| By now, virtually all
of us have been exposed to news reports about stem cells. But
exactly what are stem cells, anyway? Let’s start with the basics. In
order to get from a single cell—the fertilized egg—to a human being
made up of trillions of cells, the two processes of cell division
and cell differentiation must occur. For one cell to become
trillions, many cell divisions (each of which involves one cell
dividing into two) must obviously take place. But since it is also
obvious that all cells of the human body are not identical to each
other, the process of differentiation, whereby the descendant cells
are different from the ancestor cells, must also occur.
Differentiation occurs because external chemical cues, interacting
with the genes of a cell, can cause two descendant cells to be
slightly different from their immediate ancestor cell and from each
other. Because the external chemical cues to which a cell is exposed
depend on the exact physical location of the given cell, different
cells of an embryo or fetus are exposed to different chemical cues,
and thus differentiate into different types of cells. This is how a
single fertilized egg cell eventually gives rise to brain cells,
blood cells, heart cells, skin cells, muscle cells, etc.
Because a fertilized egg cell has the
ability, through repeated cycles of cell division and
differentiation, to eventually give rise to all of the cell types in
a human body, as well as to the cells of the structures, such as the
placenta, needed to support the developing fetus, the fertilized egg
cell is said to be totipotent. At the other end of the spectrum are
cells that, for lack of a better word, we can call “non-potent.” An
example of a “non-potent” cell might be a cheek cell, scraped from
the inside of someone’s mouth and placed into a culture dish. If
this cell were induced to divide in the culture dish, it would give
rise to two cheek cells, and then these two cells would each give
rise to two more cheek cells, and so on…. Even if the cultured cheek
cells were exposed to chemical cues in an attempt to induce them to
divide and differentiate into different types of cells, such as
heart cells, brain cells, or muscle cells, the only type of cell
that would result from any further divisions would be more cheek
cells. A cheek cell simply does not have the ability to ever give
rise to any type of cells besides other cheek cells, i.e., we can
call it “non-potent.” Its exact opposite, in terms of the
characteristic of “potency,” is the totipotent fertilized egg cell,
which can give rise to all different types of cells, including all
of the specialized cells of the human body as well as the cells of
the placenta and other structures needed to support a developing
fetus. Because of its ability to give rise to every different type
of cell needed to make a human being, i.e., because of its
totipotency, a fertilized egg can be considered to be the ultimate
stem cell.
Soon after an egg cell is fertilized,
it will divide into two cells. Like the original fertilized egg
cell, the two descendant cells are totipotent, because if they are
separated, each of them is capable of developing into a human being,
i.e. they can give rise to all of the specialized cells present in a
person as well as to the cells of the placenta and other tissues
needed to support the developing fetus. But after some more cycles
of cell division occur, the cells are no longer totipotent. A few
days after an egg is fertilized, the cycles of cell division gives
rise to a ball of cells, called a blastocyst. The blastocyst is made
up of an outer cell layer and an inner cell mass. The cells of the
outer cell layer are not totipotent; neither are the cells of the
inner cell mass. The cells of the outer cell layer are capable of
giving rise to the cells of the placenta and other tissues needed to
support the developing fetus, but are not capable of giving rise to
any other types of cells, such as brain cells, muscle cells, etc.
The cells of the inner cell mass are capable of giving rise to all
of the specialized cells of the human body, but are not capable of
giving rise to the cells of the placenta and other tissues necessary
to support the fetus. Because none of the cells of the blastocyst
are capable of giving rise to all of the specialized cells that make
up a human being as well as to the cells of the placenta and other
fetal support structures, none of these cells are totipotent. We
will ignore the cells of the outer layer of the blastocyst, but we
will call the cells of the inner cell mass of the blastocyst
pluripotent, because these cells are capable of giving rise to all
of the types of specialized cells that make up the body, except for
the cells of the fetal support structures. Since these cells are
pluripotent, they are stem cells. And to give a general definition,
stem cells are cells that when in a culture dish can divide
indefinitely (giving rise to more stem cells), but when exposed to
the proper external chemical cues, can divide and differentiate into
many different cell types. Totipotent stem cells can divide and
differentiate into all cell types, whereas pluripotent stem cells
can divide and differentiate into virtually all cell types.
The pluripotent stem cells of the
inner cell mass of the blastocyst continue to divide and
differentiate, eventually giving rise to a fully formed human being.
Through many cycles of cell division and differentiation, the
pluripotent stem cells give rise to “non-potent” specialized cells,
such as brain cells, muscle cells, blood cells, liver cells, etc.
Somewhere within those many cycles of cell division, there are a
number of different types of “intermediate” cells. These
intermediate cells are partially specialized, in that each type of
“intermediate” cell can give rise only to a certain subset of
specialized cell types, rather than to virtually all cell types, as
can the pluripotent stem cells. For example, a certain type of
“intermediate” cell can give rise to the different kinds of blood
cells, such as red blood cells, white blood cells, and platelets,
while another type of “intermediate” cell can give rise to the cells
of the different kinds of connective tissue, such as muscle,
cartilage, bone, and fat. These “intermediate” cells are called
multipotent stem cells. There are some types of multipotent stem
cells, which besides being present in fetuses are also present in
children and adults. For example, we all need to have blood stem
cells in our bone marrow, in order to constantly replenish our red
blood cells, white blood cells, and platelets.
In light of the vital role that stem
cells play in the development of a fully formed human being or other
animal from a fertilized egg, it is apparent that stem cells have
been in existence throughout hundreds of millions of years of
evolutionary history. So why have they suddenly become such a big
deal? Well, over the past few years, it has become very obvious to
medical researchers that stem cells are almost definitely going to
play a tremendous role in the advancement of medical science. In
fact, the impact that stem cells are expected to have on the field
of medicine should be at least on the scale of the medical
improvements that were brought about when vaccines and antibiotics
became standard tools of medical practice.
