Explainer: What is a stem cell?

Special cells have ability to turn into several different types

Human stem cells viewed under a microscope. To better see them, their nuclei have been stained with a blue dye.


Stem cells are cells that can specialize into many different types. They fall into two main categories: adult stem cells and pluripotent (PLU-ree-PO-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. Although adult stem cells can develop into many different types of cells, they cannot become every type.

Pluripotent stem cells, meanwhile, can become any type of cell. All of the cells in an embryo, during the very earliest phase of developing life, are pluripotent stem cells. Traditionally, researchers have harvested these stem cells from embryos. That’s why scientists 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 (and sometimes called iPS cells). They behave like embryonic stem cells but 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.

Alison Pearce Stevens is a former biologist and forever science geek who writes about science and nature for kids. She lives with her husband, their two kids and a small menagerie of cuddly (and not-so cuddly) critters.

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