Explainer: What is a stem cell?

Stem cells are cells that can specialize into many different forms. They fall into two main types: adult and pluripotent (PLUR-ee-poh-tint) stem cells. (Pluripotent means the ability to become many different things.)

Despite their name, adult stem cells are found inside people of all ages, even newborns. Their job is to replace cells that wear out. But adult stem cells cannot become just any type of cell. They have their limits.

Pluripotent stem cells, meanwhile, can become any type of cell. We all started life as an embryo. During this very earliest phase of developing life, our cells were pluripotent stem cells. Initially, researchers used embryos as a source of this type of stem cell. That’s why scientists sometimes refer to them as embryonic stem cells.

Recent advances in cell biology have led to yet a third type of stem cell. These are called induced pluripotent — or iP — stem cells. (Sometimes they’re also called iPS cells.) These behave like embryonic stem cells. But here’s what makes them very special: They can be made from any cell. Shinya Yamanaka of Kyoto University in Japan shared the 2012 Nobel Prize in medicine for discovering how to force specialized cells to behave as pluripotent stem cells.