From post-embryonic development
through the normal life of any organism, certain tissues of the body require
stem cells for normal turnover and repair.
Stem cells that are found in developed tissue, regardless of the age of
the organism at the time, are referred to as adult stem cells. The most well-known example of this are the
hematopoietic stem cells of blood. More recently, mesenchymal stem cells (MSC)
required for the maintenance of bone, muscle, and other tissues have been
discovered. Adult stem cells are multipotent; the number
of tissues that they can regenerate compares poorly with the pluripotency of
embryonic stem cells and embryonic germ cells.
However, the MSC is in fact an excellent example of the potential for
use of stem cells in human therapeutic procedures. MSCs are capable of differentiating into bone, cartilage, muscle,
fat, and a few other tissue types.
Their use for bone and cartilage replacement is undergoing FDA-approved clinical trials at the present
Adult-derived stem cell therapies
will complement, but cannot replace, therapies that may be eventually obtained
from ES cells. They do have some advantages.
For example, adult stem cells offer the opportunity to utilize small
samples of adult tissues to obtain an initial culture of a patients own cells
for expansion and subsequent implantation (this is called an autologous
transplant). This process avoids any
ethical or legal issues concerning sourcing, and also protects the patient from
viral, bacterial, or other contamination from another individual. With proper manufacturing quality controls
and testing, allogeneic adult stem cells (cells from a donor) may be practical
as well. Already in clinical use are
autologous and allogeneic transplants of hematopoietic stem cells that are
isolated from mobilized peripheral blood or from bone marrow by positive
selection with antibodies in commercial devices. In general, there is less ethical concern over their initial
source. Additionally, since they
normally differentiate into a narrower set of cell types, directing them to a
desired fate is more straightforward. However, many cells of medical interest
cannot, as of yet, be obtained from adult-derived cell types. Production of
large numbers of these cells is much more difficult than is the case for ES
cells. Based upon our present knowledge base, it appears unlikely that human
adult stem cells alone will provide all of the necessary cell types required
for the most clinically important areas of research.
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