Download stem cell therapy: an emerging science

STEM CELL THERAPY: AN EMERGING SCIENCE
Muhammad Mujahid Khan, MBBS, PhD
Department of Anatomy, College of Medicine, King Khalid University Hospital, King
Saud University, Riyadh.
Running title: Stem cell, Adult, Embryonic
Key words: Isolation, Plasticity, Surface Marker
Address for correspondence:
Dr. Muhammad Mujahid Khan, MBBS, Ph.D.
Assistant Professor, Department of Anatomy, (28), College of Medicine, King Khalid
University Hospital, King Saud University, P.O. Box 2925. Riyadh 11461. K.S.A. Tel:
009661-4671486. Fax. 009661-4671300. Email: [email protected]
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Objectives: Research on stem cells is advancing knowledge about the development of an
organism from a single cell and to how healthy cells replace damaged cells in adult
organisms. Stem cell therapy is emerging rapidly nowadays as a technical tool for tissue
repair and replacement. The purpose of this review is to provide a frame work of
understanding for the challenges behind translating fundamental stem cell biology and its
potential use into clinical therapies, also to give an overview on stem cell research to the
scientists in Saudi Arabia in general.
Methods: English-language MEDLINE publications from 1980 through January 2007
for experimental, observational and clinical studies having relation with stem cells with
different diseases were reviewed. Approximately 85 publications were reviewed based on
the relevance, strength and quality of design and methods, 36 publications were selected
for inclusion.
Conclusion: Stem cells reside in a specific area of each tissue where they may remain
undivided for several years until they are activated by disease or tissue injury. The
embryonic stem cells are typically derived from four or five day’s old embryos and they
are pluripotent. The adult tissues reported to contain stem cells include brain, bone
marrow, peripheral blood, blood vessels, skeletal muscle, skin and liver.
The promise of stem cell therapies is an exciting one, but significant technical hurdles
remain that will only be overcome through years of intensive research.
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Introduction:
Stem cell is a unique type of a cell that has a great capacity to renew itself and to give rise
to a specialized cell types. Stem cells are uncommitted and remain uncommitted until
they receive a signal to develop into a specialized cell.
Stem cells have the remarkable potential to develop into different cell types in the body.
Serving as a repair system for the body, they can divide without limit to replenish other
cells as long as the person or animal is alive. If a stem cell divides, each new cell has the
potential to either remain a stem cell or become another type of cell with a more
specialized function, such as a muscle cell, a red blood cell, or a brain cell (1).
Research on stem cells is advancing knowledge about the development of an organism
from a single cell and to how healthy cells replace damaged cells in adult organisms (2).
This promising area of science is also leading scientists to investigate the possibility of
cell-based therapies to treat disease.
Important Characteristics of Stem Cells:
Stem cells are distinguished from other types of cells by two important characteristics.
First, they are unspecialized cells that renew themselves for long periods through cell
division. The second is that under certain physiologic or experimental conditions, they
can be induced to become cells with special functions such as the beating cells of the
heart muscle or the insulin-producing cells of the pancreas (3,4).
Unique Properties of Stem Cells Stem cells differ from other kinds of cells in the body
and have three general properties regardless of their source. They are capable of dividing
and renewing themselves for long periods, and are unspecialized and can differentiate
into specialized cell types referred as stem cell plasticity (5).
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Types of Stem Cells:
There are two kinds of stem cells from animals and humans: embryonic stem cells and
adult stem cells, which have different functions and characteristics. Several ways are
discovered to obtain or derive stem cells from early mouse embryos. Many years of
detailed study of the biology of mouse stem cells led to the discovery, of how to isolate
stem cells from human embryos and adults, and grow the cells in the laboratory. The
embryos used in these studies were created for infertility purposes through in vitro
fertilization procedures and when they were no longer needed for that purpose, they were
donated for research with the informed consent of the donor.
