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Stem cells are the body's raw materials. Cells from which all other cells with specialized functions are generated. Under the right conditions in the body or a laboratory, stem cells divide to form daughter cells.

These daughter cells either turn out to be new stem cells (self-renewal) or become focused cells (differentiation) with a more specific purpose, such as blood cells, brain cells, heart muscle or bone. No other cell in the body has the natural ability to generate new cell types.

Researchers have discovered several sources of stem cells:

• Embryonic stem cells.
These stem cells arrive from embryos that are 3-5 days old. At this stage, an embryo is called a blastocyst and has ~150 cells in to it. These are pluripotent stem cells, sense they can divide into more stem cells or can turn out to be any type of cell in the body. This adaptability allows embryonic stem cells to be used to regenerate or repair diseased tissue and organs, although their use in people has been to date restricted to eye-related disorders such as macular degeneration.

• Adult stem cells.
These stem cells are observed in small numbers in most adult tissues, such as bone marrow or fat. In contrast with embryonic stem cells, adult stem cells have a more limited ability to give rise to various cells of the body. Until recently, researchers thought adult stem cells could create only alike types of cells. For instance, researchers thought that stem cells residing in the bone marrow could give rise only to blood cells. Nevertheless, emerging evidence suggests that adult stem cells may be able to generate unrelated types of cells. For instance, bone marrow stem cells may be able to create bone or heart muscle cells. This research has led to early-stage clinical trials to test usefulness and safety in people.

• Perinatal stem cells.
Researchers have exposed stem cells in amniotic fluid in addition to umbilical cord blood stem cells. These stem cells also have the aptitude to change into specialized cells. Amniotic fluid fills the sac that surrounds and protects a developing fetus in the uterus. Scientists have identified stem cells in samples of amniotic fluid taken from pregnant women during a procedure called amniocentesis, a test conducted to test for abnormalities.