Stem cellsÂ 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:Â embryonic stem cellsÂ and non-embryonicÂ "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 calledÂ human embryonic stem cells. The embryos used in these studies were created for reproductive purposes throughÂ in vitrofertilizationÂ 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, calledÂ 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 aÂ 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 forÂ cell-based therapiesÂ to treat disease, which is also referred to asÂ 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 onÂ stem cellsÂ 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, orÂ 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 ofÂ long-term self-renewal.
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Scientists are trying to understand two fundamental properties of stem cells that relate to theirÂ long-term self-renewal:
why canÂ embryonic stem cellsÂ proliferate for a year or more in the laboratory without differentiating, but mostÂ non-embryonic stem cellsÂ cannot; and
what are the factors in living organisms that normally regulate stem cellproliferationÂ 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 abnormalÂ cell divisionÂ 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 growÂ human embryonic stem cellsÂ 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.Â 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 calledÂ 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 internalÂ signalsÂ are controlled by a cell'sÂ 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 theÂ microenvironment. The interaction of signals during differentiation causes the cell's DNA to acquireÂ epigeneticÂ 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 asÂ cell-based therapiesÂ 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 aÂ 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 fertilizedÂ in vitro-in anÂ in vitroÂ fertilizationÂ clinic-and then donated for research purposes with informed consent of the donors. They areÂ notÂ derived from eggs fertilized in a woman's body.
B. How are embryonic stem cells grown in the laboratory?
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Growing cells in the laboratory is known asÂ cell culture. Human embryonic stem cells (hESCs) areÂ generated by transferringÂ cells from aÂ preimplantation-stage embryoÂ into a plastic laboratory culture dish that contains a nutrient broth known asÂ 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 aÂ feeder layer. The mouse cells in the bottom of the culture dish provide the 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 cells from the preimplantation-stage embryo are placed into a culture dish. However, if the plated 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 ofÂ subculturingÂ the cells is referred to as aÂ 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 for a prolonged period of time without differentiating, areÂ pluripotent, andÂ have not developed genetic abnormalitiesÂ are referred to as anÂ 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. 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 remainundifferentiated.
Using specific techniques to determine the presence of transcription factors that are typically produced by undifferentiated cells. Two of the mostÂ important transcription factors are Nanog and Oct4. Transcription factors help turnÂ genesÂ on and off at the right time, which is an important part of the processes of cellÂ differentiationÂ 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 aÂ 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.
D. How are embryonic stem cells stimulated to differentiate?
Directed differentiation of mouse embryonic stem cells. This figure is a flow chart showing the steps scientists take to isolate and differentiate mouse embryonic stem cells. A mouse blastocyst is shown in the upper left, with its inner cell mass (ICM) labeled. Arrows indicate removal of the ICM and plating in a tissue culture dish, labeled as "undifferentiated embryonic stem cells." The next arrow indicates the passage of time and shows that the cells in the plate have now become embryoid bodies. From this culture dish, an arrow indicates that the next step is "induce initial differentiation and select precursors." Next, two arrows show two possible fates, and the label underneath indicates that the scientists "expand precursors." The two possible precursor types are "neuronal precursors" or "pancreatic precursors." The final step indicates "complete differentiation to generate functional cells." The bottom left shows a fluorescently labeled microscope image of "dopamine- and serotonin-secreting neurons" and the bottom right shows a fluorescently labeled microscope image of "insulin-secreting pancreatic islet-like clusters."
Figure 1. Directed differentiation of mouse embryonic stem cells.Â Click hereÂ 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 formÂ 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 thedirected differentiationÂ of embryonic stem cells into some specific cell types (Figure 1). (For additional examples of directed differentiation of embryonic stem cells, refer to the NIH stem cell reports available atÂ /info/2006report/Â andÂ /info/2001report/2001report.htm.)
If scientists can reliably direct the differentiation of embryonic stem cells into specific cell types, they may be able to use the resulting, differentiated cells to treat certain diseases in the future. Diseases that might be treated by transplanting cells generated from human embryonic stem cells includeÂ Parkinson's disease, diabetes, traumatic spinal cord injury,Duchenne's muscular dystrophy, heart disease, and vision and hearing loss.
