Stem cells are defined by two characteristics. They have both the capacity to self-renew, that is, divide and give rise to more stem cells, and to differentiate, that is, to give rise to mature, specialized cells that make up our tissues and organs.
Understanding Stem Cells
Stem cells are defined by two characteristics. They have both the capacity to self-renew, that is, divide and give rise to more stem cells, and to differentiate, that is, to give rise to mature, specialized cells that make up our tissues and organs.
There are many different types of stem cells that come from different places in the body or are formed at different times in our lives. These include embryonic stem cells (see FAQ 3) that exist only at the earliest stages of development and various types of ‘tissue-specific’ or ‘adult’ stem cells (see FAQ 4) that appear during fetal development and remain in our bodies throughout life. In addition, scientists have recently been able to create induced pluripotent stem cells or iPS cells (see FAQ 5) in the laboratory. These cells, which are not found in the body but rather are engineered from specialized cells such as those from the skin, have properties similar to those of of embryonic stem cells. For more information, see FAQ 3-5.
Embryonic stem cells are pluripotent stem cells, meaning they can give rise to all cell types of the body. They can be grown indefinitely in vitro if the correct conditions are met, or if given the right cues, can be coaxed to differentiate to a variety of mature, specialized cell types. This makes them very valuable for regenerative medicine. A major focus of research is to find ways to generate cells and tissues from embryonic stem cells that can be used to test new drugs or to replace damaged organs in patients.
Embryonic stem cells are obtained from a very early stage in development, usually the blastocyst stage, which in the human forms about 5 days after fertilization of an egg. A blastocyst is a mainly hollow ball, barely visible to the naked eye. Inside is a clump of approximately 150 cells, called the inner cell mass, that would eventually form the entire body of the developing animal. Embryonic stem cells are formed by removing the cells from the inner cell mass and growing them in culture.
Mouse embryonic stem cells, first isolated in 1981, are the most widely studied. They have taught us a lot about how pluripotent cells grow and specialize, and how early development works. Mouse embryonic stem cells can be manipulated to contain specific genetic changes and then used to generate mice, which contain this change. This has led to the discovery of many genes associated with different human diseases and helped us understand how these diseases develop. Capecchi, Evans and Smithies were awarded the Nobel Prize in Physiology or Medicine, 2007 for developing this process. Read more.
Human embryonic stem cells were isolated relatively recently, in 1998. They are more difficult to work with than their mouse counterparts and currently less is known about them. However, scientists are making remarkable progress, learning about human developmental processes, modeling disease and establishing strategies that could ultimately lead to new medical treatments (read more in FAQ 7).
Many tissues contain stem cells that can replace cells that die or restore tissue after injury. Skin, muscle, intestine and bone marrow, for example, each contain their own stem cells. In the bone marrow, billions of new blood cells are made every day from blood-forming stem cells. It is not clear whether all adult organs, such as the heart, contain stem cells.
‘Adult’ or ‘tissue-specific’ stem cells are multipotent, meaning they can give rise to a limited number of mature cell types, usually corresponding to the tissues in which they reside. Some tissue-specific stem cells can only give rise to one or two mature cell types. These cells are called unipotent and bipotent, respectively. It has been proposed that some adult stem cells may have the potential to form cell types from a broader range of tissues than that from which they originate, however, this is highly controversial in the scientific community.
Tissue-specific stem cells are rare and are often difficult to isolate and grow in culture. The best-studied are the hematopoietic stem cells or blood-forming stem cells, that reside in the bone marrow and continuously produce and replace all blood cells. In addition, the bone marrow contains mesenchymal stem cells, which can give rise to cartilage, fat cells and bone.
The term ‘adult stem cells’ is often used very broadly and may include fetal and cord blood stem cells.
Induced pluripotent cells (iPS cells) are non-pluripotent cells that were induced to become pluripotent, that is, able to form all cell types of the body. In other words, a cell with a specialized function (for example, a skin cell) was ‘reprogrammed’ to an unspecialized state similar to that of an embryonic stem cell. While iPS cells and embryonic stem cells share many characteristics they are not identical.
