Stem cells are cells
which are not terminally differentiated and are therefore able
to produce cells of other types. Medical researchers hope they
can be used to repair specific tissues or to grow organs from
scratch. There are three types of stem cells: totipotent, pluripotent,
and multipotent. A single totipotent stem cell can grow into
an entire organism. Pluripotent stem cells cannot grow into
a whole organism, but they can become any other type of cell
in the body. Multipotent stem cells can only become particular
types of cells: e.g. blood cells, or bone cells.
Adult Stem Cells
Stem cells can be found in adult human beings.
Adult stem cells reproduce daily to provide certain specialized
cells - for example 200 billion red blood cells are created
each day in the body. Until recently it was thought that each
of these cells could produce just one particular type of cell
- this is called differentiation (see Morphogenesis). However
in the past few years, evidence has been gathering of stem cells
that can form in to several different forms. Bone marrow stem
cells are known to be able to transform in to liver, nerve,
muscle and kidney cells.
Adult stem cells may be even more versatile
than this. Researchers at the New York University School of
Medicine have extracted stem cells from the bone-marrow of mice
which are they say are pluripotent. Turning one type of stem
cell in to another is called transdifferentiation.
Embryonic Stem Cells
Stem cells which originate from embryos are
seen to have the most potential because of their totipotent
properties - they are able to grow in to any of the 200 cell
types in the body. Embryonic stem cells can be obtained from
a cloned embryo, created by fusing a denucleated egg-cell with
a patient's cell. The embryo produced is allowed to grow, and
stem cells are then extracted. Because they are obtained from
a clone, they are genetically compatible with the patient.
As well as having the largest medical potential,
they are also the most controversial type of stem cell because
their utilization involves the destruction of human embryos.
Some people believe that these embryos are human beings, and
therefore destroying them for any reason is effectively murder.
This belief is also the basis for 'Pro-life' opposition to abortion.
Many scientists defend the destruction of embryos citing all
the medical benefits that it is possible to achieve with them,
and saying that many would have been destroyed anyway. 'Pro-life'
groups respond that it would be possible to achieve the same
benefits from the use of adult stem cells - although most scientists
agree that we are further from using these in the same way hoped
for embryonic stem cells.
Another controversy in the use of embryonic
stem cells is the use of therapeutic cloning. This involves
the cloning of early embryos from which stem cells are harvested,
providing a larger source of the cells. Many see this as encouraging
human cloning, which they think could be dangerous or unethical.
For over 30 years, bone marrow stem cells
have been used to treat cancer patients with conditions such
as leukemia and lymphoma. These are detroyed in some chemotherapy
treatments, but if they are removed before the process and then
reinjected, the cells produce large amounts of red and white
blood cells, to keep the body healthy and help to fight infection.
Since the 1980s stem cells have been taken
from the blood instead of the bone-marrow, making the procedure
safer for older people. Although normally scarce, the number
of 'Peripheral blood cells' can be increased by a course of
drugs, which release the stem cells from the bone-marrow. These
are removed before chemotherapy, which kills most of them, and
Research injecting neural (adult) stem cells
in to the brains of rats can be astonishingly successful in
treating cancerous tumours. With traditional techniques brain
cancer can be almost impossible to treat because it spreads
so rapidly. Researchers at the Harvard Medical School injected
cells genetically engineered to convert a separately injected
non-toxic substance in to a cancer-killing agent. Within days
the cells had migrated in to the cancerous area end the injected
substance was able to reduce tumour mass by 80 per cent.
Stem cells are also apparently able to repair
muscle damaged after heart attacks. Heart attacks are due to
the coronary artery being blocked, starving tissue of oxygen
and nutrients. Days after the attack is over, the cells try
to 'remodel' themselves so they can pump harder. However, because
of the decreased blood flow this attempt is futile and results
even more muscle cells to weaken and die. Researchers at Columbia-Presbyterian
found that injecting bone-marrow stem cells in to mice which
had had heart attacks induced in them resulted in an improvement
of 33 per cent in the functioning of the heart. The damaged
tissue had regrown by 68 per cent. Clinical trials in humans
are hoped for by 2003.
Sources of Stem Cells
Blood from the placenta and unbilical cord
of new-born babies is a useful source of adult stem cells. Since
1988 these 'cord blood' stem cells have been used to treat Gunther's
disease, Hunter syndrome, Hurler syndrome, acute lymphocytic
leukemia and many more problems mostly in children. It is collected
by removing the umbilical cord, cleansing it and withdrawing
blood from the umbilical vein. This blood is then immediately
analysed for infectious agents and the tissue-type is determined.
Cord blood is stored in liquid nitrogen for later use, when
it is thawed and injected through a vein in to the patient.
This kind of treatment where the stem cells are collected from
another donor is called allogenic treatment. When the cells
are collected from the same patient they will be used on it
is called autologous.
In fact, useful sources of adult stem cells
are being found in organs all over the body. Research at McGill
University in Montreal have extracted stem cells from skin able
to differentiate in to many types of tissue including neurones,
smooth muscle cells and fat-cells. These were found in 'dermis'
- a layer of tissue beneath the skin.
In the same way that organs can be transplanted
from cadavers researchers at the Salk Insitute in California
have found that these could be used as a source of stem cells
as well. Taking stem cells from the brains of corpses they were
able to coax them in to dividing in to valuable neurons. However
whether they will function correctly when used in treatment
has not yet been determined.
In May of 2003, researchers announced that
they had successfully used embryonic stem cells to produce human
egg cells. Spokespersons stated that these egg cells could be
used in turn to produce new stem cells. If research and testing
proved that artifically created egg cells could be a viable
source for embryonic stem cells, they noted, then this would
remove the necessity of harvesting human embryos. Thus, the
controversy over donating human egg cells and embryos would
be largely dismissed.
Due to the controversy surrounding embryonic
stem cells, in November 2001 the US National Institutes of Health
announced a list of 72 approved human embryonic cell lines which
researchers are to be allowed to work with. However some scientists
declared problems they had with the list - that some cell lines
are less useful to work with than others. Also other scientists
are not convinced that the list actually contains 72 different
lines - they think some are derivations of a single line.
Controversy Over Ethical Implications
Ethicists, philosophers, theologians and clergy
are all very concerned with the ethical implications of stem
cell research. In the U.S. most Christian groups have come out
strongly against all forms of this research as they view it
as a form of abortion, which they see as murder. Jewish groups,
of all denominations, have come out in favor of this research,
as they do not view an early stage embryo as a human being.
Many Humanists, Unitarian-Universalists, and many Muslim clerics
have come out in favor of stem cell research.
In spite of this and his own personal views
on the subject, US President George W. Bush announced on August
9, 2001 that he would support federal funding of limited research
on embryonic stem cells.