The past few decades have given us
spectacular advances in the fields of molecular biology, cell
biology, and genetics. These advances have brought us to the point
where we can begin to manipulate stem cells. The manipulation of
stem cells opens up the possibility of completely curing and
treating many medical conditions, diseases, and injuries for which
current treatments are either nonexistent or less-than-satisfactory.
For example, people who have myocardial infarctions (heart attacks)
are left with areas of heart muscle that are no longer functioning.
But by utilizing stem cells, we may be able to grow new functioning
heart muscle cells to replace the patient’s damaged tissue. And
taken to the extreme, we may even be able to grow whole new hearts
for patients who require heart transplants. Similarly, we should be
able to grow new kidneys, livers, lungs, or any other organs for
patients who require transplants of those organs.
Type I diabetes (also known as
insulin-dependent diabetes or juvenile onset diabetes) is caused by
the malfunction or destruction of the pancreatic cells which are
responsible for producing insulin. Stem cells could be used to grow
new insulin-producing pancreatic cells for type I diabetes patients,
curing them so that they do not have to inject themselves with
insulin for the rest of their lives. Parkinson’s disease is due to
the death of certain brain cells that produce dopamine, one of the
neurotransmitters that brain cells use to communicate with each
other. With stem cells, it may be possible to cure Parkinson’s
disease by growing the appropriate type of brain cells for the
patient. Similarly, it is thought that Alzheimer’s disease is caused
by the death of certain brain cells that produce certain
neurotransmitters. Thus, it may be possible to cure Alzheimer’s
disease by inducing stem cells to develop into the appropriate type
of brain cells. Stroke victims often suffer disabilities as a result
of brain cell death that occurred during the stroke. Stem cells
could be used to grow and replace the cells that have died, curing
stroke victims of their disabilities.
Using stem cells, it could be
possible to cure patients who are paralyzed, by growing the nerve
cells needed to replace those that have been injured. Amyotrophic
lateral sclerosis, also known as Lou Gehrig’s disease, is due to the
death of the nerve cells that innervate the muscles. While, the
disease is currently untreatable, stem cells may provide the means
to a total cure. It should also be possible to use stem cells to
grow new skin for burn victims. The list of potential medical
applications for stem cells is seemingly endless, and the results
should be impressive enough to truly constitute a revolution in the
practice of medicine.
There is already limited use of stem
cells in treating patients today. Some cancer patients require bone
marrow transplants. These transplants involve blood stem cells
(which normally reside in the bone marrow), which are needed to
allow the patients to continue to produce red blood cells, white
blood cells, and platelets. Bone marrow transplants involve
multipotent stem cells, derived from adults, rather than pluripotent
stem cells, derived from embryos. For most of the body’s specialized
cell types, however, the multipotent stem cells which could develop
into those cell types have not yet been discovered. So at the
present time, only pluripotent stem cells, which can develop into
virtually any type of specialized cell, hold the promise of
generating all of the cell types needed to cure many of the diseases
and medical conditions that are currently incurable. In addition,
because pluripotent stem cells are found earlier along the cell
differentiation pathway than are multipotent stem cells, pluripotent
stem cells may turn out to be much more amenable to the manipulation
necessary to induce them to develop into the cells, tissues, and
organs necessary to best meet a given patient’s needs.
Like stem cells, the subject of
cloning is often subject to the media spotlight. One aspect of
cloning technology, called somatic cell nuclear transfer, may turn
out to play a very important role in the future therapeutic uses of
stem cells. A somatic cell is any cell of the body except for a cell
whose sole function is to give rise to the next generation, i.e., a
sperm cell or an egg cell. Somatic cell nuclear transfer involves
taking the nucleus (the central part of the cell, where virtually
all of the cell’s genetic information is located) from a body cell
and transplanting it into an egg cell that is from a different
person than the body cell and from which the nucleus has been
removed. The egg cell is then stimulated to divide and develop into
a blastocyst, from which pluripotent stem cells could be taken. When
the nucleus that was donated in the process comes from a patient’s
cell, the stem cells created by the process will be essentially
genetically identical to the patient’s own cells. Then any cells,
tissues, or organs that are grown from these pluripotent stem cells
should not elicit any type of rejection by the patient’s immune
system after they are transplanted into the patient. Thus, somatic
cell nuclear transfer may very well provide a way to generate stem
cells that are perfectly tailored to each individual patient.
There are currently three publicly
traded companies in the United States that are actively engaged in
stem cell research: Geron Corporation (NasdaqNM: GERN), Stem Cells,
Inc. (NasdaqNM: STEM), and Aastrom Biosciences, Inc. (NasdaqNM:
ASTM). While there will undoubtedly be other participants emerging
over the next few years, one or more of these three companies, which
already have experience in the field and which, through patent
protection, have amassed intellectual property portfolios consisting
of various aspects of stem cell technology, may very well turn out
to be among the successful leaders as the stem cell field reaches
the commercialization stage. While it is impossible to know with
certainty which companies involved in an emerging technology will
ultimately be successful, an investment in a portfolio consisting of
shares of Geron Corporation, Stem Cells, Inc., and Aastrom
Biosciences Inc., could turn out to be a very profitable investment,
indeed. And no matter which companies eventually wind up making the
most money, one thing of which we can be certain is that with the
incredible advances that stem cell research will bring to the field
of medicine, we all benefit greatly from this amazing technology.
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