The generation of mouse iPS cells was reported in 2006 (read the ‘Briefing’), and the generation of human iPS cells at the end of 2007 (read the ‘Briefing’).
Typically, iPS cells are produced by inserting copies of three or four genes known to be important in embryonic stem cells using viruses into fully mature cells, such as skin cells or immune cells. Different research groups have used slightly different combinations of genes and different methods to get them into the cell. It is not completely understood how each of these genes functions to confer pluripotency and ongoing research is addressing this question.
IPS cells hold great promise for creating patient- and disease-specific cell lines for research purposes. A great deal of work remains before these methods can be used to generate stem cells suitable for safe and effective therapies.
At this stage iPS cells cannot replace embryonic stem cells. Although the derivation of human iPS cells opens up exciting new areas of stem cell research, this technology is at a very early stage and many fundamental questions remain unanswered. While iPS cells and embryonic stem cells share many characteristics they are not identical. The similarities and differences are still being explored. Furthermore, genetic manipulation of cells is required to generate iPS cells. The long-term consequences of this manipulation, in particular when transplanted into a patient, are unknown. For these reasons, it is essential that both embryonic stem cells and iPS cells are studied.
Cancer stem cells are defined as a subpopulation of cancer cells that presented a greater renewal potential compared to the other tumor cells and the ability to reform all the characteristics of the parental tumor upon transplantation into immunodeficient mice. Cancer stem cells have been postulated to be responsible for tumor growth and tumor relapse after therapy. Many different types of human cancer contain cancer stem cells. However, in some advanced cancers, the proportion of cells with cancer stem cell properties can be very high. It remains unclear whether all cancers contain cancer stem cells, what are the exact contribution of cancer stem cells to tumor growth, and whether cancer stem cell are more resistant to chemo and radiotherapy and contribute to cancer relapse after therapy.
A stem cell line is a population of cells that can replicate themselves for long periods of time in vitro, meaning outside of the body. These cell lines are grown in incubators with specialized growth factor-containing media (liquid food source), at a temperature and oxygen/carbon dioxide mixture that approximate the conditions inside the mammalian body.
A. Insight into human development
Most knowledge about human development has been obtained through studying model organisms, such as fruit flies, worms, frogs and mice. Human embryonic stem cell lines, which can be cultured and differentiated into a variety of cells and tissues paralleling the earliest events in the development of the embryo offer a unique window into human development.
B. Study of diseases and how they develop
Experimental animal models are typically used to study human diseases in the lab. However, they do not exactly model the disease as it occurs in people. Human pluripotent stem cells, particularly patient or disease-specific lines, offer the possibility to model human disease more accurately in the lab. Read more about disease-specific or patient-specific pluripotent stem cells (see FAQ 11)
C. Regenerative medicine
Replacing diseased cells with healthy cells, an approach called regenerative medicine, is a promising application of stem cells. Currently, researchers are investigating the use of adult, fetal and embryonic stem cells as a resource for various, specialized cell types, such as nerve cells, muscle cells, blood cells and skin cells that can be used to treat various diseases. In theory, any condition in which there is tissue degeneration can be a potential candidate for stem cell therapies, including Parkinson's disease, spinal cord injury, heart disease, Type 1 diabetes, muscular dystrophies, retinal degeneration and liver diseases. However, an important consideration here is that in some cases the immune system causes the disease by destroying critical cells, such as insulin-producing cells in type I diabetes. It is therefore possible that stem cell-derived insulin-producing cells will be attacked by the immune system as well.
Additionally, some types of stem cells have been shown to migrate toward tumors or sites of injury, or to secrete various factors that influence the behavior of other cells, such as those of the immune system.These represent possible alternative approaches for the future development of stem cell-based therapies.
