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Embryo nic stem cells with fluorescent marker.]] Stem cells are primal Cells found in all multi-cellular Organisms . They retain the ability to renew themselves through Mitotic Cell Division and can Differentiate into a diverse range of specialized cell types. Research in the human stem cell field grew out of findings by Canadian scientists Ernest A. McCulloch and James E. Till in the 1960 s.12 The three broad categories of mammalian stem cells are: Embryonic Stem Cells , derived from Blastocysts , ''' Adult Stem Cells ''', which are found in adult tissues, and ''' Cord Blood Stem Cells ''', which are found in the umbilical cord. In a developing embryo, stem cells can differentiate into all of the specialized embryonic tissues. In adult organisms, stem cells and Progenitor Cell s act as a repair system for the body, replenishing specialized cells. As stem cells can be grown and Transformed into specialized cells with characteristics consistent with cells of various tissues such as muscles or nerves through Cell Culture , their use in Medical Therapies has been proposed. In particular, embryonic Cell Line s, Autologous embryonic stem cells generated through Therapeutic Cloning , and highly plastic adult stem cells from the Umbilical Cord blood or Bone Marrow are touted as promising candidates.3 STEM CELL PROPERTIES Defining properties The rigorous definition of a stem cell requires that it possesses two properties:
These properties can be illustrated '' In Vitro '', using methods such as Clonogenic Assay s, where the progeny of single cell is characterized.45 However, ''in vitro'' culture conditions can alter the behavior of cells, making it unclear whether the cells will behave in a similar manner '' In Vivo ''. Considerable debate exists whether some proposed adult cell populations are truly stem cells. Potency definitions ''Potency'' specifies the differentiation potential (the potential to differentiate into different cell types) of the stem cell.
EMBRYONIC STEM CELLS See Also: Embryonic stem cell Embryonic stem cell lines (ES cell lines) are cultures of cells derived from the epiblast tissue of the inner cell mass (ICM) of a when given sufficient and necessary stimulation for a specific cell type. They do not contribute to the extra-embryonic membranes or the Placenta . Nearly all research to date has taken place using mouse embryonic stem cells (mES) or human embryonic stem cells (hES). Both have the essential stem cell characteristics, yet they require very different environments in order to maintain an undifferentiated state. Mouse ES cells are grown on a layer of gelatin and require the presence of Leukemia Inhibitory Factor (LIF). , Mouse Embryonic Stem (ES) Cell Culture-Current Protocols in Molecular Biology Human ES cells are grown on a feeder layer of mouse embryonic Fibroblasts (MEF's) and require the presence of basic Fibroblast Growth Factor (bFGF or FGF-2).[http://stemcells.nih.gov/research/NIHresearch/scunit/culture.asp , Culture of Human Embryonic Stem Cells (hESC) NIH Without optimal culture conditions or genetic manipulation6 embryonic stem cells will rapidly differentiate. A human embryonic stem cell is also defined by the presence of several transcription factors and cell surface proteins. The transcription factors Oct-4 , Nanog , and Sox2 form the core regulatory network which ensures the suppression of genes that lead to differentiation and the maintenance of pluripotency.7 The cell surface proteins most commonly used to identify hES cells are the glycolipids SSEA3 and SSEA4 and the keratan sulfate antigens Tra-1-60 and Tra-1-81. The molecular definition of a stem cell includes many more proteins and continues to be a topic of research.8 After 20 years of research, there are no approved treatments or human trials using embryonic stem cells. Their tendency to produce tumors and malignant carcinomas, cause transplant rejection, and form the wrong kinds of cells are just a few of the hurdles that embryonic stem cell researchers still face.9 Many nations currently have Moratoria on either ES cell research or the production of new ES cell lines. Because of their combined abilities of unlimited expansion and pluripotency, embryonic stem cells remain a theoretically potential source for regenerative medicine and tissue replacement after injury or disease. ADULT STEM CELLS See Also: Adult stem cell The term Adult stem cell refers to any cell which is found in a developed organism that has two properties: the ability to divide and create another cell like itself and also divide and create a cell more differentiated than itself. Also known as ''' Somatic ''' (from Greek Σωματικóς, ''of the body'') stem cells, they can be found in children, as well as adults10. Pluripotent adult stem cells are rare and generally small in number but can be found in a number of tissues including umbilical cord blood.11 Most adult stem cells are lineage restricted ( Multipotent ) and are generally referred to by their tissue origin ( Mesenchymal Stem Cell , adipose-derived stem cell, Endothelial Stem Cell , etc.)1213 A great deal of adult stem cell research has focused on clarifying their capacity to divide or ''self-renew'' indefinitely and their differentiation potential.14 In mice, pluripotent stem cells can be directly generated from adult fibroblast cultures.15 While embryonic stem cell potential remains untested, adult stem cell treatments have been used for many years to successfully treat leukemia and related bone/blood cancers through bone marrow transplants. Bone Marrow Transplant The use of adult stem cells in research and therapy is not as Controversial as Embryonic Stem Cells , because the production of adult stem cells does not require the destruction of an Embryo . Consequently, more US government funding is being provided for adult stem cell research[http://www.hhs.gov/news/press/2004pres/20040714b.html ,USDHHS Stem Cell FAQ 2004. LINEAGE See Also: Stem cell line To ensure self-renewal, stem cells undergo two types of cell division (see ''Stem cell division and differentiation'' diagram). Symmetric division gives rise to two identical daughter cells both endowed with stem cell properties. Asymmetric division, on the other hand, produces only one stem cell and a Progenitor Cell with limited self-renewal potential. Progenitors can go through several rounds of cell division before terminally Differentiating into a mature cell. It is possible that the molecular distinction between symmetric and asymmetric divisions lies in differential segregation of cell membrane proteins (such as Receptors ) between the daughter cells.16 An alternative theory is that stem cells remain undifferentiated due to environmental cues in their particular niche. Stem cells differentiate when they leave that niche or no longer receive those signals. Studies in Drosophila germarium have identified the signals dpp and adherins junctions that prevent germarium stem cells from differentiating1718. The signals that lead to reprogramming of cells to an embryonic-like state are also being investigated. These signal pathways include several Transcription Factor s including the Oncogene C-Myc . Initial studies indicate that transformation of mice cells with a combination of these anti-differentiation signals can reverse differentiation and may allow adult cells to become pluripotent.19 However, the need to transform these cells with an oncogene may prevent the use of this approach in therapy. TREATMENTS See Also: Stem cell treatments Medical researchers believe that stem cell therapy has the potential to radically change the treatment of human disease. A number of adult stem cell therapies already exist, particularly Bone Marrow Transplant s that are used to treat Leukemia .20 In the future, medical researchers anticipate being able to use technologies derived from stem cell research to treat a wider variety of diseases including Cancer , Parkinson's Disease , Spinal Cord Injuries , and Muscle damage, amongst a number of other impairments and conditions.2122 However, there still exists a great deal of social and scientific uncertainty surrounding stem cell research, which could possibly be overcome through public debate and future research. Stem cells, however, are already used extensively in research, and some scientists do not see cell therapy as the first goal of the research, but see the investigation of stem cells as a goal worthy in itself. 23. CONTROVERSY SURROUNDING STEM CELL RESEARCH See Also: Stem cell controversy There exists a widespread controversy over stem cell research that emanates from the techniques used in the creation and usage of stem cells. Human Embryonic Stem Cell research is particularly controversial because, with the present state of technology, starting a Stem Cell Line requires the destruction of a Human Embryo and/or Therapeutic Cloning . However, recently, it has been shown in principle that embryonic stem cell lines can be generated using a single-cell biopsy similar to that used in Preimplantation Genetic Diagnosis that may allow stem cell creation without embryonic destruction.http://abcnews.go.com/Health/wireStory?id=3307505 Opponents of the research argue that embryonic stem cell technologies are a Slippery Slope to Reproductive Cloning and can fundamentally devalue human life. Those in the Pro-life movement argue that a human embryo is a human life and is therefore entitled to protection. Contrarily, supporters of embryonic stem cell research argue that such research should be pursued because the resultant treatments could have significant medical potential. It is also noted that excess embryos created for In Vitro Fertilisation could be donated with consent and used for the research. The ensuing debate has prompted authorities around the world to seek regulatory frameworks and highlighted the fact that stem cell research represents a Social and Ethical challenge. KEY STEM CELL RESEARCH EVENTS
STEM CELL FUNDING & POLICY DEBATE IN THE US
SEE ALSO
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