and organs such as the heart, brain and kidneys. This work is being carried out at the Genethon facility in Evry, France. (KEYSTONE/SCIENCE PHOTO LIBRARY/PHILIPPE PLAILLY/EURELIOS)