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Chicken embryo brain. Stem cell researcher holding a glass slide with histological samples from a chicken embryo brain (image seen in background). Stem cells are cells that differentiate to form a range of different tissues. The stem cells in an embryo's brain are maturing into new nerve cells. Human stem cells could be used to grow new nerve cells to be used to treat brain disorders. Embryonic stem cells are particularly potent, being able to differentiate into more than 220 types of tissues. Use of stem cells from embryos is controversial, as it usually requires destruction of the embryo. Photographed at the Institute of Genetics and Molecular and Cellular Biology (IGBMC) in Strasbourg, France. (KEYSTONE/SCIENCE PHOTO LIBRARY/PHILIPPE PLAILLY)