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Oct. 12, 2006 — What are adult stem
cells and where are they found?
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Response
by Fernando Camargo
Whitehead Fellow |
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An adult stem cell is an undifferentiated
cell that is found among specialized cells in a tissue
or organ, can renew itself, and can differentiate to
produce the major specialized cell types of that 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. Unlike embryonic stem cells,
which are defined by their origin (the inner cell mass
of a 4-day-old embryo), the origin of adult stem cells
in most mature tissues is unknown.
The history of research on adult stem cells began about
40 years ago. In the 1960s, researchers discovered that
the bone marrow contains a very rare population of cells
that could generate all of the cells of our immune system
and all the red cells in our blood. Furthermore, if
these “hematopoietic” stem cells were transplanted
into mice that had already received a strong dose of
radiation, they allowed the mice to survive an otherwise
lethal procedure.
These early basic observations laid the foundation for
the development and the implementation of human bone
marrow stem cell transplants. Recently, people have
been able to characterize two other populations of stem
cells that reside in bone marrow—mesenchymal stem
cells, which gives rise mainly to skeletal and cartilage
cell types, and endothelial progenitor cells, which
are important for the generation of blood vessels.
Also in the 1960s, scientists who were studying rats
discovered two regions of the brain that contained dividing
cells that become nerve cells. Despite these early results,
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
contains stem cells that are able to generate neuronal
and non-neuronal cell types.
In the last 10 years, intense research has confirmed
that most adult tissues contain a resident adult stem
cell that is responsible for the growth and regeneration
of that tissue. Stem cells have been isolated from the
liver, muscle, intestine, skin, breast, cornea and lung.
It is still highly controversial whether other tissues
such as the heart or the pancreas contain a cell capable
of performing bona fide stem cell functions.
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