to form functional desired tissues or organs in vivo. Often, the survival and function of seeded cells, as well as interactions between the biomatrices and host tissues can be explored using the IVIS system 113, 116, 144, 151-156. In a 2008 report published in PNAS154, Jennifer Elisseeff’s group at Johns Hopkins University used a combination of chondrocyte-secreted morphogenetic factors and hydrogels to commit mesenchymal stem cells (derived from human embryonic stem cells) into a chrondrogenic lineage and eventually form cartilaginous tissue in mice. As seen in the figure below, they relied on IVIS imaging to validate the survival, viability and differentiation of MSCs seeded on a biodegradable polyester scaffold containing hydroxyapatite. Figure 4. Imaging viability of bioluminescent mesenchymal stem cells seeded on a biodegradable scaffold (Hwang et al, PNAS, 105, p20641, 2008). Conclusion In summary, the IVIS is a valuable tool for stem cell research as it enables spatiotemporal and longitudinal monitoring of stem cell processes, cell viability and homing patterns, and therapeutic function in living animals. These preclinical readouts provide insightful cues to clinicians on the safety and efficacy of stem cell and regenerative medicine therapies to treat human diseaseF or centuries, scientists have known that certain animals can regenerate missing parts of their bodies. Humans actually share this ability with animals like the starfish and the newt. Although we can’t replace a missing leg or a finger, our bodies are constantly regenerating blood, skin, and other tissues. The identity of the powerful cells that allow us to regenerate some tissues was first revealed when experiments with bone marrow in the 1950s established the existence of stem cells in our bodies and led to the development of bone marrow transplantation, a therapy now widely used in medicine. This discovery raised hope in the medical potential of regeneration. For the first time in history, it became possible for physicians to regenerate a damaged tissue with a new supply of healthy cells by drawing on the unique ability of stem cells to create many of the body’s specialized cell types. Once they had recognized the medical potential of regeneration through the success of bone marrow transplants, scientists sought to identify similar cells within the embryo. Early studies of human development had demonstrated that the cells of the embryo were capable of producing every cell type in the human body. Scientists were able to extract embryonic stem cells from mice in the 1980s, but it wasn’t until 1998 that a team of scientists from the University of Wisconsin–Madison became the first group to isolate human embryonic stem cells and keep them alive in the laboratory. The team knew that they had in fact isolated stem cells because the cells could remain unspecialized for long periods of time, yet maintained the ability to transform into a variety of specialized cell types, including nerve, gut, muscle, bone, and cartilage cells. Stem cell research is being pursued in the hope of achieving major medical breakthroughs. Scientists are striving to create therapies that rebuild or replace damaged cells with tissues grown from stem cells and offer hope to people suffering from cancer, diabetes, cardiovascular disease, spinal-cord injuries, and many other disorders. Both adult and embryonic stem cells may also provide a route for scientists to develop valuable new methods of drug discovery and testing. They are also powerful tools for doing the research that leads to a better understanding of the basic biology of the human body. By drawing on expert scientists, doctors, bioethicists, and others, the National Academies have examined the potential of stem cell technologies for medicine and provided a forum for discussing the ethical implications and moral dilemmas of stem cell research. 2 3 Ultimately, every cell in the human body can be traced back to a fertilized egg that came into existence from the union of egg and sperm. But the body is made up of over 200 different types of cells, not just one. All of these cell types come from a pool of stem cells in the early embryo. During early development, as well as later in life, various types of stem cells give rise to the specialized or differentiated cells that carry out the specific functions of the body, such as skin, blood, muscle, and nerve cells. Over the past two decades, scientists have been gradually deciphering the processes by which unspecialized stem cells become the many specialized cell types in the body. Stem cells can regenerate themselves or produce specialized cell types. This property makes stem cells appealing for scientists seeking to create medical treatments that replace lost or damaged cells. WHAT IS A STEM CELL? 4 Stem cells are found in all of us, from the early stages of human development to the end of life. All stem cells may prove useful for medical research, but each of the different types has both promise and limitations. Embryonic stem cells, which can be derived from a very early stage in human development, have the potential to produce all of the body’s cell types. Adult stem cells, which are found in certain tissues in fully developed humans, from babies to adults, may be limited to producing only certain types of specialized cells. Recently, scientists have also identified stem cells in umbilical cord blood and the placenta that can