Umbilical cord blood is rich in hematopoietic or blood stem cells and is currently used as an alternative to bone marrow transplantation. Umbilical cord blood can be collected non-invasively from the umbilical cord and placenta after birth, tested and stored frozen in tissue banks ready to use. The host-donor match required for transplantation is less stringent, increasing the number of patients that could use each sample. However, as there may not be a sufficient number of stem cells in a single cord blood sample to treat an adult, cord blood stem cells are more often used to treat children. This highlights the need to develop better methods to expand adult stem cells outside the body.
The development of patient-specific or disease-specific pluripotent stem cells has great therapeutic promise for two reasons. First, these cells could provide a powerful new tool for studying the basis of human disease and for discovering new drugs. Second, patient-specific pluripotent cells could be developed into a cell type needed to treat that patient. When transplanted into the original donor, they would be recognized as 'self', thereby avoiding the problems of rejection and immunosuppression that occur with transplants from donors, even when donor and recipient are related.
Somatic cell nuclear transfer (SCNT) is a technique in which the nucleus of a somatic cell (that is any cell of the body apart from the sperm or egg), is transferred into an egg that has had its original nucleus removed. The egg now has the same DNA, or genetic material, as the donor somatic cell. Given the right signals, the egg can be coaxed into developing as if it had been fertilized. The egg would divide to form 2 cells, then 4 cells, then 8 cells and so on until the blastocyst of around 150 cells is formed. Embryonic stem cells can be derived from this blastocyst to create cell lines that are genetically identical to the donor somatic cell.
Because of the potential to use such embryonic stem cells for therapy in the future, this technique is also sometimes called ‘therapeutic cloning’. This is different from ‘reproductive cloning’ (See FAQ 14).
First, these cells could provide a powerful new tool for studying the basis of human disease and for discovering new drugs. Second, the resulting embryonic stem cells could be developed into mature cell types. After transplantation into the original donor, these would be recognized as 'self', thereby avoiding the problems of rejection and immunosuppression that occur with transplants from unrelated donors. It is possible however that the recent development of iPS cells will supersede the use of somatic nuclear transfer as it is technically easier and does not require the use of eggs, avoiding ethical issues with human egg procurement.
If an egg generated by somatic cell nuclear transfer (see ‘What is somatic cell nuclear transfer?’) was implanted into the womb of an animal, an individual would be born that has identical genetic material as the donor somatic cell and might be referred to as a ‘clone.' The procedure is referred to as ‘reproductive cloning’ and is fraught with profound technical and biological problems. Cloned offspring have been generated in a number of animal species, including sheep, mouse, pig, cow, rat, rabbit, dog, cat and horse, but the cloned animals have exhibited a range of serious developmental and physiological defects. As such, the overwhelming consensus of the world’s scientific and medical communities is that at this time human reproductive cloning should be banned.
The goal of regenerative medicine is to repair organs or tissues that are damaged by disease, aging or trauma, such that function is restored or improved.
The term regenerative medicine is often used nowadays to describe medical treatments and research that use stem cells (either adult or embryonic) to restore the function of organs or tissues. This can be achieved in different ways; first, by administering stem cells, or specific cells that are derived from stem cells in the laboratory; or second, by administering drugs that coax stem cells that are already present in tissues to more efficiently repair the tissue involved.
Stem cell research contributes to a fundamental understanding of how organisms develop and grow, and how tissues are maintained throughout adult life. This knowledge is required to understand what goes wrong during disease and injury and ultimately how these conditions might be treated. Of particular interest in this context is research on human embryonic stem cells, as these cells open a window into unique aspects of human development and biology, and therefore will lead to novel insights into human disease. Research on human embryonic stem cells, somatic cell nuclear transfer, induced pluripotent stem cells (iPS cells) and ‘adult’ or tissue-specific stem cells needs to continue in parallel. All are part of a research effort that seeks to expand our knowledge of how cells function, what fails in the disease process, and how the first stages of human development occur. It is this combined knowledge that will ultimately generate safe and effective therapies.
