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What are human embryonic stem cells?
Stem cells are cells that have the remarkable potential to develop into many different cell types in the body. Serving as a sort of repair system for the body, they can theoretically divide without limit to replenish other cells for as long as the person or animal is still alive. When a stem cell divides, each "daughter" cell has the potential to either remain a stem cell or become another type of cell with a more specialized function, such as a muscle cell, a red blood cell, or a brain cell.
A more detailed primer on stem cells can be found at Stem Cell Basics.
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What classes of stem cells are there?
There are three classes of stem cells: totipotent, multipotent, and pluripotent.
- A fertilized egg is considered totipotent, meaning that its potential is total; it gives rise to all the different types of cells in the body.
- Pluripotent stem cells can give rise to any type of cell in the body except those needed to develop a fetus.
- Stem cells that can give rise to a small number of different cell types are generally called multipotent.
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Where do stem cells come from?
There are several sources of stem cells. Pluripotent stem cells can be isolated from human embryos that are a few days old. Cells from these embryos can be used to create pluripotent stem cell "lines" —cell cultures that can be grown indefinitely in the laboratory. Pluripotent stem cell lines have also been developed from fetal tissue (older than 8 weeks of development).
In late 2007, scientists identified conditions that would allow some specialized adult human cells to be reprogrammed genetically to assume a stem cell-like state. These stem cells are called induced pluripotent stem cells (iPSCs). IPSCs 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 three 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.
Non-embryonic, or "adult" stem cells have been identified in many organs and tissues. Typically there is a very small number of stem cells in each tissue, and these cells have a limited capacity for proliferation, thus making it difficult to generate large quantities of these cells in the laboratory. Stem cells are thought to reside in a specific area of each tissue (called a "stem cell niche") where they may remain quiescent (non-dividing) for many years until they are activated by a normal need for more cells, or by disease or tissue injury.
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Why do scientists want to use stem cell lines?
Once a stem cell line is established from a cell in the body, it is essentially immortal, no matter how it was derived. That is, the researcher using the line will not have to go through the rigorous procedure necessary to isolate stem cells again. Once established, a cell line can be grown in the laboratory indefinitely and cells may be frozen for storage or distribution to other researchers.
Stem cell lines grown in the lab provide scientists with the opportunity to "engineer" them for use in transplantation or treatment of diseases. For example, before scientists can use any type of tissue, organ, or cell for transplantation, they must overcome attempts by a patient's immune system to reject the transplant. In the future, scientists may be able to modify human stem cell lines in the laboratory by using gene therapy or other techniques to overcome this immune rejection. Scientists might also be able to replace damaged genes or add new genes to stem cells in order to give them characteristics that can ultimately treat diseases.
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Why are doctors and scientists so excited about human embryonic stem cells?
Stem cells have potential in many different areas of health and medical research. To start with, studying stem cells will help us to understand how they transform into the dazzling array of specialized cells that make us what we are. Some of the most serious medical conditions, such as cancer and birth defects, are due to problems that occur somewhere in this process. A better understanding of normal cell development will allow us to understand and perhaps correct the errors that cause these medical conditions.
Another potential application of stem cells is making cells and tissues for medical therapies. Today, donated organs and tissues are often used to replace those that are diseased or destroyed. Unfortunately, the number of people needing a transplant far exceeds the number of organs available for transplantation. Pluripotent stem cells offer the possibility of a renewable source of replacement cells and tissues to treat a myriad of diseases, conditions, and disabilities including Parkinson's disease, amyotrophic lateral sclerosis, spinal cord injury, burns, heart disease, diabetes, and arthritis.
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Have human embryonic stem cells been used successfully to treat any human diseases yet?
Scientists have only been able to do experiments with human embryonic stem cells (hESCs) since 1998, when a group led by Dr. James Thomson at the University of Wisconsin developed a technique to isolate and grow the cells. Although hESCs are thought to offer potential cures and therapies for many devastating diseases, research using them is still in its early stages.
In late January 2009, the California-based company Geron received FDA clearance to begin the first human clinical trial of cells derived from human embryonic stem cells.
