“An adult stem cell is an undifferentiated cell (not
having changed to become a specialized cell type) 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.
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.”
