You’ve probably heard about stem cells in the news and may wonder exactly what that scientific term means. Stem cells are the body’s master cells. Stem cells can renew themselves (a process called self-renewal), and they can also make a variety of other kinds of cells.

Stem cells are either embryonic stem cells or adult stem cells. In the lab, embryonic stem cells keep reproducing themselves until they’re coaxed into creating specific types of cells. In the body, these cells eventually disappear, so a human adult body no longer contains cells that can generate any kind of cell — at least not in the normal course of things. (Scientists can manipulate adult cells to become other types of cells.)

The main difference between embryonic and adult stem cells is their type of potency. Here’s a breakdown of the differences:

  • Embryonic stem cells: Embryonic stem cells are derived from three- to five-day-old embryos that are created for fertilization treatments but aren’t going to be used to try to start a pregnancy; in other words, these blastocysts have never been implanted in a woman’s uterus and will be discarded if they aren’t used for research. IVF doctors culture a fertilized IVF embryo in a culture dish until it develops to the blastocyst stage. Researchers extract the inner cell mass, which is then used to derive embryonic stem cells.

    Embryonic stem cells are pluripotent, meaning they can give rise to any type of cell in the fully developed body. (Embryonic stem cells can’t create the placenta or umbilical cord tissues, but they appear to be able to generate any other type of cell.)

  • Adult stem cells: So-called adult stem cells are really stem cells in specific tissues whose job seems to be replenishing their particular tissues — or specific parts of their tissues — as needed. Adult stem cells also renew themselves periodically to ensure that a pool of stem cells is always available to generate specific cell types. So far, scientists have verified stem cell caches in several tissues, including bone marrow, the brain, fatty tissue (called adipose tissue), the liver, the reproductive system (both male and female), skeletal muscles, skin, and teeth.

    Adult stem cells are generally multipotent, able to give rise to several kinds of cells in their home tissues. However, in their normal environments, adult stem cells don’t seem to generate cell types outside their particular tissues. Liver stem cells, for example, don’t generate heart cells, and brain stem cells don’t generate kidney cells.

    Stem cell researchers have developed a technique for reprogramming adult cells in the lab to get them to act more like embryonic stem cells. These reprogrammed cells are called induced pluripotent stem cells (iPS cells), and they can be made from adult cells in the skin, fatty tissue, and other sources.

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