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these first steps have been achieved, work on animals plays an important role in furthering basic research and developing medical applications. This work is necessary to form the foundation of knowledge that will point the way to medical advances. Identifying Stem Cells As early as 1961, scientists knew that adult bone marrow contained cells that could make all of the blood cell types. But it wasn’t until 1988 that those stem cells were isolated as purepopulations. Why did it take so long? The techniques for identifying stem cells have only recently been
developed. Partly, this is because adult stem cells are, by their very nature, inconspicuous in shape, size, and function. They also tend to hide deep in tissues and are present only in very low numbers, making their identification and isolation like finding a needle in a haystack. How do scientists know when they have found a stem cell? Every cell displays an array of proteins on its surface; different cell types have different proteins. Scientists can use these surface proteins as “markers” that characterize individual cell types—a type of “molecular ID.” For example, using molecules that recognize and attach to specific surface proteins and that can fluoresce under certain wavelengths of light, scientists can visually tell the difference between a blood stem cell and a mature white blood cell. Unfortunately, not all stem cells can now be identified in this manner because scientists have not yet identified markers for all stem cell types. Scientists also identify stem cells by observing their behavior in the laboratory: stem cells must be able to remain unspecialized and self-renew for long periods of time. 9WORKING WITH STEM CELLSFluorescent markers can be used to identify stem cells hidden among ordinary adult cells. Here, human embryonic stem cells are recognized by the marker proteins they express (green). Courtesy of Paul J. Tesar, Laboratory of Molecular Biology, NINDS and the NIH Stem Cell Unit.Scientists believe that there might be more types of adult stem cells than the handful that have already been identified, but finding them is a difficult process. Culturing Cell Lines and Stimulating Stem Cells to Differentiate Cell culture is a term that refers to the growth and maintenance of cells in a controlled environment outside of an organism. A successful stem cell culture is one that keeps the cells healthy, dividing, and unspecialized. The culturing of stem cells is the first step in establishing a stem cell line—a propagating collection of genetically identical cells. Cell lines are important because they provide a long-term supply of multiplying cells that can be shared among scientists for research and therapy development. The National Academies report Stem Cells and the Future of Regenerative Medicine (2001) described some of the challenges of maintaining cell lines: “Over time, all cell lines…change, typically accumulating harmful genetic mutations. There is no reason to expect stem cell lines to behave differently. While there is much that can be learned using existing
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identity of the powerful cells, existence of stem cells