What are Stem Cells ?
The prospect of human stem cell research has emerged as a frontier scientific and bioethical issue. Ever since investigators reported that they had succeeded in isolating and culturing human embryonic stem cells in 1998, 1 research into these cells has attracted immense scientific and public interest. This new found ability to grow human stem cells has opened the door to the generation of brand new cells and tissues that might be used to treat those with such serious diseases as Parkinson’s, heart disease, and diabetes. Moreover, it has improved our prospects for understanding the processes of early human development and offers a novel way of testing drugs for their safety and effectiveness without involving human subjects. Given the immense promise of stem cell research, it is important to understand just what stem cells are, where they come from, and what they might do before we go on to explore several major ethical and public policy issues that their application to human beings raises. Stem cells exhibit two characteristics found in no other cells of the human body: they can renew themselves by cell division and, although they are unspecialized, can differentiate into various cell types, such as those of bone, muscle, and blood. These cells are central to human development and health for two main reasons: they have a role in the formation and maintenance of our bodily organs and they travel to diseased or injured tissue sites to inject a stream of new cells to replace those that have gone awry. Stem cells have been discovered in human adults and infants, as well as in human embryos, fetuses, placentas, and umbilical cord blood. They are also found in a wide variety of animals, including mice, rabbits, monkeys, and fruit flies.
Stem Cell Research
Scientific investigators are growing human stem cells in cultures in the laboratory with several different goals in mind. The first aim of stem cell research is to use stem cells as the basis of new therapies for those with serious diseases. Medical researchers hope to learn how to guide the differentiation of stem cells into specific types of cells and then transfer these stem cell offshoots to patients to provide lifelong treatments for such conditions as diabetes, spinal cord injury, and heart failure. Researchers are also exploring the use of stem cells for the treatment of degenerative diseases associated with aging, such as Alzheimer’s disease and Parkinson’s disease. The transferred cells would not just stop the progression of disease, as do current methods of medical treatment, but would also restore lost cellular, tissue, and organ functions. Such cell-based therapy is therefore often referred to as ‘‘regenerative medicine’’.
In their efforts to develop stem cell–based therapies, some scientists are attempting to pair stem cell research with research into human genes. This involves taking stem cells from patients, modifying them genetically to correct genes associated with inherited disorders, and then reintroducing them into these patients. Inserting modified genes into stem cells would provide a more precise way to repair tissue in patients than current methods of direct gene transfer by means of viral vectors or other means. Medical researchers could, for instance, insert a normal copy of a gene into differentiating stem cells that are missing that gene or in which that gene is malfunctioning in the laboratory and, when satisfied that the genetically modified cells meet appropriate tests for safety and efficacy, could then transfer them to the bodies of patients with such conditions as hemophilia and muscular dystrophy. Indeed, some predict that, in the future, it will be possible through the combined use of stem cell research, gene transfer, and tissue engineering not only to provide patients with treatments that will last a lifetime but alsoto create whole organs to replace those that fail.
A second major aim of stem cell research is to gain increased understanding of the processes of human development. Researchers hope to learn how a single cell, the early embryo, divides, grows, and gives rise to the trillions of cells and hundreds of tissues that make up the human body. Gaining a more complete grasp of how the human organism develops could open the door to understanding how healthy cells replace damaged cells and how cellular proliferation is regulated in space and time. Since many cancers arise from disturbances of normal developmental processes, the study of human stem cells is already informing cancer research. Further, studies of how abnormal cell divisions that may occur during the first few days of embryonic growth lead to chromosomal and developmental disorders in newborn children could lead medical researchers to develop treatments for such birth anomalies. Stem cell research could also advance knowledge about the causes of infertility and help diminish premature pregnancy loss.
A third goal of stem cell research, scientists point out, is to provide new ways to test drugs for efficacy, toxicity, and safety. Investigators currently assess new pharmaceuticals in animals before they begin trials of these drugs in human beings. Yet there are significant differences between animal and human physiology. Consequently, animal studies do not necessarily reveal how these new drugs will affect humans. Indeed, some assays performed on animals may overstate the toxicity of drugs. For such reasons, it has been proposed that human stem cells could be used for drug screening and testing. Drugs would be applied to specialized cells derived from human stem cells, such as those of the heart and liver, and these cells would then be evaluated for evidence of drug efficacy and toxicity. This would help weed out dangerous compounds before they were used in clinical testing in humans. If results indicated that further testing was warranted, human trials of such drugs could be initiated.
A word of caution is in order. The field of stem cell research is fraught with challenges. There are many issues related to the derivation, expansion, manipulation, characterization, and testing of stem cells for efficacy, toxicity, disposition to tumor and cancer formation, and immune responses that must be resolved before these cells or cells derived from them can be used to treat patients. It is important to realize that, as things stand as of this writing, few stem cells progeny are ready for safe and ethical use in treating human beings. Some believe that in another five years, clinical researchers will be ready to transplant specialized stem cells or their offshoots into patients to replace cells that are diseased or carrydeleterious mutations. Others argue that it will take much longer to achieve these goals, since considerably more information about the basic biology of human stem cells needs to be developed before they can be used more broadly to treat human beings safely.