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 pure populations. 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. 9 WORKING WITH STEM CELLS Fluorescent 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 stem cell lines…such concerns necessitate continued monitoring of these cells as well as the development of new stem cell lines in the future.” Once they have established a stable stem cell line, scientists start the process of causing the stem cells to differentiate into specialized cell types. The cellular environment in which stem cells naturally reside provides scientists with clues about how to make them differentiate in a culture dish. For example, in the bone marrow, where blood stem cells reside, bone cells send physical and chemical signals that tell the blood stem cells when to differentiate. Scientists are just beginning to understand these signals and have developed ways to mimic the natural processes in cell cultures. Usually, the technology involves adding certain proteins to the cell culture and, in some cases, introducing specific genes into the stem cells. It will be essential that scientists are sure that stem cells have fully differentiated before they can use them for medical applications. If completely undifferentiated stem cells (such as embryonic stem cells) are implanted directly into an organism, they can cause a type of tumor called a teratoma, which scientists have observed in experiments using mice. Semi-specialized adult stem cells and differentiated cells derived from embryonic stem cells are unlikely to cause teratomas. The Role of Animals in Stem Cell Research For medical research, as well as for research that explores the basic processes in the development of organisms and diseases, scientists often rely on animals. Implanting human cells into animals 10 WORKING WITH STEM CELLS Center Photo: Scientists can test whether they have successfully caused embryonic stem cells to differentiate by labeling for specific marker proteins found in specialized cells. Courtesy of Dr. Daniel Anderson, MIT. such as mice has long been common practice in order to test the safety and effectiveness of new drugs, procedures, and medical devices before clinical testing in human volunteers. For stem cell research, scientists use animals to make sure the stem cells are able to incorporate into the tissue, do not cause any harmful consequences, and function in concert with the rest of the body. For example, before using stem cells to replace the pancreatic cells that are destroyed by type I diabetes in humans, scientists will transplant human stem cells into a mouse to see whether the stem cells yield healthy, insulin-producing cells. If their methods prove successful in mice, scientists may eventually