The SAGE Encyclopedia of Stem Cell Research. Группа авторов
witnessed in some vertebrates, but, according to the scientific community, none has yet been observed in humans. In cases where certain adult stem cells could be “reprogrammed” into other types of cells, such capability provides a way to repair damaged cell types in the aftermath of a disease. It has been proven quite recently through experimentation that insulin-producing cells damaged or lost due to diabetes can possibly regain functionality through the reprogramming of other pancreatic cells.
In addition to “lineage reprogramming,” adult somatic cells can be reengineered to behave like embryonic stem cells (ESCs) through the insertion of pluripotency genes.
For many years now, research on adult stem cells has triggered a great deal of interest and excitement among the community of scientists and clinicians. This is certainly due to the capacity these cells demonstrate to self-renew indefinitely, thus constituting a renewable source of tissue and cell replacement in the treatment of a series of diseases, and the potential regeneration of entire organs from a few cells.
For over 40 years, bone marrow transplants based on the use of adult stem cells have made it possible to successfully treat cancers such as lymphomas, myelomas, and leukemia and genetic diseases such as thalassemias. This progress has opened new doors to regenerative therapy and has led to a surge in clinical trials relying on the use of adult stem cells. These developments are providing great hope for the treatment of conditions such as diabetes, myocardial infarction, congestive heart failure, and so forth.
A new trend in the field of stem cell biology is the adoption of all-inclusive concepts illustrated by the definition of a flurry of new scientific terms. Notwithstanding this tendency, recent studies suggest that functional differences do exist between stem and so-called progenitor cells.
Stem cells are called progenitors when they do not have the capacity to self-renew. In the past, developmental biologists referred to ancestral embryonic cells as precursors. For any particular cell in the embryo, there exists an ancestor (progenitor or precursor) cell that gives rise to it. The notion of transit-amplifying cells (TAC) also needs to be clarified in this constantly shifting biological landscape. The difference between TACs and progenitors is not always clear.
Transit-amplifying cells might be defined as dedicated progenitors among adult stem cells. Progenitor cells are known to have potential uses in medicine. Researchers from the Boston Children’s Hospital are currently reviewing the potential of muscle and blood progenitor cells in building blood vessels and heart valves, for instance.
Adult stem cells are located in a specific microenvironment known as a niche, in which both adult stem cells and TAC stay under control during their differentiation and self-renewing processes. If the differentiation of adult stem cells can be monitored in a laboratory environment, such cells may serve as a basis of transplantation-based therapies, especially since, unlike embryonic stem cells, the use of human adult stem cells in research is not considered controversial: the cells are extracted from adult tissue samples rather than young human embryos bound to have been destroyed.
It is true that adult stem cells do not carry the same ethical concerns, or generate the same level of controversy, as embryonic stem cells. However, the practical challenges involved in their use are real. As scientists continue to explore ways to successfully harvest adult stem cells, the public awaits new therapies for some of the more severe afflictions. Looking at the potential clinical applications of stem cells, scientists have reached the conclusion that stem cell treatments have the potential to make great impact on the general well-being of populations and individuals, physically, psychologically, and economically.
Approximately 130 million people suffer today from some kind of degenerative, chronic disease. In this context, stem cell therapies hold great promise, in particular in the treatment of many conditions affecting the nervous system that usually result from a loss of nerve cells. However, because mature nerve cells cannot divide, they cannot be relied on to replace lost cells. These types of conditions do not offer any therapeutic options outside of the regeneration of damaged or lost nerve tissue. This is true of Parkinson’s disease, in which nerve cells secreting dopamine die; Alzheimer’s disease cells, in which neurotransmitters are depleted; or amyotrophic sclerosis; in which motor nerve cells responsible to activate muscles are destroyed.
In primary immunodeficiency diseases such as AIDS, adult stem cells offer the promise of treating such conditions through stem cell therapy. Pluripotent stem cells, the master cells capable of generating cells from the three basic body layers, such as adult stem cells, can self-renew. In so doing, they are able to regenerate the missing immune cells at the basis of nearly all primary immunodeficiency illnesses. The transplantation of reconstituted stem cells using normal genes could therefore restore the immune function and provide a new quality of life to affected individuals.
What are the challenges to the use of adult stem cells?
For many years, adult stem cells have been used therapeutically in the form of bone marrow transplants. Nevertheless, the scientific community is still facing today a series of challenges that need to be overcome before stem cells can be deemed ready to effectively treat a wider range of diseases.
Researchers are still attempting to grasp the unique molecular and genetic basis for the phenomenon enabling these cells to replicate endlessly. As we can see, challenges pertaining to the immune system constitute a significant impediment to the reliable application of stem cell therapies.
Cell therapy surely provides exceptional prospects to disease treatment, yet the value of these technological accomplishments will only be fully realized once the therapeutic techniques underway are carefully applied to patients through clinical programs capable of ensuring efficacious and tangible results at a reasonable cost.
Morenike Trenou
Independent Scholar
See Also: Embryonic Stem Cells, Methods to Produce; Pluripotent Stem Cells, Embryonic; Pluripotent Stem Cells, Germ; Stem Cell Markers.
Further Readings
Abraham, E. J., C. A. Leech, J. C. Lin, et al. “Insulinotropic Hormone Glucagon-Like Peptide-1 Differentiation of Human Pancreatic Islet-Derived Progenitor Cells Into Insulin-Producing Cells.” Endocrinology, v.143 (2002).
Alison, M. R., R. Poulsom, R. Jeffery, et al. “Hepatocytes From Non-Hepatic Adult Stem Cells.” Nature, v.406/6793 (2002).
American Diabetes Association. “Economic Costs of Diabetes in the U.S. in 2007.” Diabetes Care, v.31 (2008).
Audet, J., C. L. Miller, S. Rose-John, et al. “Distinct Role of gp130 Activation in Promoting Self-Renewal Divisions by Mitogenically Stimulated Murine Hematopoietic Stem Cells.” Proceedings of the National Academy of Sciences of the USA, v.98/4 (February 13, 2001).
Baeyens, L., S. De Breuck, J. Lardon, et al. “In Vitro Generation of Insulin-Producing Beta Cells From Adult Exocrine Pancreatic Cells.” Diabetologia, v.48 (2005).
Baum, C. M., I. L.Weissman, A. S. Tsukamoto, et al. “Isolation of a Candidate Human Hematopoietic Stem-Cell Population.” Proceedings of the National Academy of Sciences of the USA, v.89 (1992).
Bittner, R. E., C. Schöfer, K. Weipoltshammer, et al. “Recruitment of Bone-Marrow-Derived Cells by Skeletal and Cardiac Muscle in Adult Dystrophic mdx Mice.” Anatomy and Embryology (Berl), v.199/5 (1999).
Boiani, M., S. Eckardt, H. R. Schöler, et al. “Oct4 Distribution and Level in Mouse Clones: Consequences for Pluripotency.” Genes and Development, v.16/10 (2002).
Centers for Disease Control and Prevention. “National Diabetes Fact Sheet: General Information and National Estimates on Diabetes in the United States, 2005.” http://www.cdc.gov/diabetes/pubs/pdf/ndfs_2005.pdf (Accessed April 24, 2008).
Chen, J., C. M. Astle, and D. E. Harrison. “Development and Aging of Primitive Hematopoietic Stem Cells in BALB/cBy Mice.” Experimental Hematology, v./5 (1999).
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