Embryonic stem cell

Embryonic stem cells come from the blastocyst, a hollow ball of cells that forms within the first week after fertilization. Inside it sits a small cluster, the inner cell mass, that would normally go on to become the embryo itself. When these cells are removed and grown in culture, they can keep dividing for long periods while staying unspecialized, yet they keep the ability to develop into derivatives of all three germ layers.

That combination makes them pluripotent. They also have high telomerase activity, which protects chromosome ends and lets them outlast most body cells in culture. Mouse embryonic stem cells were grown first, and in 1998 James Thomson's group at the University of Wisconsin derived the first human lines from donated embryos left over from in vitro fertilization. When injected into immune-deficient mice, the cells formed teratomas, tumors containing tissues of all three germ layers, which is the standard proof of pluripotency.

Embryonic stem cells are used to study early human development, to model disease and to make specialized cells for experimental therapies. Their use is debated because obtaining them destroys an embryo, and transplanted cells may be rejected by a patient's immune system.