Bioethics is the study of the social, moral and ethical issues in the fields of scientific research, medical treatment and, more generally, in the life sciences. With advancing technology come new and exciting insights into scientific processes and diseases; at the same time, new ethical issues arise.
One of the most commonly asked questions in stem cell research is whether it is ethical to use human embryos to procure embryonic stem cells. However, there are many additional ethical issues that pertain to stem cell research. For example, in order to produce induced pluripotent stem cells and embryonic stem cells from somatic cell nuclear transfer, biological material must be collected from living donors. There are many questions regarding the rights of these donors.
Furthermore, wherever there is the potential for medical treatments, there are complex decisions that need to be made about how and when to begin trial treatments in humans, and if the trials are successful how to make such treatments fairly available.
FAQs About Stem Cell Therapy
A bone marrow transplant, also called a hematopoietic stem cell transplant, is a medical procedure used to treat conditions of the blood such as leukemia, sickle cell anemia, or some metabolic conditions. It relies on the hematopoietic (blood) stem cells that are present in the bone marrow that are the precursors to all blood cells. Doctors have been transferring blood stem cells by bone marrow transplant for more than 40 years. Advanced techniques for collecting or "harvesting" blood stem cells are now used. Cord blood, like bone marrow, is stored as a source of blood stem cells and is used as an alternative to bone marrow in transplantation.
Other stem cell applications are the use of skin progenitor cells for burns, and the use of limbal stem cells, which reside in the cornea, for injury of the cornea. Despite intensive research, no therapies are available yet using embryonic stem cells, although two clinical trials, to treat spinal cord injury and a certain type of blindness respectively, were approved in 2010.
However, with the exception of the treatments discussed here, the use of cell therapies remains at an experimental stage and has not been shown to be safe or eff=ective.
A list of clinical trials using stem cells can be found at. http://clinicaltrials.gov/ct2/home. There are about 3,861 clinical trials listed in the database, not all of which are active.
Some of the promise of stem cell therapy has been realized. A prime example is bone marrow transplantation. Even here, however, manyproblems remain to be solved. Challenges facing stem cell therapy include the following:
Adult stem cells
Tissue-specific stem cells in adult individuals tend to be rare. Furthermore, while they can regenerate themselves in an animal or person they are generally very difficult to grow and to expand in the laboratory. Because of this, it is difficult to obtain sufficient numbers of many adult stem cell types for study and clinical use. Hematopoietic or blood-forming stem cells in the bone marrow, for example, only make up one in a hundred thousand cells of the bone marrow. They can be isolated, but can only be expanded a very limited amount in the laboratory. Fortunately, large numbers of whole bone marrow cells can be isolated and administered for the treatment for a variety of diseases of the blood. Skin stem cells can be expanded however, and are used to treat burns. For other types of stem cells, such as mesenchymal stem cells, some success has been achieved in expanding the cells in vitro, but application in animals has been difficult. One major problem is the mode of administration. Bone marrow cells can be infused in the blood stream, and will find their way to the bone marrow. For other stem cells, such as muscle stem cells, mesenchymal stem cells and neural stem cells, the route of administration in humans is more problematic. It is believed, however, that once healthy stem cells find their niche, they will start repairing the tissue. In another approach, attempts are made to differentiate stem cells into functional tissue, which is then transplanted. A final problem is rejection. If stem cells from the patients are used, rejection by the immune system is not a problem. However, with donor stem cells, the immune system of the recipient will reject the cells, unless the immune system is suppressed by drugs. In the case of bone marrow transplantation, another problem arises. The bone marrow contains immune cells from the donor. These will attack the tissues of the recipient, causing the sometimes deadly graft-versus-host disease.
Pluripotent stem cells
All embryonic stem cell lines are derived from very early stage embryos, and will therefore be genetically different from any patient. Hence, immune rejection will be major issue. For this reason, iPS cells, which are generated from the cells of the patient through a process of reprogramming, are a major breakthrough, since these will not be rejected. A problem however is that many iPS cell lines are generated by insertion of genes using viruses, carrying the risk of transformation into cancer cells. Furthermore, undifferentiated embryonic stem cells or iPS cells form tumors when transplanted into mice. Therefore, cells derived from embryonic stem cells or iPS cells have to be devoid of the original stem cells to avoid tumor formation. This is a major safety concern.
