The answers to the following questions were written and reviewed
by a panel of scientists who specialize in stem cell research ~
Q&A borrowed from The International Society
for Stem Cell Research (ISSCR).
1. What are stem cells? Stem cells are the foundation cells
for every organ, tissue and cell in the body. They are like a blank
microchip that can ultimately be programmed to perform particular
tasks. Stem cells are undifferentiated or "blank" cells that have
not yet fully specialized. Under proper conditions, stem cells
begin to develop into specialized tissues and organs. Additionally,
stem cells can self-renew, that is they can divide and give rise to
more stem cells.
There are many different types of stem cells. These include
embryonic stem cells that exist only at the earliest stages of
embryonic development; as embryonic stem cells can form all cell
types of the body, they are referred to as ‘pluripotent’ stem
cells. There are various types of ‘adult’ or ‘tissue-specific’ stem
cells that exist in a number of different fetal and adult tissues.
These stem cells generally can only form a limited number of cell
types corresponding with their tissues of origin; they are called
‘multipotent’ stem cells.
2. Where do stem cells come from? Embryonic stem cells are
derived from the inner cell mass of a blastocyst: the fertilized
egg, called the zygote, divides and forms two cells; each of these
cells divides again, and so on. Soon there is a hollow ball of
about 150 cells called the blastocyst that contains two types of
cells, the trophoblast and the inner cell mass. Embryonic stem
cells are obtained from the inner cell mass.
Stem cells can also be found in small numbers in various tissues in
the fetal and adult body. For example, blood stem cells are found
in the bone marrow that give rise to all specialized blood cell
types. Such tissue-specific stem cells have not yet been identified
in all vital organs, and in some tissues like the brain, although
stem cells exist, they are not very active, and thus do not readily
respond to cell injury or damage.
Stem cells can also be obtained from other sources, for example,
the umbilical cord of a newborn baby is a source of blood stem
cells. Recently, scientists have also discovered the existence of
cells in baby teeth and in amniotic fluid that may also have the
potential to form multiple cell types. Research on these cells is
at a very early stage.
Recently, cells with properties similar to embryonic stem cells,
referred to as induced pluripotent stem cells (iPS cells) have been
engineered from somatic cells (see ‘What is are induced pluripotent
stem cells?’).
3. What is a stem cell line? A stem cell line is a
population of cells that can replicate themselves for long periods
of time in vitro, meaning outside of the body. These cell lines are
grown in incubators with specialized growth factor-containing media
(liquid food source), at a temperature and oxygen/carbon dioxide
mixture resembling that found in the mammalian body.
4. What is an embryonic stem cell? Embryonic stem cells are
those grown from the cells that make up the inner cell mass of the
blastocyst. Embryonic stem cells have been derived from a variety
of animals, including human, and are described as ‘pluripotent’-
that is, they are capable of generating any and all cells in the
body under the right conditions.
Embryonic stem cell lines can be grown indefinitely in vitro if the
correct conditions are met. Importantly, these cells continue to
retain their ability to form different, specialized cell types once
they are removed from the conditions that keep them in an
undifferentiated, or unspecialized, state.
The most widely studied are mouse embryonic stem cells. Mouse
embryonic stem cells have taught us a lot about how pluripotent
cells grow and specialize, and how embryonic development works.
Indeed, mouse embryonic stem cells are a critical research tool for
studying the function of individual genes and modeling human
diseases. Mouse embryonic stem cells can be manipulated to contain
specific genetic changes then used to generate mice which contain
this change. Capecchi, Evans and Smithies were awarded the Nobel
Prize in Physiology or Medicine, 2007 for developing this process.
Read more.
Human embryonic stem cells were isolated relatively recently, in
1998. They are more difficult to work with than their mouse
counterparts and currently less is known about them. However,
scientists are making remarkable progress, learning about human
developmental processes, modeling disease and establishing
strategies that could ultimately lead to therapies to replace or
restore damaged tissues using these human cells.
5. What is an adult (tissue-specific) stem cell? Perhaps
better referred to as a tissue-specific stem cell, these cells are
found in tissues that have already developed. Tissue-specific stem
cells can be isolated from many tissues, including brain. The most
common source of tissue-specific stem cells is the bone marrow,
located in the center of some bones. There are different types of
stem cells found in the bone marrow, including hematopoietic or
blood stem cells, endothelial stem cells, and mesenchymal stem
cells. It is well established that hematopoietic stem cells form
blood, that endothelial stem cells form the vascular system
(arteries and veins), and that mesenchymal stem cells form bone,
cartilage, muscle, fat, and fibroblasts.
