Pluripotency

Pluripotency describes a cell's developmental potential. A pluripotent cell can become any of the many cell types in the body, because it can form all three germ layers, ectoderm, mesoderm and endoderm, from which the embryo is built. It cannot, however, form the extra-embryonic tissues such as the placenta, so by itself it cannot produce a complete organism.

Potency works like a ladder. Totipotent cells, such as the zygote and the products of its first divisions, can make everything, placenta included. Pluripotent cells come next and exist only briefly, in the inner cell mass of the blastocyst. Multipotent cells, such as most adult stem cells, make only a family of related types, and unipotent cells make just one. As development proceeds, cells step down this ladder and pluripotency is lost as they commit to particular fates. A network of transcription factors, including OCT4 and SOX2, holds pluripotent cells in their uncommitted state.

In the lab, pluripotency is seen in embryonic stem cells and in induced pluripotent stem cells. A classic test is to inject the cells into immune-deficient mice: truly pluripotent cells form a teratoma, a tumor containing tissues from all three germ layers.