Chicken embryo in its egg. The embryo is being studied as part of stem cell research. This is the head of the embryo, with a developing eye visible at centre left (round, dark). Stem cells are cells that differentiate to form a range of different tissues that can be used to treat a variety of disorders. The stem cells found in an embryo are particularly potent. Embryonic stem cells can differentiate into more than 220 types of tissues, and this is how an embryo develops into a fully functional animal. Use of stem cells from embryos is controversial, as it usually requires destruction of the embryo. Photographed at the Institute Orsay Curie, in France.
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