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| From OncoLog,
September 2003, Vol. 48, No. 9 |
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Understanding the Buzz over Stem Cells
 A
human stem cell is very much like a queen bee’s egg. A beehive must
have the right numbers of the right types of bees to survive and produce
honey. Although the queen’s eggs are all exactly alike, the worker
bees treat the honey with special ingredients to control what types of
bees develop: workers, drones, guards, or another queen. Therefore, each
egg has the potential to become any type of bee in the hive. This same
potential exists in stem cells.
Embryonic
Stem Cells
Embryonic
stem cells can become any of the more than 200 cell types in the human
body. This unique characteristic gives them tremendous potential as a
treatment for many different diseases and conditions, which is why they
are the most interesting stem cells to scientists.
Embryonic
stem cells have not yet been used to treat humans, but scientists hope
to eventually use them to develop replacement cells and tissues for parts
of the body that are damaged, diseased, or worn out. Once scientists discover
how to control the types of cells that embryonic stem cells become and
overcome other obstacles, they will be able to transplant specialized
cells derived from embryonic stem cells into patients to treat many conditions,
such as Parkinson’s disease, diabetes, spinal cord injury, and heart
failure.
The
use of human embryonic stem cells for research and therapy is controversial,
however. Embryonic stem cells used for research typically come from four-
to five-day-old embryos, whose development must be terminated to extract
the stem cells (30-40 cells). The embryos come from in vitro fertilization
clinics, where the eggs of women who are having difficulty becoming pregnant
are fertilized in a laboratory. Not all of the fertilized eggs from each
woman who is treated at the clinic are implanted into the woman’s
womb, and some of the “extra” embryos are used in stem cell
research. The point is that each healthy embryo has the potential to become
a baby.
Adult
Stem Cells
Fortunately,
this moral dilemma does not exist with the use of adult stem cells. Unlike
embryonic stem cells, adult stem cells are scattered throughout the tissues
of the body, and they are far more difficult to isolate and grow in culture
than are embryonic stem cells. However, much like embryonic stem cells,
adult stem cells are unspecialized cells that can generate specialized
cells, such as nerve cells, bone cells, liver cells, and muscle cells.
Adult stem cells can proliferate, or replicate themselves, for years and
continue to produce differentiated, or specialized, cells as needed. When
tissue is damaged, adult stem cells produce cells to replace the damaged
ones. Adult stem cells generally produce the cell types of the tissue
in which they live. Until recently, they were thought to be able to produce
only those cell types, but new evidence shows that at least some adult
stem cells can produce the cells of other tissues. For example, adult
bone marrow or peripheral blood–derived stem cells can produce heart,
skeletal muscle, skin, brain, and lung cells.
Stem
cells found in bone marrow have been transplanted from healthy donors
to sick patients for more than 40 years to treat many blood disorders
and leukemia and lymphoma. In studies of a very limited number of patients,
adult stem cells have even been used to treat patients with diabetes or
advanced kidney cancer. Recently, researchers at M. D. Anderson Cancer Center discovered that cells derived from stem cells donated for bone
marrow transplants became functioning liver cells. Further testing revealed
donor-derived stem cells in the liver, skin, and gut tissue of transplant
recipients. This discovery means that stem cells circulating in the blood
might be used in treatments to repair many different kinds of tissues
or organs.
The
potential of stem cell research is as unlimited as that of the stem cells
themselves. Knowing the difference between embryonic and adult stem cells,
their origins, and their potential uses in medicine will help guide decisions
about their research and development. For more information on this topic or for questions about M. D. Anderson’s treatments, programs, or services, call askMDAnderson at (877) MDA-6789.
Other
articles in OncoLog, September 2003 issue:
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