Embryonic Stem Cells:
As their name suggests, the embryonic stem cells are derived from embryos, and in
human, specifically, are derived from embryos that develop from eggs that have been
fertilized in vitro—in an in vitro fertilization clinic—and then donated for research
purposes with informed consent of the donors. Unlike in mouse they are not derived from
eggs fertilized in the body. The human embryonic stem cells are typically derived from
four or five days old embryos, and are a hollow microscopic ball of cells called the
blastocyst, and in mice are derived on embryonic day 4.0 (6). The blastocyst includes
three structures: the trophoblast, which is the layer of cells that surrounds the blastocyst;
the blastocoel, which is the hollow cavity inside the blastocyst, and the inner cell mass,
which is a group of approximately 30 cells at one end of the blastocoel (7).
Adult stem cells:
An adult stem cells or somatic stem cells are undifferentiated cell found among
differentiated cells in a tissue or organ, can renew itself, and can differentiate to yield the
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major specialized cell types of the tissue or organ (8). The primary roles of adult stem
cells in a living organism are to maintain and repair the tissue in which they are found 4.
Unlike embryonic stem cells, the origin of adult stem cells in mature tissues is unknown
so far. Research on adult stem cells has recently revealed that they are found in many
more tissues than they once thought possible (9). This finding has led scientists to ask
whether adult stem cells could be used for transplants. In fact, adult blood forming stem
cells from bone marrow have been used in transplants for more than 30 years. Certain
kinds of adult stem cells seem to have the ability to differentiate into a number of
different cell types, given the right conditions (5). If this differentiation of adult stem
cells can be controlled in the laboratory, these cells may become the basis of therapies for
many serious common diseases (10).
It was discovered earlier that the bone marrow contains at least two kinds of stem cells.
First is called hematopoietic stem cells, forms all the types of blood cells in the body.
Second is bone marrow stromal cells which are a mixed cell population that generates
bone, cartilage, fat, and fibrous connective tissue.
Scientists, who were studying rats, discovered two regions of the brain that contained
dividing cells, which become nerve cells (11). Despite these reports, most scientists
believed that new nerve cells could not be generated in the adult brain. It was not until the
1990s that scientists agreed that the adult brain does contain stem cells that are able to
generate the brain's three major cell types—astrocytes and oligodendrocytes, which are
non-neuronal cells, and neurons, or nerve cells (11,12,13).
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Cell culture:
Human or animal embryonic stem cells are isolated by transferring the inner cell mass
into a plastic culture dish that contains a culture medium. The cells divide and spread
over the surface of the dish. The inner surface of the culture dish is typically coated with
mouse embryonic skin cells that have been treated so they will not divide. This coating
layer of cells is called a feeder layer. The reason for having the mouse cells in the bottom
of the culture dish is to give the inner cell mass cells a sticky surface to which they can
attach. Also, the feeder cells release nutrients into the culture medium. Recently,
scientists have begun to devise ways of growing embryonic stem cells without the mouse
feeder cells (12). This is a significant scientific advancement because of the risk that
viruses or other macromolecules in the mouse cells may be transmitted to the human
cells.
Embryonic stem cells that have proliferated in cell culture for six or more months without
differentiating, are pluripotent, and appear genetically normal are referred to as an
embryonic stem cell line.
Once cell lines are established, or even before that stage, batches of them can be frozen
and shipped to other laboratories for further culture and experimentation.
At various points during the process of generating embryonic stem cell lines, scientists
test the cells to see whether they exhibit the fundamental properties that make them
embryonic stem cells (14). This process is called characterization.
Laboratories that grow human or animal embryonic stem cell lines use several kinds of
tests:
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a) Growing and subculturing of stem cells for many months. This ensures that the
cells are capable of long term self renewal.
b) Using specific techniques to determine the presence of surface markers that are
found only on undifferentiated cells. The presence of a protein called Oct-4 that
undifferentiated cells specifically make (15).
c) Examining the chromosomes under a microscope.
d) Determining whether the cells can be subcultured after freezing and replating.
Testing whether the human embryonic stem cells are pluripotent by: (i) allowing the cells
to differentiate spontaneously in cell culture, (ii) manipulating the cells so they will
differentiate to form specific cell types, (iii) injecting the cells into an immunosuppressed
mouse to test for the formation of a benign tumor called a teratoma, an indication that the
embryonic stem cells are capable of differentiating into multiple cell types.
