Stem Cells have the remarkable potential to develop into many different cell types
in the body during early life and growth. In addition, in many tissues
they serve as a sort of internal repair system, dividing essentially
without limit to replenish other cells as long as the person or animal
is still alive. When a stem cell divides, each new cell has the
potential either to 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.
Stem cells are distinguished from other cell types by two important
characteristics. First, they are unspecialized cells capable of
renewing themselves through cell division, sometimes after long periods
of inactivity. Second, under certain physiologic or experimental
conditions, they can be induced to become tissue- or organ-specific
cells with special functions. In some organs, such as the gut and bone
marrow, stem cells regularly divide to repair and replace worn out or
damaged tissues. In other organs, however, such as the pancreas and the
heart, stem cells only divide under special conditions.
Until recently, scientists primarily worked with two kinds of stem cells from animals and humans: Embryonic Stem Cells and non-embryonic Somatic Or Adult Stem Cells.
The functions and characteristics of these cells will be explained in
this document. Scientists discovered ways to derive embryonic stem
cells from early mouse embryos nearly 30 years ago, in 1981. The
detailed study of the biology of mouse stem cells led to the discovery,
in 1998, of a method to derive stem cells from human embryos and grow
the cells in the laboratory. These cells are called human embryonic stem cells The embryos used in these studies were created for reproductive purposes through in vitro fertilization
procedures. When they were no longer needed for that purpose, they were
donated for research with the informed consent of the donor. In 2006,
researchers made another breakthrough by identifying conditions that
would allow some specialized adult cells to be "reprogrammed"
genetically to assume a stem cell-like state. This new type of stem
cell, called induced pluropotent stem cells will be discussed in a later section of this document. Stem cells are important for living organisms for many reasons. In the 3- to 5-day-old embryo, called a blastocyst,
the inner cells give rise to the entire body of the organism, including
all of the many specialized cell types and organs such as the heart,
lung, skin, sperm, eggs and other tissues. In some adult tissues, such
as bone marrow, muscle, and brain, discrete populations of adult stem
cells generate replacements for cells that are lost through normal wear
and tear, injury, or disease.
Given their unique regenerative abilities, stem cells offer new
potentials for treating diseases such as diabetes, and heart disease.
However, much work remains to be done in the laboratory and the clinic
to understand how to use these cells for cell based therapies to treat disease, which is also referred to as regenerative or repairative medicine. Laboratory studies of stem cells enable scientists to learn about
the cells’ essential properties and what makes them different from
specialized cell types. Scientists are already using stem cells in the
laboratory to screen new drugs and to develop model systems to study
normal growth and identify the causes of birth defects.
Research on stem cells continues to advance knowledge about how an organism develops from a
single cell and how healthy cells replace damaged cells in adult
organisms. Stem cell research is one of the most fascinating areas of
contemporary biology, but, as with many expanding fields of scientific
inquiry, research on stem cells raises scientific questions as rapidly
as it generates new discoveries.
What are the unique properties of all stem cells?
Stem cells differ from other kinds of cells in the body. All stem
cells—regardless of their source—have three general properties: they
are capable of dividing and renewing themselves for long periods; they
are unspecialized; and they can give rise to specialized cell types.
Stem cells are capable of dividing and renewing themselves for long periods.
Unlike muscle cells, blood cells, or nerve cells—which do not normally
replicate themselves—stem cells may replicate many times, or proliferate.
A starting population of stem cells that proliferates for many months
in the laboratory can yield millions of cells. If the resulting cells
continue to be unspecialized, like the parent stem cells, the cells are
said to be capable of long term self renewal.
Scientists are trying to understand two fundamental properties of stem cells that relate to their long term self renewal:
why can embryonic stem cells proliferate for a year or more in the laboratory without differentiating, but most non-embryonic stem cells cannot; and
what are the factors in living organisms that normally regulate stem cell proliferationand self-renewal?
Discovering the answers to these questions may make it possible to
understand how cell proliferation is regulated during normal embryonic
development or during the abnormal cell division
that leads to cancer. Such information would also enable scientists to
grow embryonic and non-embryonic stem cells more efficiently in the
laboratory.
The specific factors and conditions that allow stem cells to remain
unspecialized are of great interest to scientists. It has taken
scientists many years of trial and error to learn to derive and
maintain stem cells in the laboratory without them spontaneously
differentiating into specific cell types. For example, it took two
decades to learn how to grow human embryonic stem cells
in the laboratory following the development of conditions for growing
mouse stem cells. Therefore, understanding the signals in a mature
organism that cause a stem cell population to proliferate and remain
unspecialized until the cells are needed. Such information is critical
for scientists to be able to grow large numbers of unspecialized stem
cells in the laboratory for further experimentation.
Stem cells are unspecialized.One of the fundamental
properties of a stem cell is that it does not have any tissue-specific
structures that allow it to perform specialized functions. For example,
a stem cell cannot work with its neighbors to pump blood through the
body (like a heart muscle cell), and it cannot carry oxygen molecules
through the bloodstream (like a red blood cell). However, unspecialized
stem cells can give rise to specialized cells, including heart muscle
cells, blood cells, or nerve cells.
Stem cells can give rise to specialized cells. When unspecialized stem cells give rise to specialized cells, the process is called differentiation.
