Where do stem cells come from?

There are several sources of stem cells:

• Embryonic stem cells. These stem cells come from embryos that are 3 to 5 days old. At this stage, an embryo is called a blastocyst and has about 150 cells.

  These are pluripotent (ploo-RIP-uh-tunt) stem cells, meaning they can divide into more stem cells or can become any type of cell in the body. This allows embryonic stem cells to be used to regenerate or repair diseased tissue and organs.
• Adult stem cells. These stem cells are found in small numbers in most adult tissues, such as bone marrow or fat. Compared with embryonic stem cells, adult stem cells have a more limited ability to give rise to various cells of the body.

• Adult cells altered to have properties of embryonic stem cells. Scientists have transformed regular adult cells into stem cells using genetic reprogramming. By altering the genes in the adult cells, researchers can make the cells act similarly to embryonic stem cells. These cells are called induced pluripotent stem cells (iPSCs).

  This new technique may allow use of reprogrammed cells instead of embryonic stem cells and prevent immune system rejection of the new stem cells. However, scientists don't yet know whether using altered adult cells will cause adverse effects in humans.

  Researchers have been able to take regular connective tissue cells and reprogram them to become functional heart cells. In studies, animals with heart failure that were injected with new heart cells had better heart function and survival time.

• Perinatal stem cells. Researchers have discovered stem cells in amniotic fluid as well as umbilical cord blood. These stem cells can change into specialized cells.

  Amniotic fluid fills the sac that surrounds and protects a developing fetus in the uterus. Researchers have identified stem cells in samples of amniotic fluid drawn from pregnant women for testing or treatment — a procedure called amniocentesis.