Adult stem cells such as blood-forming stem cells in bone marrow (called hematopoietic stem cells, or HSCs) are currently the only type of stem cell commonly used to treat human diseases. Doctors have been transferring HSCs in bone marrow transplants for over 40 years and advances in techniques of collecting, or "harvesting" HSCs have been made. HSCs are now used to reconstitute the immune system after leukemia, lymphoma, or various blood or autoimmune disorders have been treated with chemotherapy.
The clinical potential of adult stem cells has also been demonstrated in the treatment of other human diseases that include diabetes and advanced kidney cancer. However, these newer uses have involved studies with a very limited number of patients.
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What will be the best type of stem cell to use for therapy?
Pluripotent stem cells, while having great therapeutic potential, face formidable technical challenges. First, scientists must learn how to control their development into all the different types of cells in the body. Second, the cells now available for research are likely to be rejected by a patient's immune system. Another serious consideration is that the idea of using stem cells from human embryos or human fetal tissue troubles many people on ethical grounds.
Until recently, there was little evidence that multipotent adult stem cells could change course and provide the flexibility that researchers need in order to address all the medical diseases and disorders they would like to. New findings in animals, however, suggest that even after a stem cell has begun to specialize, it may be more flexible than previously thought.
There are currently several limitations to using adult stem cells. Although many different kinds of multipotent stem cells have been identified, adult stem cells that could give rise to all cell and tissue types have not yet been found. Adult stem cells are often present in only minute quantities and can therefore be difficult to isolate and purify. There is also evidence that they may not have the same capacity to multiply as embryonic stem cells do. Finally, adult stem cells may contain more DNA abnormalities—caused by sunlight, toxins, and errors in making more DNA copies during the course of a lifetime. These potential weaknesses might limit the usefulness of adult stem cells.
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I have Parkinson’s Disease. Is there a clinical trial that I can participate in that uses stem cells as therapy?
The public may search a database of NIH-sponsored clinical trials at www.clinicaltrials.gov. Enter the search terms of interest (in this case, Parkinson's Disease and stem cells) to search for applicable clinical trials.
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Where can I donate umbilical cord stem cells?
NIH cannot accept donated umbilical cord stem cells from the general public. The National Marrow Donor Program maintains a Web page on donating cord blood at http://www.marrow.org/HELP/Donate_Cord_Blood_Share_Life/index.html, and the International Cord Blood Society has one at http://www.cordblood.org/index.php?rm=common_page&id=10.
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I am a scientist funded by the NIH. How many cell lines are available to me, and how do I get them?
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I'm interested in purchasing more than one cell line from the NIH Stem Cell Registry. What is known about the status of the cell lines and their availability?
Many of the cell lines have been characterized as embryonic stem cells by detecting expression of surface antigen markers specific to embryonic stem cells, determining if the cells are pluripotent, and demonstrating that the cells are undifferentiated. A number of scientific publications have described the characterization of human embryonic stem cells. Although the characterization approaches may differ across laboratories, an example of the strategies used can be found in Thomson et al. (1998), Science, 282,1145–1147.
The National Stem Cell Bank and the individual providers of the federally eligible cells are working to make them available to researchers. This includes developing quality control measures to grow and reproduce the cell lines in sufficient numbers, having the administrative structure to receive and process requests, and establishing material transfer agreements with research purchasers. The National Stem Cell Bank and the individual providers of federally eligible cell lines have the most up-to-date information on availability. A list of these sources and contact information is available on the NIH Stem Cell Registry.
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Who owns the cells?
The stem cell lines remain the property of the individual stem cell providers, as listed on the NIH Stem Cell Registry. Researchers may negotiate a material transfer agreement (MTA) with either the National Stem Cell Bank on behalf of the individual providers, or directly with the cell providers in order to specify their rights and responsibilities concerning resulting data, publications, and potential patents. Examples of MTAs negotiated between the Department of Health and Human Services/NIH and various stem cell line providers are listed by provider on the NIH Stem Cell Registry.
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What policies govern use of stem cell lines from WiCell Research Institute?
WiCell has published FAQs About WiCell's Policies on the Use of Its hESC Lines (136k PDF file; get Adobe Reader) to address this question.