A second major challenge is differentiation of pluripotent cells into cells or tissues that are functional in an adult patient and that meet the standards that are required for ‘transplantation grade’ tissues and cells.
A major advantage of pluripotent cells is that they can be grown and expanded indefinitely in the laboratory. Therefore, in contrast to adult stem cells, cell number will be less of a limiting factor. Another advantage is that given their very broad potential, several cell types that are present in an organ might be generated. Sophisticated tissue engineering approaches are therefore being developed to reconstruct organs in the lab.
While results from animal models are promising, the research on stem cells and their applications to treat various human diseases is still at a preliminary stage. As with any medical treatment, a rigorous research and testing process must be followed to ensure long-term efficacy and safety.
Many clinics from all over the world offer stem cell therapies for a variety of diseases. However, many of these treatments are unproven, and in addition, these treatments tend to be very expensive. Please visit our Web site www.closerlookatstemcells.org for more information on unproven stem cell treatments.
The process used to turn scientific knowledge into real world medical treatments is called clinical translation. Before being marketed or adopted as standard of care, most medicines are tested through clinical trials. Read more on How Science Becomes Medicine.
An experimental, or investigational, treatment is one that is still being developed and has not yet been shown in careful clinical trials to be safe and effective. As stated in FAQ 9, only a few stem cell treatments, such as bone marrow transplantation to treat blood diseases, are widely accepted by the medical community. Read more on www.closerlookatstemcells.org.
A clinical trial is a research study designed to answer specific questions about a new treatment or a new way of using current treatments. Clinical trials are used to establish whether new treatments are safe and effective. It is very important to understand that the new treatment may not be better than, or even as good as, existing treatments.
Most drugs and treatments that are widely accepted by the medical community have been tested in clinical trials. If a trial is successful, defined as finding that the treatment is shown to be safer, more effective, or cheaper than previously available treatments, the experimental treatment can become standard treatment.
The fact that a procedure is experimental does not automatically mean that it is part of a research study or clinical trial. A responsible clinical trial can be characterized by a number of key features. First, preclinical data (data obtained in the laboratory, preferably in animal models) supporting that the treatment under study is likely to be safe and effective are required. Next, oversight by an independent group such as an Institutional Review Board or medical ethics committee that protect patients’ rights is necessary. In many countries the trial is assessed and approved by a national regulatory agency, such as the European Medicines Agency (EMA) or the U.S. Food and Drug Administration (FDA).
The trial itself is designed to answer specific questions about a new treatment or a new way of using current treatments, often with a control group to which the group of people receiving the new treatment is compared. Typically, the cost of the new treatment and trial monitoring is defrayed by the company developing the treatment or by local or national government funding.
Responsibly conducted clinical trials are critical in the development of new treatments as they allow us to learn whether these treatments are safe and effective. The ISSCR supports participation in responsible clinical trials after careful consideration of the issues highlighted on this site and after discussion with a trusted physician.
Every medical procedure has risks. A goal of clinical trials is to determine whether the potential benefit of a treatment outweighs the risks. A possible risk of some stem cell treatments may be the development of tumors or cancers. For example, when cells are grown in culture (a process called expansion), the cells may lose the normal mechanisms that control growth. A particular danger of pluripotent cells is that, if undifferentiated, they may form tumors called teratomas. Other possible risks include infection, tissue rejection, and complications arising from the medical procedure itself.
While your own cells are less likely to be rejected by your immune system, this does not necessarily mean the cells are safe to use as a therapeutic treatment. The methods used to isolate, modify, grow or transplant the cells may alter the cells, could cause infection or introduce other unknown risks. Transplanting cells into a different part of the body than they originated from may have unforeseen risk, complications or unpredictable outcomes.