Bone marrow is a spongy tissue found inside bones. The bone marrow in the breast bone, skull, hips, ribs and spine contains stem cells that produce the body's blood cells. These blood cells include white blood cells (leukocytes), which fight infection; red blood cells (erythrocytes), which carry oxygen to and remove waste products from organs and tissues; and platelets, which enable the blood to dot
In patients with leukemia, aplastic anemia, and some immune deficiency diseases, the stem cells in the bone marrow malfunction, producing an excessive number of defective or immature blood cells (in the case of leukemia) or low blood cell counts (in the case of aplastic anemia). The immature or defective blood cells interfere with the production of normal blood cells, accumulate in the bloodstream and may invade other tissues.
Large doses of chemotherapy and/or radiation are required to destroy the abnormal stem cells and abnormal blood cells. These therapies, however, not only kill the abnormal cells but can destroy normal cells found in the bone marrow as well. Similarly, aggressive chemotherapy used to treat some lymphomas and other cancers can destroy healthy bone marrow. A bone marrow transplant enables physicians to treat these diseases with aggressive chemotherapy and/or radiation by allowing replacement of the diseased or damaged bone marrow after the chemotherapy/radiation treatment.
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While bone marrow transplants do not provide 100 percent assurance that the disease will not recur, a transplant can increase the likelihood of a cure or at least prolong the period of disease-free survival for many patients.
TYPES OF TRANSPLANTS
In a bone marrow transplant, the patient's diseased bone marrow is destroyed and healthy marrow is infused into the patient's blood-stream. In a successful transplant, the new bone marrow migrates to the cavities of the large bones, engrafts and begins producing normal blood cells.
If bone marrow from a donor is used, the transplant is called an "allogeneic" BMT, or "syngeneic" BMT if the donor is an identical twin. In an allogeneic BMT, the new bone marrow infused into the patient must match the genetic makeup of the patient's own marrow as perfectly as possible. Special blood tests are conducted to determine whether or not the donor's bone marrow matches the patient's. If the donor's bone marrow is not a good genetic match, it will perceive the patient's body as foreign material to be attacked and destroyed. This condition is known as graft-versus-host disease (GVHD) and can be life-threatening. Alternatively, the patient's immune system may destroy the new bone marrow. This is called graft rejection.
There is a 35 percent chance that a patient will have a sibling whose bone marrow is a perfect match. If the patient has no matched sibling, a donor may be located in one of the international bone marrow donor registries, or a mis-matched or autologous transplant may be considered.
In some cases, patients may be their own bone marrow donors. This is called an autologous BMT and is possible if the disease afflicting the bone marrow is in remission or if the condition being treated does not involve the bone marrow (e.g. breast cancer, ovarian cancer, Hodgkin's disease, non-Hodgkin's lymphoma, and brain tumors). The bone marrow is extracted from the patient prior to transplant and may be "purged" to remove lingering malignant cells (if the disease has afflicted the bone marrow).
I. Introduction: What are stem cells, and why are they important?
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.
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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 pluripotent stem cells (iPSCs), 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 reparative 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.
II. 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:
1. why can embryonic stem cells proliferate for a year or more in the laboratory without differentiating, but most non-embryonic stem cells cannot; and
2. what are the factors in living organisms that normally regulate stem cell proliferation and 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.
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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.
III. What are embryonic stem cells?
A. 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.
B. 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.Â 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.
C. How are embryonic stem cells stimulated to differentiate?
Figure 1. Directed differentiation of mouse embryonic stem cells. Click here for larger image. (Â© 2001 Terese Winslow)
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 differentiation of embryonic stem cells into some specific cell types .
What is cord blood stem cell transplantation?
Blood contained in the placenta and umbilical cord of
newborn babies is emerging as a new source of stem
cells. Cord blood contains significant numbers of stem
cells and has advantages over bone marrow or adult
blood stem cell transplantation for certain patients.
Umbilical cord blood stem cell transplantation has
transformed a waste product of the birth process into a
What diseases may be treated with cord blood stem
The first cord blood stem cell transplant was performed
in 1988 in Paris, France. The patient was a boy with
Fanconi's syndrome (a rare and serious type of
anemia) who is alive and healthy today. Since that first
transplant, cord blood stem cell transplants have been
successfully performed on patients (mostly children)
with severe aplastic anemia, Gunther's disease, Hunter
syndrome, Hurler syndrome, acute lymphocytic
leukemia (ALL), acute myelogenous leukemia (AML),
myelodysplasia, chronic myelogenous leukemia (CML),
juvenile chronic myelogenous leukemia (JCML),
neuroblastoma, non-Hodgkin's lymphoma, thalassemia,
Wiskott-Aldrich syndrome, and X-linked lymphoproliferative
syndrome. To date, more than 800 cord
blood stem cell transplants have been performed
worldwide. Approximately 75% of these have been
done with unrelated donors.
How is cord blood collected, stored, and used for
The most commonly used procedure for collecting cord
blood is relatively simple. Immediately after a baby is
delivered, the umbilical cord is clamped. The baby is
then removed from the area and the placenta is placed
in a sterile supporting structure with the umbilical cord
hanging through the support. The cord is then cleansed
with povidone-iodine (Betadine) and alcohol, and a
needle is inserted into the umbilical vein. Blood is drawn
through the needle into a standard blood collection bag
containing nutrients and a solution to keep the blood
from clotting (anticoagulant solution). Blood is then
collected by gravity drainage, yielding an average 75
milliliters (mL) of blood.
A second method involves collecting the cord blood
after delivery of the child, while the placenta is still in the
mother. Theoretically, this method may be advantageous
by beginning the collection earlier (before the blood has
a chance to clot), and by using the contractions of the
uterus to enhance blood collection. However, the
technique is more intrusive, with the potential to
interfere with the mother's care after delivery.
Theoretically, allogeneic stem cell transplants may be
more successful for patients with certain cancers
because of a lower risk of disease relapse than is the
case with autologous transplants. However, this is
unproven and varies with different disease states.
The advantages of cord blood stem cell transplant
Potential advantages of cord blood stem cell
transplantation over bone marrow transplants include:
â€¢ large potential donor pool;
â€¢ rapid availability, since the cord blood has been
prescreened and tested;
â€¢ greater racial diversity can be attained in the banks by
focusing collection efforts on hospitals where children
of underrepresented ethnic backgrounds are born;
â€¢ no risk or discomfort for the donor;
â€¢ rare contamination by viruses and
â€¢ lower risk of graft-versus-host disease (where the
donor's cells attack the patient's organs and tissues),
even for recipients with a less-than-perfect tissue