While it has been theorized that some adult stem cells may have a
broader potential to form different cell types than was previously
suspected (for example, cells from the bone marrow may contribute
to regeneration of damaged livers, hearts and other organs), this
is highly controversial in the scientific community. Currently, it
is not clear whether stem cells from adult tissues or umbilical
cord blood are truly pluripotent. The comparison of human embryonic
stem cells to adult stem cells is currently a very active area of
research.
6. What are ‘induced pluripotent cells’ or iPS cells?
Induced pluripotent cells (iPS cells) are non-pluripotent cells
that were engineered (‘induced’) to become pluripotent, that is,
able to form all cell types of the body. In other words, a cell
with a specialized function (for example a skin cell) was
‘reprogrammed’ to an unspecialized state similar to that of an
embryonic stem cell. While iPS cells and embryonic stem cells share
many characteristics they are not identical.
The generation of mouse iPS cells was reported in 2006 (read the
‘Briefing’
), and the generation of human iPS cells at the end of 2007 (read
the ‘Briefing’
).
Currently, iPS cells are produced by inserting copies of three-four
genes into specialized cells known to be important in embryonic
stem cells using viruses. Different groups have used slightly
different combinations of genes. It is not completely understood
how each of these genes functions to confer pluripotency and
ongoing research is addressing this question.
The technology used to generate iPS cells holds great promise for
creating patient- and disease-specific cell lines for research
purposes. However, a great deal of work remains before these
methods can be used to generate stem cells suitable for safe and
effective therapies.
7. What are the potential uses of human stem cells? Stem
cell research contributes to a fundamental understanding of how
organisms develop and grow, and how tissues are maintained
throughout adult life. This is knowledge that is required to work
out what goes wrong during disease and injury and ultimately how
these conditions might be treated. The development of a range of
human tissue-specific and embryonic stem cell lines will provide
researchers with the tools to model disease, test drugs and develop
increasingly effective therapies.
Replacing diseased cells with healthy cells, a process called cell
therapy, is a promising use of stem cells in the treatment of
disease; this is similar to organ transplantation only the
treatment consists of transplanting cells instead of organs.
Currently, researchers are investigating the use of adult, fetal
and embryonic stem cells as a resource for various, specialized
cell types, such as nerve cells, muscle cells, blood cells and skin
cells that can be used to treat various diseases.
In theory, any condition in which there is tissue degeneration can
be a potential candidate for stem cell therapies, including
Parkinson's disease, spinal cord injury, stroke, burns, heart
disease, Type 1 diabetes, osteoarthritis, rheumatoid arthritis,
muscular dystrophies and liver diseases.
In addition, retinal regeneration with stem cells isolated from the
eyes can lead to a possible cure for damaged or diseased eyes and
may one day help reverse blindness. Bone marrow transplantation
(transfers blood stem cells) is a well-established treatment for
blood cancers and other blood disorders.
8. What are the obstacles that must be overcome before the
potential uses of stem cells in cell therapy will be realized?
Here are just a few of the challenges that lie ahead. Firstly, a
source of stem cells must be found. The process of identifying,
isolating and growing the right kind of stem cell, for example a
rare cell in the adult tissue, is painstaking. In general,
embryonic and fetal stem cells are believed to be more versatile
than tissue-specific stem cells. Secondly, once stem cells are
identified and isolated, the right conditions must be developed so
that the cells differentiate into the specialized cells required
for a particular therapy. This too will require a great deal of
experimentation. Thirdly, a system that delivers the cells to the
right part of the body must be developed and the cells once there
must be encouraged to integrate and function in concert with the
body's natural cells. Furthermore, just as in organ transplants,
the body's immune system must be suppressed to minimize the immune
reaction set off by the transplanted cells.
While results from animal models are promising, the research on
stem cells and their applications to treat various human diseases
is still at a preliminary stage. As with any medical treatment, a
rigorous research and testing process must be followed to ensure
long-term efficacy and safety.
9. Are stem cells currently used in therapies today?
Hematopoietic stem cells (HSCs) or blood stem cells, present in the
bone marrow are the precursors to all blood cells. Blood stem cells
are currently the only type of stem cells commonly used for
therapy. Doctors have been transferring blood stem cells by bone
marrow transplant for more than 40 years. Advanced techniques for
collecting or "harvesting" HSCs are now used to treat leukemia,
lymphoma and several inherited blood disorders. Cord blood, like
bone marrow, is stored as a source of HSCs and is being used
experimentally as an alternative to bone marrow in
transplantation.