IV. What are adult stem cells?
An adult stem cell is thought to be anÂ undifferentiatedÂ cell, found among differentiated cells in a tissue or organ that can renew itself and can differentiate to yield some or all of the major specialized cell types of the tissue or organ. The primary roles ofÂ adult stem cellsÂ in a living organism are to maintain and repair the tissue in which they are found. Scientists also use the termÂ somatic stem cellÂ instead of adult stem cell, where somatic refers to cells of the body (not the germ cells, sperm or eggs). Unlike embryonic stem cells, which are defined by their origin (cells from theÂ preimplantation-stage embryo), the origin of adult stem cells in some mature tissues is still under investigation.
Research on adult stem cells has generated a great deal of excitement. Scientists have found adult stem cells in many more tissues than they once thought possible. This finding has led researchers and clinicians to ask whether adult stem cells could be used for transplants. In fact, adult hematopoietic, or blood-forming, stem cells from bone marrow have been used in transplants for 40 years. Scientists now have evidence that stem cells exist in the brain and the heart. If the differentiation of adult stem cells can be controlled in the laboratory, these cells may become the basis of transplantation-based therapies.
The history of research on adult stem cells began about 50 years ago. In the 1950s, researchers discovered that the bone marrow contains at least two kinds of stem cells. One population, calledÂ hematopoietic stem cells, forms all the types of blood cells in the body. A second population, calledÂ bone marrow stromal stem cellsÂ (also calledmesenchymal stem cells, or skeletal stem cells by some), were discovered a few years later. These non-hematopoietic stem cells make up a small proportion of theÂ stromal cellpopulation in the bone marrow, and can generate bone, cartilage, fat, cells that support the formation of blood, and fibrous connective tissue.
In the 1960s, scientists who were studying rats discovered two regions of the brain that contained dividing cells that ultimately become nerve cells. Despite these reports, most scientists believed that the adult brain could not generate new nerve cells. 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-astrocytesÂ andoligodendrocytes, which are non-neuronal cells, andÂ neurons, or nerve cells.
A. Where are adult stem cells found, and what do they normally do?
Adult stem cells have been identified in many organs and tissues, including brain, bone marrow, peripheral blood, blood vessels, skeletal muscle, skin, teeth, heart, gut, liver, ovarian epithelium, and testis. They are thought to reside in a specific area of each tissue (called a "stem cell niche"). In many tissues, current evidence suggests that some types of stem cells are pericytes, cells that compose the outermost layer of small blood vessels. Stem cells may remain quiescent (non-dividing) for long periods of time until they are activated by a normal need for more cells to maintain tissues, or by disease or tissue injury.
Typically, there is a very small number of stem cells in each tissue, and once removed from the body, their capacity to divide is limited, making generation of large quantities of stem cells difficult. Scientists in many laboratories are trying to find better ways to grow large quantities of adult stem cells inÂ cell cultureÂ and to manipulate them to generate specific cell types so they can be used to treat injury or disease. Some examples of potential treatments include regenerating bone using cells derived from bone marrow stroma, developing insulin-producing cells for typeÂ 1 diabetes, and repairing damaged heart muscle following a heart attack with cardiac muscle cells.
B. What tests are used for identifying adult stem cells?
Scientists often use one or more of the following methods to identify adult stem cells: (1) label the cells in a living tissue with molecular markers and then determine the specialized cell types they generate; (2) remove the cells from a living animal, label them in cell culture, and transplant them back into another animal to determine whether the cells replace (or "repopulate") their tissue of origin.
Importantly, it must be demonstrated that a single adult stem cell can generate a line of genetically identical cells that then gives rise to all the appropriate differentiated cell types of the tissue. To confirm experimentally that a putative adult stem cell is indeed a stem cell, scientists tend to show either that the cell can give rise to these genetically identical cells in culture, and/or that a purified population of these candidate stem cells can repopulate or reform the tissue after transplant into an animal.
