Stem Cell Research
Stem cells are cells that can divide for an infinite period of time when being grown outside of the body, and which can differentiate into various types of specialized cells. When fertilization of an egg with sperm occurs, the resulting fertilized cell has the capability to form an entire organism. The cell is described as being totipotent (having total potential). After some time, as rounds of cell division occur, specialization of cells occurs. But, early in fetal development, before the developing mass of cells attaches itself to the wall of the uterus, some cells still retain the ability to form virtually every type of cell in the body. These cells are pluripotent (capable of differentiating into many types of cells but not all types required for fetal development). With continued fetal development, further specialization of pluripotent stem cells results in multipotent stem cells--cells that give rise to cells having a particular function, such as blood cells and various types of skin cells. Indeed, life would be impossible without blood stem cells, which function to replenish blood cell supply throughout life.
Stem cell research is concerned primarily with the pluripotent cells. The field is relatively new. James Thomson reported in Science in late 1998 his success in maintaining undifferentiated embryonic stem cells in their undifferentiated state in lab culture.
Stem cells can be obtained from human embryos at the so-called blastocyst stage (a stage very early in fetal development, only a few division cycles after fertilization). As well, cells can be obtained from fetal tissue from terminated pregnancies. The latter procedure has precipitated much discourse. In August 2001, United States president George W. Bush announced that he would support very limited federal funding of research using stem cells from human embryos. It was a compromise that did not completely satisfy parties on either side of the controversial issue.
Another potential means of obtaining pluripotent stem cells may be a technique called somatic cell nuclear transfer. In the technique involves the physical removal of its nucleus from an egg cell. The nucleus is the specialized area of the cell that contains the organized pieces of genetic material called the chromosomes. The material left behind in the egg cell contains nutrients and other energy-producing materials necessary for development of the embryo. Then, a somatic cell--any cell other than an egg or a sperm cell--is placed next to the denucleated egg cell, and the two cells are chemically fused together. After a requisite number of cell divisions, pluripotent stem cells can, at least in theory thus far, be recovered and used.
Pluripotent stem cells are important to science and to advances in health care. At the most fundamental level, study of these cells could advance the understanding of the processes of cellular development, such as the orchestrated mechanisms by which genes are turned on and off during development and growth. Some of the most serious medical conditions, such as cancer and birth defects, are due to abnormal cell specialization and cell division. Pluripotent stem cells could also be used to screen new drugs, eliminating the need to use living subjects for the early phases of drug discovery. The most far-reaching potential application of the stem cells is the generation of cells and tissue that could be used for so-called cell therapies. Potentially stem cells may function as a kind of universal human donor cell, which could serve as raw material for whatever diseased cell requires replacing. Such donor cells would have to be genetically engineered so as not to form the cell surface molecules that would alert the recipient's immune system. The cells could be used for replacement of defective or diseased cells without the danger of transplantation rejection that occurs presently.
Potential applications include the replacement of defective heart tissue and replacement of malfunctioning insulin producing cells in Type I diabetes. In the last several years, several lines of research have produced concrete results showing the potential of stem cells in cell therapy. Genetic engineering of stem cells may be promising as a cancer eradication strategy. In rats, neural stem cells genetically engineered to convert a compound into a cancer-killing agent have been found to selectively target and destroy cancerous cells in the brain. Elsewhere, neural stem cells have also been shown capable of integration into the diseased retina of rats and of taking on some of the characteristics of retinal cells. This holds the promise that stem cell therapy may aid in repairing retinal damage. Other researchers have demonstrated, again in rats, that stem cells in the brain were able to repair damaged areas and restore function when stimulated by a growth-inducing protein. If replicated in humans, then stem cell treatments for stroke, nervous system and spinal cord injury and diseases such as Parkinson's and Alzheimer's that are marked by degeneration of nerve cells.
Another application of stem cells has been to form a chimera—an animal that grows from an embryo in which stem cells from another animal have been inserted. Some of the chimera's cells have one set of parents, and some cells have another set of parents. "Knockout" mice, research animals lacking specific genes, are chimeras. While theoretically conceivable, human chimeras are not contemplated.
Researchers have claimed success at reprogramming multipotent cells for a function other than that they were programmed for. Specifically, adult skin cells from cattle were reverted to stem cells and then transformed into heart cells. Other studies involved neural stem cells from mice and bone marrow cells from rats have also indicated that functional reprogramming of adult cells may be feasible. These breakthrough studies hold forth the potential of using cells from adults to treat diseases, rather than extracting embryonic cells. For the present, however, there are barriers to the use of adult stem cells. Such cells may not be present in all tissues of the body. More knowledge of the locations of adult stem cells is still required. Secondly, adult stem cells are present in minute quantities, are difficult to isolate and their number decreases with age. The time necessary to locate, harvest, and grow the cells to usable numbers may be too long for practical purposes. Finally, adult stem cells may contain DNA abnormalities, which have accumulated as a result of a lifetime of exposure to DNA-altering agents such as sunlight and toxic chemicals. Further research may overcome these limitations, allowing stem cells obtained from adults to be used in cell therapy.
This is the complete article, containing 1,063 words
(approx. 4 pages at 300 words per page).