New clinical applications for stem cells are currently being tested
therapeutically for the treatment of musculoskeletal abnormalities,
cardiac disease, liver disease, autoimmune and metabolic disorders
(amyloidosis), chronic inflammatory diseases (lupus) and other
advanced cancers. However, these new therapies have been offered
only to a very limited number of patients.
10. Why is cord blood a valuable resource? Cord blood is
rich in hematopoietic or blood stem cells and is currently being
used as an experimental alternative to bone marrow transplantation.
The collection process is completely non-invasive, the host-donor
match required for transplantation is less stringent and cord blood
has fewer mature immune cells and thus poses a lower risk of graft
vs. host disease.
11. Why are researchers interested in developing
disease-specific or patient-specific pluripotent stem cells?
The development of patient-specific or disease-specific pluripotent
stem cells has great therapeutic promise for three reasons.
Firstly, these cells could provide a powerful new tool for studying
the basis of human disease and for discovering new drugs. Secondly,
the resulting embryonic stem cells could be developed into a needed
cell type, and if transplanted into the original donor, would be
recognized as 'self', thereby avoiding the problems of rejection
and immunosuppression that occur with transplants from unrelated
donors.
12. What is somatic cell nuclear transfer (SCNT)? Somatic
cell nuclear transfer (SCNT) is a technique in which the nucleus of
a somatic cell, that is any cell of the body apart from the sperm
or egg, is transferred into an egg that has had its original
nucleus removed. The egg now has the same DNA, or genetic material,
as the donor somatic cell. Given the right signals, the egg can be
coaxed into developing as if it had been fertilized. The egg would
divide to form 2 cells, then 4 cells, then 8 cells and so on until
the blastocyst is formed. Embryonic stem cells can be derived from
this blastocyst to create cell lines that are genetically identical
to the donor somatic cell.
13. Why derive embryonic stem cell lines following somatic cell
nuclear transfer (SCNT)? The derivation of patient-specific
human embryonic stem cell lines using this technique (see ‘What is
somatic cell nuclear transfer?’) Firstly, these cells could provide
a powerful new tool for studying the basis of human disease and for
discovering new drugs. Secondly, the resulting embryonic stem cells
could be developed into a needed cell type, and if transplanted
into the original donor, would be recognized as 'self', thereby
avoiding the problems of rejection and immunosuppression that occur
with transplants from unrelated donors.
14. Can induced pluripotent cells replace research on embryonic
stem cells or somatic cell nuclear transfer? No. The derivation
of human induced pluripotent stem cells opens up exciting new areas
of stem cell research, however, this technology is at a very early
stage and many fundamental questions remain. While iPS cells and
embryonic stem cells share many characteristics they are not
identical. The similarities and differences are still being
explored.
Research on human embryonic stem cells, somatic cell nuclear
transfer and ‘adult’ or tissue-specific stem cells needs to
continue in parallel. All are part of a research effort that seeks
to expand our knowledge of how cells function, what fails in the
disease process, and how the first stages of human development
occur. It is this combined knowledge that will ultimately generate
safe and effective therapies.
15. What is reproductive cloning? If an egg generated by
somatic cell nuclear transfer (see ‘What is somatic cell nuclear
transfer?’) was implanted into the womb of an animal, an individual
would be born that has identical genetic material as the donor
somatic cell and might be referred to as a ‘clone’. The procedure
is referred to as ‘reproductive cloning’ and is fraught with
profound technical and biological problems. The overwhelming
consensus of the world’s scientific and medical communities is that
at this time human reproductive cloning should be banned.
16. What is regenerative medicine? The goal of regenerative
medicine is to repair organs or tissues that are damaged by
disease, aging or trauma, such that function is restored, or at
least improved.
The term regenerative medicine is often used nowadays to describe
medical treatments and research that use stem cells (either adult
or embryonic) to restore the function of organs or tissues. This
can be achieved in different ways; first, by administering stem
cells, or specific cells that are derived from stem cells in the
laboratory; or second, by administering drugs that coax stem cells
that are already present in tissues to more efficiently repair the
involved tissue.
17. What is bioethics? Bioethics is the study of the moral
and ethical issues in the fields of scientific research, medical
treatment and, more generally, in the life sciences. With advancing
technology come new and exciting insights into scientific processes
and diseases; at the same time, new ethical issues arise.
By NATASHA SINGER and DUFF WILSON
Published: December 12, 2009
SOURCE: http://www.nytimes.com/2009/12/13/business/13drug.html?em
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