C. What is known about adult stem cell differentiation?
"Hematopoietic and stromal cell differentiation." The figure shows a long bone, with marrow in its center and an enlargement of the bone/marrow interface in a boxed inset, with cell types identified. Cell types shown include the osteocytes embedded in the noncellular bone matrix, the osteoclast, pericytes around tiny blood vessels, adipocytes, and stromal cells. Using arrows, the artist has drawn illustrations of the lineages of marrow and stromal cells. Marrow lineage: a hematopoietic stem cell gives rise to a multipotent stem cell, which can divide to produce one of two possible cell types: (1) a myeloid progenitor cell, which is capable of producing neutrophils, basophils, eosinophils, monocytes/macrophages, platelets, and red blood cells or (2) a lymphoid progenitor cell, which gives rise to natural killer (NK) cells, T lymphocytes, and B lymphocytes. Stromal lineage: a stromal stem cell gives rise to bone cells, including pre-osteoblasts, osteoblasts, lining cells, and osteocytes. The artist has also indicated two other cell types that the bone marrow may be capable of producing: skeletal muscle stem cells, and hepatocyte stem cells. Each possible lineage is followed by a question mark, to indicate that scientists do not agree whether or not bone marrow is capable of producing these two cell types.
Figure 2. Hematopoietic and stromal stem cell differentiation.Â Click hereÂ for larger image. (Â© 2001 Terese Winslow)
As indicated above, scientists have reported that adult stem cells occur in many tissues and that they enter normalÂ differentiationÂ pathways to form the specialized cell types of the tissue in which they reside.
Normal differentiation pathways of adult stem cells.Â In a living animal, adult stem cells are available to divide, when needed, and can give rise to mature cell types that have characteristic shapes and specialized structures and functions of a particular tissue. The following are examples of differentiation pathways of adult stem cells (Figure 2) that have been demonstratedÂ in vitroÂ orÂ in vivo.
Hematopoietic stem cells give rise to all the types of blood cells: red blood cells, B lymphocytes, T lymphocytes, natural killer cells, neutrophils, basophils, eosinophils, monocytes, and macrophages.
Mesenchymal stem cellsÂ give rise to a variety of cell types: bone cells (osteocytes), cartilage cells (chondrocytes), fat cells (adipocytes), and other kinds of connective tissue cells such as those in tendons.
Neural stem cellsÂ in the brain give rise to its three major cell types: nerve cells (neurons) and two categories of non-neuronal cells-astrocytes andoligodendrocytes.
Epithelial stem cells in the lining of the digestive tract occur in deep crypts and give rise to several cell types: absorptive cells, goblet cells, paneth cells, and enteroendocrine cells.
Skin stem cells occur in the basal layer of the epidermis and at the base of hair follicles. The epidermal stem cells give rise to keratinocytes, which migrate to the surface of the skin and form a protective layer. The follicular stem cells can give rise to both the hair follicle and to the epidermis.
Transdifferentiation.Â A number of experiments have reported that certain adult stem cell types can differentiate into cell types seen in organs or tissues other than those expected from the cells' predicted lineage (i.e., brain stem cells that differentiate into blood cells or blood-forming cells that differentiate into cardiac muscle cells, and so forth). This reported phenomenon is called transdifferentiation.
Although isolated instances of transdifferentiation have been observed in some vertebrate species, whether this phenomenon actually occurs in humans is under debate by the scientific community. Instead of transdifferentiation, the observed instances may involve fusion of a donor cell with a recipient cell. Another possibility is that transplanted stem cells are secreting factors that encourage the recipient's own stem cells to begin the repair process. Even when transdifferentiation has been detected, only a very small percentage of cells undergo the process.
