Stem cells have the remarkable potential to develop into many different cell types in the
body. Serving as a sort of repair system for the body, they can theoretically divide without
limit to replenish other cells as long as the person or animal is still alive. When a stem cell
divides, each new cell has the potential to either remain a stem cell or become another type
of cell with a more specialized function, such as a muscle cell, a red blood cell, or a brain cell.

Stem cells have two important characteristics that distinguish them from other types of cells.
First, they are unspecialized cells that renew themselves for long periods through cell
division. The second is that under certain physiologic or experimental conditions, they can be
induced to become cells with special functions such as the beating cells of the heart muscle
or the insulin-producing cells of the pancreas.

Stem cells are cells that have the remarkable potential to develop into many different cell
types in the body. Serving as a sort of repair system for the body, they can theoretically
divide without limit to replenish other cells for as long as the person or animal is still alive.
When a stem cell divides, each "daughter" cell has the potential to either remain a stem cell
or become another type of cell with a more specialized function, such as a muscle cell, a red
blood cell, or a brain cell.

An adult stem cell is an undifferentiated cell found
among differentiated cells in a tissue or organ, can
renew itself, and can differentiate to yield the major
specialized cell types of the tissue or organ.
The primary roles of adult stem cells in a living
organism are to maintain and repair the tissue in
which they are found. Some scientists now use the
term somatic stem cell instead of adult stem cell.
Unlike embryonic stem cells , which are defined by
their origin (the inner cell mass of the blastocyst ),
the origin of adult stem cells in mature tissues is
unknown.

Research on adult stem cells has recently generated
a great deal of excitement. Scientists have found
adult stem cells in many more tissues than they
once thought possible. This finding has led scientists
to ask whether adult stem cells could be used for
transplants. In fact, adult blood forming stem cells from bone marrow have been used in
transplants for 30 years. Certain kinds of adult stem cells seem to have the ability to
differentiate into a number of different cell types, given the right conditions. If this
differentiation of adult stem cells can be controlled in the laboratory, these cells may become
the basis of therapies for many serious common diseases.

The history of research on adult stem cells began about 40 years ago. In the 1960s,
researchers discovered that the bone marrow contains at least two kinds of stem cells. One
population, called hematopoietic stem cells , forms all the types of blood cells in the body.
A second population, called bone marrow stromal cells , was discovered a few years later.
Stromal cells are a mixed cell population that generates bone, cartilage, fat, and fibrous
connective tissue.

Also in the 1960s, scientists who were studying rats discovered two regions of the brain that
contained dividing cells, which become nerve cells. Despite these reports, most scientists
believed that new nerve cells could not be generated in the adult brain. It was not until the
1990s that scientists agreed that the adult brain does contain stem cells that are able to
generate the brain's three major cell types? astrocytes and oligodendrocytes , which are
non-neuronal cells, and neurons , or nerve cells.

Pluripotent stem cells, while having great therapeutic potential, face formidable technical
challenges. First, scientists must learn how to control their development into all the different
types of cells in the body. Second, the cells now available for research are likely to be
rejected by a patient's immune system. Another serious consideration is that the idea of
using stem cells from human embryos or human fetal tissue troubles many people on ethical
grounds.

Until recently, there was little evidence that multipotent adult stem cells could change course
and provide the flexibility that researchers need in order to address all the medical diseases
and disorders they would like to. New findings in animals, however, suggest that even after
a stem cell has begun to specialize, it may be more flexible than previously thought.

There are currently several limitations to using adult stem cells. Although many different
kinds of multipotent stem cells have been identified, adult stem cells that could give rise to all
cell and tissue types have not yet been found. Adult stem cells are often present in only
minute quantities and can therefore be difficult to isolate and purify. There is also evidence
that they may not have the same capacity to multiply as embryonic stem cells do. Finally,
adult stem cells may contain more DNA abnormalities?caused by sunlight, toxins, and errors
in making more DNA copies during the course of a lifetime. These potential weaknesses
might limit the usefulness of adult stem cells.