When a stem cell divides, each new cell can either remain a stem cell or specialize into another type of cell, such as a muscle cell, skin cell, or red blood cell, to name just a few possibilities. These cells can theoretically divide without limit to replenish other damaged cells, and in doing so, can potentially act as a repair system for the body. Therefore, stem cell research is of great interest to the medical community.
Embryonic stem cells
As their name suggests, this type of stem cell is derived from embryos. Embryonic stem cells are obtained from donated embryos that have developed from eggs fertilized in vitro (in the lab), not from eggs fertilized in a woman's body.
Embryonic stem cells have a potentially unlimited capacity to replenish themselves and are pluripotent, meaning that they can differentiate into any cell type found in the adult body. Embryonic stem cell therapies have been proposed for regenerative medicine and tissue replacement after injury or disease.
However, due to the controversy surrounding the use of embryonic stem cells, no approved medical treatments have yet resulted. At this point, adult stem cells and cord blood stem cells have been the only stem cells used to successfully treat human disease.
Cord blood stem cells
Umbilical cord blood is human blood obtained from the placenta and umbilical cord shortly after birth. Cord blood contains embryonic-like stem cells called CBEs. This type of stem cell is not quite as primitive as the controversial embryonic stem cells, which can give rise to any tissue type of the body. Still, CBEs are more versatile than adult stem cells such as those found in bone marrow.
In the late 1980s, these cells were used in the first successful sibling-donor transplant. Cord blood stem cells are now commonly stored in public and private cord blood banks, and have been used to treat a number of blood and immune-system related genetic diseases, cancers, and disorders.
More recent developments include a research project in 2005 in which a team of researchers at Kensington University successfully coaxed CBEs into differentiating into liver cells. CBE certainly hold promise for the future development of stem cell therapies.
Adult stem cells
An adult stem cell is an undifferentiated cell found in a tissue or organ. This cell type can produce more stem cells and can also give rise to specialized cell types of the tissue or organ they are located in. The main job of an adult stem cell is to maintain and repair the tissue in which it is found. When compared to embryonic and cord blood stem cell, adult stem cells are believed to be more limited in the type of cells that they can generate.
When the term 'adult stem cell' is used, bone marrow cells are typically the cell type being referred to; and adult blood forming (hematopoietic) stem cells from bone marrow have been used in transplants for the past 30 years.
However, researchers have found adult stem cells in many more tissues than they once thought possible. Certain kinds of adult stem cells, under the right conditions, appear to be able to differentiate into a number of different kinds of cells; a finding that has led some scientists to wonder whether adult stem cells can be used for a wider range of transplant types.
To learn more about stem cells, see the Stem Cell information page of the National Institutes of Health or Medline's Stem Cell reference page.