In a variation of transdifferentiation experiments, scientists have recently demonstrated that certain adult cell types can be "reprogrammed" into other cell types in vivo using a well-controlled process of genetic modification (see Section VI for a discussion of the principles of reprogramming). This strategy may offer a way to reprogram available cells into other cell types that have been lost or damaged due to disease. For example, one recent experiment shows how pancreatic beta cells, the insulin-producing cells that are lost or damaged in diabetes, could possibly be created by reprogramming other pancreatic cells. By "re-starting" expression of three critical beta-cell genes in differentiated adult pancreatic exocrine cells, researchers were able to create beta cell-like cells that can secrete insulin. The reprogrammed cells were similar to beta cells in appearance, size, and shape; expressed genes characteristic of beta cells; and were able to partially restore blood sugar regulation in mice whose own beta cells had been chemically destroyed. While not transdifferentiation by definition, this method for reprogramming adult cells may be used as a model for directly reprogramming other adult cell types.
In addition to reprogramming cells to become a specific cell type, it is now possible to reprogram adult somatic cells to become like embryonic stem cells (induced pluripotent stem cells, iPSCs) through the introduction of embryonic genes. Thus, a source of cells can be generated that are specific to the donor, thereby avoiding issues of histocompatibility, if such cells were to be used for tissue regeneration. However, like embryonic stem cells, determination of the methods by which iPSCs can be completely and reproducibly committed to appropriate cell lineages is still under investigation.
D. What are the key questions about adult stem cells?
Many important questions about adult stem cells remain to be answered. They include:
How many kinds of adult stem cells exist, and in which tissues do they exist?
How do adult stem cells evolve during development and how are they maintained in the adult? Are they "leftover" embryonic stem cells, or do they arise in some other way?
Why do stem cells remain in an undifferentiated state when all the cells around them have differentiated? What are the characteristics of their "niche" that controls their behavior?
Do adult stem cells have the capacity to transdifferentiate, and is it possible to control this process to improve its reliability and efficiency?
If the beneficial effect of adult stem cell transplantation is a trophic effect, what are the mechanisms? Is donor cell-recipient cell contact required, secretion of factors by the donor cell, or both?
What are the factors that control adult stem cell proliferation and differentiation?
What are the factors that stimulate stem cells to relocate to sites of injury or damage, and how can this process be enhanced for better healing?
V. What are the similarities and differences between embryonic and adult stem cells?
Human embryonicÂ andÂ adult stem cellsÂ each have advantages and disadvantages regarding potential use forÂ cell-based regenerative therapies. One major difference between adult and embryonic stem cells is their different abilities in the number and type of differentiated cell types they can become.Â Embryonic stem cellsÂ can become all cell types of the body because they areÂ pluripotent. Adult stem cells are thought to be limited to differentiating into different cell types of their tissue of origin.
Embryonic stem cells can be grown relatively easily in culture. Adult stem cells are rare in mature tissues, so isolating these cells from an adult tissue is challenging, and methods to expand their numbers inÂ cell cultureÂ have not yet been worked out. This is an important distinction, as large numbers of cells are needed for stem cell replacement therapies.
Scientists believe that tissues derived from embryonic and adult stem cells may differ in the likelihood of being rejected after transplantation. We don't yet know whether tissues derived from embryonic stem cells would cause transplant rejection, sinceÂ the first phase 1 clinical trialÂ testing the safety of cells derived from hESCS has only recently been approved by the United States Food and Drug Administration (FDA).
Adult stem cells, and tissues derived from them, are currently believed less likely to initiate rejection after transplantation. This is because a patient's own cells could be expanded in culture, coaxed into assuming a specific cell type (differentiation), and then reintroduced into the patient. The use of adult stem cells and tissues derived from the patient's own adult stem cells would mean that the cells are less likely to be rejected by the immune system. This represents a significant advantage, as immune rejection can be circumvented only by continuous administration of immunosuppressive drugs, and the drugs themselves may cause deleterious side effects
VI. What are induced pluripotent stem cells?
Induced pluripotent stem cells (iPSCs)Â are adult cells that have been genetically reprogrammed to an embryonic stem cell-like state by being forced to express genes and factors important for maintaining the defining properties of embryonic stem cells. Although these cells meet the defining criteria for pluripotent stem cells, it is not known if iPSCs and embryonic stem cells differ in clinically significant ways. Mouse iPSCs were first reported in 2006, and human iPSCs were first reported in late 2007. Mouse iPSCs demonstrate important characteristics of pluripotent stem cells, including expressing stem cell markers, forming tumors containing cells from all three germ layers, and being able to contribute to many different tissues when injected into mouse embryos at a very early stage in development. Human iPSCs also express stem cell markers and are capable of generating cells characteristic of all threeÂ germ layers.
