Stem cells are found in all of us,
from the early stages of human development to the end of
life. All stem cells may prove useful for medical research,
but each of the different types has both promise and
limitations. Embryonic stem cells, which can be derived
from a very early stage in human development, have the
potential to produce all of the body's cell types. Adult
stem cells, which are found in certain tissues in fully
developed humans, from babies to adults, may be limited
to producing only certain types of specialized cells.
Recently, scientists have also identified stem cells in
umbilical cord blood and the placenta that can give rise
to the various types of blood cells.
Embryonic Stem Cells [top]
A
blastocyst (BLAST-oh-sist), is a pre-implantation
embryo that develops 5 days after the fertilization of an
egg by a sperm. It contains all the material necessary for
the development of a complete human being. The blastocyst
is a mostly hollow sphere of cells that is smaller than
the period at the end of this sentence. In its interior is the
inner cell mass, which is composed of 30-34 cells that are
referred to by scientists as pluripotent because they can
differentiate into all of the cell types of the body.
In
comon usage, "embryo" can refer to all stages of development
from fertilization until a somewhat ill-defined
stage when it is called a fetus. Scientists use terms such as
"morula" and "blastocyst" to refer to precise, specific
stages of pre-implantation development. In order to be as
precise as possible, this booklet uses the scientific terms
when describing scientific concepts but uses the term
"embryo" where more precision seemed likely to confuse
rather than clarify.
In normal development, the blastocyst would implant
in the wall of the uterus to become the embryo and
continue developing into a mature organism. Its outer
cells would begin to form the placenta and the inner
cell mass would begin to differentiate into the progressively
more specialized cell types of the body.
When the blastocyst is used for stem cell research,
scientists remove the inner cell mass and place these
cells in a culture dish with a nutrient-rich liquid
where they give rise to embryonic stem cells.
Embryonic stem cells seem to be more flexible than
stem cells found in adults, because they have the
potential to produce every cell type in the human
body. They are also generally easier to collect, purify
and maintain in the laboratory than adult stem cells.
Scientists can induce embryonic stem cells to replicate
themselves in an undifferentiated state for very
long periods of time before stimulating them to create
specialized cells. This means that just a few
embryonic stem cells can build a large bank of stem
cells to be used in experiments. However, such
undifferentiated stem cells could not be used directly
for tissue transplants because they can cause a
type of tumor called a teratoma. To be used for therapies,
embryonic stem cells would first need to be
differentiated into specialized cell types.
Some find embryonic stem cell research to be morally
objectionable, because when scientists remove the
inner cell mass, the blastocyst no longer has the potential
to become a fully developed human being.
Sources of Embryonic Stem Cells [top]
In Vitro Fertilization: [top]
The largest potential
source of blastocysts for stem cell research is from
in
vitro fertilization (IVF) clinics. The process of IVF
requires the retrieval of a woman's eggs via a surgical
procedure after undergoing an intensive regimen of
"fertility drugs," which stimulate her ovaries to produce
multiple mature eggs.
When IVF is used for
reproductive purposes, doctors typically fertilize all of
the donated eggs in order to maximize their chance of
producing a viable blastocyst that can be implanted in
the womb. Because not all the fertilized eggs are
implanted, this has resulted in a large bank of
"excess" blastocysts that are currently stored in freezers
around the country. The blastocysts stored in IVF
clinics could prove to be a major source of embryonic stem cells for use in medical research. However,
because most of these blastocysts were created before
the advent of stem cell research, most donors were not
asked for their permission to use these left-over blastocysts
for research.
The IVF technique could potentially
also be used to produce blastocysts specifically
for research purposes. This would facilitate the isolation
of stem cells with specific genetic traits necessary
for the study of particular diseases. For example, it
may be possible to study the origins of an inherited
disease like cystic fibrosis using stem cells made from
egg and sperm donors who have this disease. The creation
of stem cells specifically for research using IVF
is, however, ethically problematic for some people
because it involves intentionally creating a blastocyst
that will never develop into a human being.
Nuclear Transfer: [top] The process called
nuclear
transfer offers another potential way to produce
embryonic stem cells. In animals, nuclear transfer
has been accomplished by inserting the nucleus of
an already differentiated adult cell-for example,
a skin cell-into a donated egg that has had its
nucleus removed. This egg, which now contains the
genetic material of the skin cell, is then stimulated to
form a blastocyst from which embryonic stem cells
can be derived. The stem cells that are created in this
way are therefore copies or "clones" of the original
adult cell because their nuclear DNA matches that
of the adult cell.
As of the summer of 2006, nuclear transfer has not
been successful in the production of human embryonic
stem cells, but progress in animal research suggests
that scientists may be able to use this technique to
develop human stem cells in the future.
Scientists believe that if they are able to use nuclear
transfer to derive human stem cells, it could allow
them to study the development and progression of
specific diseases by creating stem cells containing
the genes responsible for certain disorders. In the
future, scientists may also be able to create "personalized"
stem cells that contain only the DNA of
a specific patient. The embryonic stem cells created
by nuclear transfer would be genetically matched to
a person needing a transplant, making it far less
likely that the patient's body would reject the new
cells than it would be with traditional tissue transplant
procedures.
Although using nuclear transfer to produce stem
cells is not the same as reproductive cloning, some
are concerned about the potential misapplication of
the technique for reproductive cloning purposes.
Other ethical considerations include egg donation,
which requires informed consent, and the possible
destruction of blastocysts.
Producing Embryonic Stem Cells
Using Nuclear Transfer Is Not the
Same as Reproductive Cloning
The use of nuclear transfer to develop disease-specific
stem cells can be called
research cloning, and the use
of this technique for personalized tissue transplants is
sometimes called
therapeutic cloning.
These terms must
be carefully distinguished from
reproductive cloning,
in which the intent is to implant a cloned embryo in a
female's womb and allow it to develop fully into
an individual. This was the technique by which Dolly the
sheep was made and is now widely used for reproductive
cloning in animals. In humans, however, reproductive
cloning has been actively discouraged by most in the
scientific community. The National Academies concluded,"
Human reproductive cloning should not now be
practiced. It is dangerous and likely to fail" in the
2002 report
Scientific and Medical Aspects of Human
Reproductive Cloning.
Adult Stem Cells [top]
Adult stem cells are hidden deep within organs,
surrounded by millions of ordinary cells, and may
help replenish some of the body's cells when needed.
In fact, some adult stem cells are currently being
used in therapies. They have been found in several
organs that need a constant supply of cells, such as
the blood, skin, and lining of the gut, and have also
been found in surprising places like the brain, which
is not known to readily replenish its cells.
Unlike
embryonic stem cells, adult stem cells are already
somewhat specialized. For example, blood stem
cells normally only give rise to the many types of
blood cells, and nerve stem cells can only make the
various types of brain cells. Recent research however,
suggests that some adult stem cells might be more
flexible than previously thought, and may be made
to produce a wider variety of cell types. For example,
some experiments have suggested that blood stem cells
isolated from adult mice may also be able to produce
liver, muscle, and skin cells, but these results are not
yet proven and have not been demonstrated with
human cells. Nevertheless, scientists are working on
finding a way to stimulate adult stem cells, or even
other types of adult cells, to be more versatile. If
they succeed, it could provide another source of
unspecialized stem cells.
Comparison of the Different Sources of Stem Cells [top]
Click to Enlarge
This Web page is based on
Understanding Stem Cells: An Overview of the Science and Issues from the National Academies.
