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Adult Stem Cell Studies From
the National Institutes of Health we learn that:
"Adult stem cells have been identified in many organs
and tissues. One important point to understand about adult stem cells is
that there are a very small number of stem cells in each tissue. Stem
cells are thought to reside in a specific area of each tissue where they
may remain quiescent (non-dividing) for many years until they are
activated by disease or tissue injury. The adult tissues reported to
contain stem cells include brain, bone marrow, peripheral blood, blood
vessels, skeletal muscle, skin and liver.
Scientists in many laboratories are trying to find
ways to grow adult stem cells in cell culture and manipulate them to
generate specific cell types so they can be used to treat injury or
disease. Some examples of potential treatments include replacing the
dopamine-producing cells in the brains of Parkinson's patients,
developing insulin-producing cells for type I diabetes and repairing
damaged heart muscle following a heart attack with cardiac muscle cells.
Also, a single adult stem cell should be able to
generate a line of genetically identical cells—known as a clone—which
then gives rise to all the appropriate differentiated cell types of the
tissue. Scientists tend to show either that a stem cell can give rise to
a clone of cells in cell culture, or that a purified population of
candidate stem cells can repopulate the tissue after transplant into an
animal. Recently, by infecting adult stem cells with a virus that gives
a unique identifier to each individual cell, scientists have been able
to demonstrate that individual adult stem cell clones have the ability
to repopulate injured tissues in a living animal.
As indicated above, scientists have reported that
adult stem cells occur in many tissues and that they enter normal
differentiation pathways to form the specialized cell types of the
tissue in which they reside. Adult stem cells may also exhibit the
ability to form specialized cell types of other tissues, which is known
as transdifferentiation or plasticity.
Normal differentiation pathways of adult stem cells.
In a living animal, adult stem cells can divide for a long period and
can give rise to mature cell types that have characteristic shapes and
specialized structures and functions of a particular tissue."
"The following are examples of differentiation
pathways of adult stem cells (above figure):
Hematopoietic stem cells give rise to all the types
of blood cells: red blood cells, B lymphocytes, T lymphocytes, natural
killer cells, neutrophils, basophils, eosinophils, monocytes,
macrophages, and platelets.
Bone marrow stromal cells (mesenchymal stem cells)
give rise to a variety of cell types: bone cells (osteocytes),
cartilage cells (chondrocytes), fat cells (adipocytes), and other
kinds of connective tissue cells such as those in tendons.
Neural stem cells in the brain give rise to its
three major cell types: nerve cells (neurons) and two categories of
non-neuronal cells—astrocytes and oligodendrocytes.
Epithelial stem cells in the lining of the
digestive tract occur in deep crypts and give rise to several cell
types: absorptive cells, goblet cells, Paneth cells, and
enteroendocrine cells.
Skin stem cells occur in the basal layer of the
epidermis and at the base of hair follicles. The epidermal stem cells
give rise to keratinocytes, which migrate to the surface of the skin
and form a protective layer. The follicular stem cells can give rise
to both the hair follicle and to the epidermis.
Adult stem cell plasticity and transdifferentiation.
A number of experiments have suggested that certain adult stem cell
types are pluripotent (ability of a single stem cell to develop into
many different cell types of the body).
This ability to differentiate into multiple cell types is called
plasticity or transdifferentiation (image below)."
Testimony of Dr. David A.
Prentice, Ph.D.
Senior Fellow for Life Sciences, Family Research Council
Affiliated Scholar, Center for Clinical Bioethics, Georgetown University
Medical Center
House Health and Government Operations Committee
and
House Appropriations Committee
Maryland Legislature
March 2, 2005
. . . A wealth of
scientific papers published over the last few years document that adult
stem cells are a much more promising source of stem cells for
regenerative medicine. Adult stem cells actually do show pluripotent
capacity in generation of tissues, meaning that they can generate many,
if not all, of the different tissues of the body. In a paper published
in May 2001, the researchers found that one adult bone marrow stem cell
could regenerate not only marrow and blood, but also form liver, lung,
digestive tract, skin, heart, muscle.
Many published references
also show that adult stem cells can multiply in culture for extensive
periods of time, retaining their ability to differentiate, and providing
sufficient numbers of cells for clinical treatments. More importantly,
adult stem cells have been shown to be effective in treating animal
models of disease, including such diseases as diabetes,
stroke,
spinal cord injury,
Parkinson’s disease,
and retinal degeneration.
Moreover, adult stem cells
are already being used clinically to treat many diseases in human
patients. These include as reparative treatments with various cancers,
autoimmune diseases such as multiple sclerosis, lupus, and arthritis,
anemias including sickle cell anemia, and immunodeficiencies.
Adult stem cells are also
being used to treat patients by formation of cartilage, growing new
corneas to restore sight to blind patients, treatments for stroke, and
several groups are using adult stem cells with patients to repair damage
after heart attacks. Early clinical trials have shown initial success in
patient treatments for Parkinson’s disease and spinal cord injury. And,
the first FDA-approved trial to treat juvenile diabetes in human
patients is ready to begin at Harvard Medical School, using adult cells.
An advantage of using adult stem cells is that in most cases the
patient’s own stem cells can be used for the treatment, circumventing
the problems of immune rejection, and without tumor formation.
The mechanism for these
amazing regenerative treatments is still unclear. Adult stem cells in
some cases appear capable of interconversion between different tissue
types, known as transdifferentiation. In some tissues, adult stem cells
appear to fuse with the host tissue and take on that tissue’s
characteristics, facilitating regeneration. And in some studies, the
adult stem cells do not directly contribute to the regenerating tissue,
but instead appear to stimulate the endogenous cells of the tissue to
begin repair.
Whatever the mechanism, the
adult cells are successful at regenerating damaged tissue.
A representative list of
diseases currently in patient trials with adult stem cells is given as
Appendix A below.
In summary, adult stem
cells have been shown by the published evidence to be a more promising
alternative for patient treatments, with a vast biomedical potential.
Adult stem cells have proven success in the laboratory dish, in animal
models of disease, and in current clinical treatments. Adult stem cells
also avoid problems with tumor formation, transplant rejection, and
provide realistic excitement for patient treatments.
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