Although additional research is needed, iPSCs are already useful tools for drug development and modeling of diseases, and scientists hope to use them in transplantation medicine. Viruses are currently used to introduce the reprogramming factors into adult cells, and this process must be carefully controlled and tested before the technique can lead to useful treatments for humans. In animal studies, the virus used to introduce the stem cell factors sometimes causes cancers. Researchers are currently investigating non-viral delivery strategies. In any case, this breakthrough discovery has created a powerful new way to "de-differentiate" cells whose developmental fates had been previously assumed to be determined. In addition, tissues derived from iPSCs will be a nearly identical match to the cell donor and thus probably avoid rejection by the immune system. The iPSC strategy creates pluripotent stem cells that, together with studies of other types of pluripotent stem cells, will help researchers learn how to reprogram cells to repair damaged tissues in the human body.
VII. What are the potential uses of human stem cells and the obstacles that must be overcome before these potential uses will be realized?
There are many ways in which human stem cells can be used in research and the clinic. Studies ofÂ human embryonic stem cellsÂ will yield information about the complex events that occur during human development. A primary goal of this work is to identify howundifferentiatedÂ stem cells become the differentiated cells that form the tissues and organs. Scientists know that turningÂ genesÂ on and off is central to this process. Some of the most serious medical conditions, such as cancer and birth defects, are due to abnormalÂ cell divisionÂ andÂ differentiation. A more complete understanding of the genetic and molecular controls of these processes may yield information about how such diseases arise and suggest new strategies for therapy. Predictably controlling cell proliferation and differentiation requires additional basic research on the molecular and genetic signals that regulate cell division and specialization. While recent developments with iPS cells suggest some of the specific factors that may be involved, techniques must be devised to introduce these factors safely into the cells and control the processes that are induced by these factors.
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 humanÂ pluripotentÂ 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. 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. Current knowledge of the signals controlling differentiation falls short of being able to mimic these conditions precisely to generate pure populations of differentiated cells for each drug being tested.
Perhaps the most important potential application of human stem cells is the generation of cells and tissues that could be used forÂ cell-based therapies. 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. 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 Alzheimer's diseases, spinal cord injury, stroke, burns, heart disease, diabetes, osteoarthritis, and rheumatoid arthritis.
Heart muscle repair with adult stem cells. This figure is divided into two panels, with each illustrating a possible means by which adult stem cells could help regenerate damaged heart muscle. On the left, a mouse heart is being injected with a syringe of green-labeled adult stem cells. Next, a magnifying glass shows a close-up of the damaged heart muscle cells (greyish-black) next to an area of healthy heart muscle (pink). Arrows indicate that the adult stem cells are intermingling with the heart muscle fibers. On the right, a mouse is shown being injected in the tail blood vessels with a syringe of pink human bone marrow stem cells. The magnifying glass in this panel again shows a close-up of the damaged heart muscle cells (greyish-black) next to an area of healthy heart muscle (pink). The pink human bone marrow stem cells intermingle with the heart muscle fibers and the text indicates that they induce new blood vessel formation in the damaged heart muscle and also cause proliferation of existing heart blood vessels.
Figure 3. Strategies to repair heart muscle withÂ adult stem cells.Â Click hereÂ for larger image.
Â© 2001 Terese Winslow
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. Preliminary research in mice and other animals indicates that bone marrow stromal cells, transplanted into a damaged heart, can have beneficial effects. Whether these cells can generate heart muscle cells or stimulate the growth of new blood vessels that repopulate the heart tissue, or help via some other mechanism is actively under investigation. For example, injected cells may accomplish repair by secreting growth factors, rather than actually incorporating into the heart. Promising results from animal studies have served as the basis for a small number of exploratory studies in humans (for discussion, see call-out box, "Can Stem Cells Mend a Broken Heart?"). Other recent studies inÂ cell culturesystems indicate that it may be possible to direct theÂ differentiationÂ of embryonic stem cells or adult bone marrow cells into heart muscle cells (Figure 3).