As long as the embryonic stem cells in culture are grown under certain conditions, they
can remain undifferentiated (unspecialized). But if cells are allowed to clump together to
form embryoid bodies, they begin to differentiate spontaneously. They can form muscle
cells, nerve cells, and many other cell types. Although spontaneous differentiation is a
good indication that a culture of embryonic stem cells is healthy.
So, to generate cultures of specific types of differentiated cells eg: heart muscle cells,
blood cells, or nerve cells, scientists try to control the differentiation of embryonic stem
cells. They change the chemical composition of the culture medium, alter the surface of
the culture dish, or modify the cells by inserting specific genes for the directed
differentiation of embryonic stem cells into some specific cell types (16).
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If direct differentiation of embryonic stem cells into specific cell types can reliably be
done, it may be possible to use the resulting, differentiated cells to treat certain diseases
at some point in the future. Diseases that might be treated by transplanting cells generated
from human embryonic and adult stem cells include Parkinson's disease, diabetes,
traumatic spinal cord injury, Purkinje cell degeneration, Duchenne's muscular dystrophy,
heart disease, and vision and hearing loss (17).
Functions of Adult Stem Cells:
A very small number of adult stem cells are identified in many organs and tissues. Stem
cells are thought to reside in a specific area of each tissue where they may remain
undivided for several years until they are activated by disease or tissue injury. The adult
tissues reported to contain stem cells include brain, bone marrow, peripheral blood, blood
vessels, skeletal muscle, skin and liver.
Scientists are trying to grow and manipulate adult stem cells to generate specific cell
types so they can be used to treat injury or disease. Some examples of potential
treatments include replacing the dopamine-producing cells in the brains of Parkinson's
patients, developing insulin-producing cells for type I diabetes (18) and repairing
damaged heart muscle following a heart attack with cardiac muscle cells.
Tests Used to Identify Adult Stem Cells:
One or more of the following three methods are used to identify and test adult stem cells:
(i) Determining the specialized cell types by labeling them in living tissues with
molecular markers (19); (ii) removing the cells from a living animal, labeling them in cell
culture, and transplanting them back into another animal to determine whether the cells
repopulate their tissue of origin; and (iii) isolating, growing, and manipulating cells, by
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adding growth factors or introducing new genes, to determine what differentiated cell
types they can become. Certain adult stem cells are pluripotent and may also exhibit the
ability to form specialized cell types of other tissues, which is known as
transdifferentiation or plasticity (5).
Also, a single adult stem cell should be able to generate a line of genetically identical
cells, known as a clone, which then gives rise to all the appropriate differentiated cell
types of the tissue (20). Scientists tend to show either that a stem cell can give rise to a
clone of cells in cell culture, or that a purified population of candidate stem cells can
repopulate the tissue after transplant into an animal (21). Recently, by infecting adult
stem cells with a virus that gives a unique identifier to each individual cell, scientists
have been able to demonstrate that individual adult stem cell clones have the ability to
repopulate injured tissues in a living animal.
Current research is aimed at determining the mechanisms that underlie adult stem cell
plasticity. If such mechanisms can be identified and controlled, existing stem cells from a
healthy tissue might be induced to repopulate and repair a diseased tissue (22).
Similarities and Differences between Embryonic and Adult Stem Cells:
Human embryonic and adult stem cells each have advantages and disadvantages
regarding potential use for cell-based regenerative therapies. Of course, adult and
embryonic stem cells differ in the number and type of differentiated cell types they can
become. Embryonic stem cells can become all cell types of the body because they are
pluripotent. Adult stem cells are generally limited to differentiating into different cell
types of their tissue of origin. However, some evidence suggests that adult stem cell
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plasticity may exist; increasing the number of cell types a given adult stem cell can
become (23).
Large numbers of embryonic stem cells can be relatively easily grown in culture, while
adult stem cells are rare in mature tissues and methods for expanding their numbers in
cell culture have not yet been worked out (24). This is an important distinction, as large
numbers of cells are needed for stem cell replacement therapies.
A potential advantage of using stem cells from an adult is that the patient's own cells
could be expanded in culture and then reintroduced into the patient (25,26). The use of
the patient's own adult stem cells would mean that the cells would not be rejected by the
immune system. This represents a significant advantage as immune rejection is a difficult
problem that can only be circumvented with immunosuppressive drugs.
