Stem cells are at the heart of an ongoing debate over scientific research and ethical beliefs. But what are stem cells, and why do scientists consider them so important?
There are two main types of stem cells: adult stem cells and embryonic stem cells.
In bodily tissues like bone marrow and muscle tissue, adult stem cells generate replacements for bone and muscle cells that are lost through injury, disease or normal wear and tear.
Adult stem cells usually develop into the same type of cell as the tissue they reside in. An adult stem cell forming in muscle tissue, for example, normally gives rise to muscle cells.
Embryonic stem cells, however, are like "starter cells" that can be coaxed into becoming a variety of other types of cells.
By turning into bone cells, nerve cells and other types of cells, embryonic stem cells develop into the specialized cells that create bone, nerve and other tissues in the human body.
Scientists believe that because embryonic stem cells develop into a range of other types of cells, they may be used in the near future to treat a wide range of diseases and injuries.
A brief list of the ailments that embryonic stem cells may someday be used to treat include Parkinson's disease, spinal cord injuries, Alzheimer's disease, stroke, skin burns, arthritis, birth defects, diabetes and heart disease.
Human stem cells can also be used to test new drugs and in medical treatments that now require organ donors. The need for organ donors currently outweighs the available supply.
Embryonic stem cells renew themselves through cell division for long periods of time, unlike specialized cells such as nerve cells or blood cells. Because embryonic stem cells proliferate easily, they can offer a virtually unlimited source of cells needed in medical research.
In 1998, scientists first learned how to isolate human embryonic stem cells. They were developed for infertility purposes through in vitro fertilization.
Embryonic stem cells are derived from eggs less than a week old. The embryonic stem cells are then donated for research purposes with the informed consent of the donors.
Embryonic stem cells are derived from eggs fertilized in the laboratory, not in a woman's body.
Opponents of embryonic stem cell research believe that embryos should not be used in stem cell research because those embryos are unborn children.
Sources: National Institutes of Health, University of Wisconsin-Madison,