Can Stem Cells Mend a Broken Heart?: Stem Cells for the Future Treatment of Heart Disease
Cardiovascular disease (CVD), which includes hypertension, coronary heart disease, stroke, and congestive heart failure, has ranked as the number one cause of death in the United States every year since 1900 except 1918, when the nation struggled with an influenza epidemic. Nearly 2600 Americans die of CVD each day, roughly one person every 34 seconds. Given the aging of the population and the relatively dramatic recent increases in the prevalence of cardiovascular risk factors such as obesity and type 2 diabetes, CVD will be a significant health concern well into the 21st century.
Cardiovascular disease can deprive heart tissue of oxygen, thereby killing cardiac muscle cells (cardiomyocytes). This loss triggers a cascade of detrimental events, including formation of scar tissue, an overload of blood flow and pressure capacity, the overstretching of viable cardiac cells attempting to sustain cardiac output, leading to heart failure, and eventual death. Restoring damaged heart muscle tissue, through repair or regeneration, is therefore a potentially new strategy to treat heart failure.
The use of embryonic and adult-derived stem cells for cardiac repair is an active area of research. A number of stem cell types, including embryonic stem (ES) cells, cardiac stem cells that naturally reside within the heart, myoblasts (muscle stem cells), adult bone marrow-derived cells including mesenchymal cells (bone marrow-derived cells that give rise to tissues such as muscle, bone, tendons, ligaments, and adipose tissue), endothelial progenitor cells (cells that give rise to the endothelium, the interior lining of blood vessels), and umbilical cord blood cells, have been investigated as possible sources for regenerating damaged heart tissue. All have been explored in mouse or rat models, and some have been tested in larger animal models, such as pigs.
A few small studies have also been carried out in humans, usually in patients who are undergoing open-heart surgery. Several of these have demonstrated that stem cells that are injected into the circulation or directly into the injured heart tissue appear to improve cardiac function and/or induce the formation of new capillaries. The mechanism for this repair remains controversial, and the stem cells likely regenerate heart tissue through several pathways. However, the stem cell populations that have been tested in these experiments vary widely, as do the conditions of their purification and application. Although much more research is needed to assess the safety and improve the efficacy of this approach, these preliminary clinical experiments show how stem cells may one day be used to repair damaged heart tissue, thereby reducing the burden of cardiovascular disease.
In people who suffer from typeÂ 1 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 persons with diabetes.
To realize the promise of novel cell-based therapies for such pervasive and debilitating diseases, scientists must be able to manipulate stem cells so that they possess the necessary characteristics for successful differentiation, transplantation, and engraftment. The following is a list of steps in successful cell-based treatments that scientists will have to learn to control to bring such treatments to the clinic. To be useful for transplant purposes, stem cells must be reproducibly made to:
Proliferate extensively and generate sufficient quantities of tissue.
Differentiate into the desired cell type(s).
Survive in the recipient after transplant.
Integrate into the surrounding tissue after transplant.
Function appropriately for the duration of the recipient's life.
Avoid harming the recipient in any way.
Also, to avoid the problem of immune rejection, scientists are experimenting with different research strategies to generate tissues that will not be rejected.
To summarize, stem cells offer exciting promise for future therapies, but significant technical hurdles remain that will only be overcome through years of intensive research.
VIII. Where can I get more information?
For a more detailed discussion of stem cells, see theÂ NIH's Stem Cell Reports. Check the Frequently Asked QuestionsÂ page for quick answers to specific queries. The navigation table at right can connect you to the information you need.
The following websites, which are not part of the NIH Stem Cell Information site, also contain information about stem cells. The NIH is not responsible for the content of these sites.
Stem cell information for the public from the International Society for Stem Cell Research (ISSCR).
Medline Plus is a consumer health database that includes news, health resources, clinical trials, and more
A United Kingdom-based resource for the general public that discusses the use of stem cells in medical treatments and therapies.
A commercial, online newsletter that features stories about stem cells of all types.