Embryonic stem cells from a donor introduced into a patient could cause transplant
rejection. However, whether the recipient would reject donor embryonic stem cells has
not been determined in human experiments.
Potential uses of human stem cells and the hurdles that must be overcome:
There are many ways in which human stem cells can be used in basic and clinical
researches. However, there are many technical hurdles between the promise of stem cells
and the realization of these uses, which will only be overcome by continued intensive
stem cell research.
Studies of human embryonic stem cells may yield information about the complex events
that occur during human development. A primary goal of these studies is to identify how
undifferentiated stem cells become differentiated. Some of the most serious medical
conditions, such as cancer and birth defects, are due to abnormal cell division and
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differentiation. A better understanding of the genetic and molecular controls of these
processes may yield information about how such diseases arise and suggest new
strategies for therapy. A significant hurdle to this use and most uses of stem cells is that
scientists do not yet fully understand the signals that turn specific genes on and off to
influence the differentiation of the stem cell.
Human stem cells could also be used to test new drugs. For example, new medications
could be tested for safety on differentiated cells generated from human pluripotent cell
lines. Other kinds of cell lines are already used in this way. Cancer cell lines, for
example, are used to screen potential anti-tumor drugs. But, the availability of pluripotent
stem cells would allow drug testing in a wider range of cell types. However, to screen
drugs effectively, the conditions must be identical when comparing different drugs.
Therefore, scientists will have to be able to precisely control the differentiation of stem
cells into the specific cell type on which drugs will be tested.
Perhaps the most important potential application of human stem cells is the generation of
cells and tissues that could be used for cell-based therapies (27). Today, donated organs
and tissues are often used to replace ailing or destroyed tissue, but the need for
transplantable tissues and organs far outweighs the available supply (28). Stem cells,
directed to differentiate into specific cell types, offer the possibility of a renewable source
of replacement cells and tissues to treat diseases including Parkinson's and Alzheimer's
diseases, spinal cord injury, stroke, burns, heart disease, diabetes, osteoarthritis, and
rheumatoid arthritis (29,30).
For example, it may become possible to generate healthy heart muscle cells in the
laboratory and then transplant those cells into patients with chronic heart disease (31,32).
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Preliminary research in mice and other animals indicates that bone marrow stem cells,
transplanted into a damaged heart, can generate heart muscle cells and successfully
repopulate the heart tissue (33). Other recent studies in cell culture systems indicate that
it may be possible to direct the differentiation of embryonic stem cells or adult bone
marrow cells into heart muscle cells or into hepatocytes (34).
In people who suffer from type I diabetes, the cells of the pancreas that normally produce
insulin are destroyed by the patient's own immune system. New studies indicate that it
may be possible to direct the differentiation of human embryonic stem cells in cell culture
to form insulin-producing cells that eventually could be used in transplantation therapy
for diabetics (18).
To realize the promise of novel cell-based therapies for such pervasive and debilitating
diseases, scientists must be able to easily and reproducibly manipulate stem cells so that
they possess the necessary characteristics for successful differentiation, transplantation
and engraftment (35). For the successful cell-based treatments, scientists will have to
learn to precisely control to bring such treatments to the clinic. To be useful for transplant
purposes, stem cells must be reproducibly made to:
a) Proliferate extensively and generate sufficient quantities of tissue.
b) Differentiate into the desired cell type(s).
c) Survive in the recipient after transplant.
d) Integrate into the surrounding tissue after transplant.
e) Function appropriately for the duration of the recipient's life.
f) Avoid harming the recipient in any way.
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Also, to avoid the problem of immune rejection, scientists are experimenting with
different research strategies to generate tissues that will not be rejected (36).
Conclusion:
To summarize, the promise of stem cell therapies is an exciting one, but significant
technical hurdles remain that will only be overcome through years of intensive research.
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Acknowledgements
The author is thankful to Dr. Hasem H. Darwish for the critical review of the manuscript.
He is also indebted to Dr. Sultan Ayub Meo for technical assistance
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