While differentiating, the cell usually goes through several stages,
becoming more specialized at each step. Scientists are just beginning
to understand the signals inside and outside cells that trigger each
stem of the differentiation process. The internal signals are controlled by a cell's genes,
which are interspersed across long strands of DNA, and carry coded
instructions for all cellular structures and functions. The external
signals for cell differentiation include chemicals secreted by other
cells, physical contact with neighboring cells, and certain molecules
in the microenvironment. The interaction of signals during differentiation causes the cell's DNA to acquire epigenetic marks that restrict DNA expression in the cell and can be passed on through cell division.
Many questions about stem cell differentiation remain. For example,
are the internal and external signals for cell differentiation similar
for all kinds of stem cells? Can specific sets of signals be identified
that promote differentiation into specific cell types? Addressing these
questions may lead scientists to find new ways to control stem cell
differentiation in the laboratory, thereby growing cells or tissues
that can be used for specific purposes such as cell based therapies or drug screening.
Adult stem cells typically generate the cell types of the tissue in
which they reside. For example, a blood-forming adult stem cell in the
bone marrow normally gives rise to the many types of blood cells. It is
generally accepted that a blood-forming cell in the bone marrow—which
is called a hematopoietic stem cell—cannot
give rise to the cells of a very different tissue, such as nerve cells
in the brain. Experiments over the last several years have purported to
show that stem cells from one tissue may give rise to cell types of a
completely different tissue. This remains an area of great debate
within the research community. This controversy demonstrates the
challenges of studying adult stem cells and suggests that additional
research using adult stem cells is necessary to understand their full
potential as future therapies.
What are embryonic stem cells?
What stages of early embryonic development are important for generating embryonic stem cells?
Embryonic stem cells, as their name suggests, are derived from embryos. Most embryonic stem
cells 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. They are not derived from eggs fertilized in a woman's body. The embryos from which human embryonic stem cells are derived are typically four or five days old and are a hollow microscopic ball of cells called the blastocyst. The blastocyst includes three structures: the trophoblast, which is the layer of cells that surrounds the blastocoel, a hollow cavity inside the blastocyst; and the inner cell mass, which is a group of cells at one end of the blastocoel that develop into the embryo proper.
How are embryonic stem cells grown in the laboratory?
Growing cells in the laboratory is known as cell culture. Human embryonic stem cells are isolated by transferring the inner cell mass into a plastic laboratory culture dish that contains a nutrient broth known as 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 mouse cells in the bottom of the culture dish provide the inner
cell mass cells a sticky surface to which they can attach. Also, the
feeder cells release nutrients into the culture medium. Researchers
have devised ways to grow embryonic stem cells without mouse feeder
cells. This is a significant scientific advance because of the risk
that viruses or other macromolecules in the mouse cells may be
transmitted to the human cells.
The process of generating an embryonic stem cell line is somewhat
inefficient, so lines are not produced each time an inner cell mass is
placed into a culture dish. However, if the plated inner cell mass
cells survive, divide and multiply enough to crowd the dish, they are
removed gently and plated into several fresh culture dishes. The
process of re-plating or subculturing the cells is repeated many times
and for many months. Each cycle of subculturing the cells is referred to as a passage. Once
the cell line is established, the original cells yield millions of
embryonic stem 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.
At any stage in the process, batches of cells can be frozen and shipped
to other laboratories for further culture and experimentation.
What laboratory tests are used to identify embryonic stem cells?
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. This
process is called characterization.
Scientists who study human embryonic stem cells have not yet agreed
on a standard battery of tests that measure the cells' fundamental
properties. However, laboratories that grow human embryonic stem cell
lines use several kinds of tests, including:
Growing and subculturing the stem cells for many months. This ensures
that the cells are capable of long-term growth and self-renewal.
Scientists inspect the cultures through a microscope to see that the
cells look healthy and remain undifferentiated.
Using specific techniques to determine the presence of transcription
factors that are typically produced by undifferentiated cells. Two of
the most important transcription factors are Nanog and Oct4.
Transcription factors help turn genes on and off at the right time, which is an important part of the processes of cell differentiation
and embryonic development. In this case, both Oct 4 and Nanog are
associated with maintaining the stem cells in an undifferentiated
state, capable of self-renewal.
Using specific techniques to determine the presence of paricular cell
surface markers that are typically produced by undifferentiated cells.
Examining the chromosomes under a microscope. This is a method to
assess whether the chromosomes are damaged or if the number of
chromosomes has changed. It does not detect genetic mutations in the
cells.
Determining whether the cells can be re-grown, or subcultured, after freezing, thawing, and re-plating.
Testing whether the human embryonic stem cells are pluripotent by 1)
allowing the cells to differentiate spontaneously in cell culture; 2)
manipulating the cells so they will differentiate to form cells
characteristic of the three germ layers; or 3) injecting the cells into
a mouse with a suppressed immune system to test for the formation of a
benign tumor called a teratoma.
Since the mouse’s immune system is suppressed, the injected human stem
cells are not rejected by the mouse immune system and scientists can
observe growth and differentiation of the human stem cells. Teratomas
typically contain a mixture of many differentiated or partly
differentiated cell types—an indication that the embryonic stem cells
are capable of differentiating into multiple cell types.
How are embryonic stem cells stimulated to differentiate?
As long as the embryonic stem cells in culture are grown under
appropriate 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, it is not an efficient way to produce cultures of
specific cell types.
So, to generate cultures of specific types of differentiated
cells—heart muscle cells, blood cells, or nerve cells, for
example—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. Through years of experimentation, scientists have
established some basic protocols or "recipes" for the directed differentation of embryonic stem cells into some specific cell types
