As Americans consider recent medical discoveries pertaining to stem-cell research and the ethical and political issues surrounding it, conversations on the news, around water coolers and even at dinner tables have begun including such complex terms as “DNA abnormalities,” “cell differentiation” and “somatic cell nuclear transfer.”
Even President George W. Bush joined the conversation this month when he entered living rooms around the nation via television to outline his decision on the controversial issue of embryonic stem-cell research. The federal government will fund research of the approximately 60 existing stem-cell lines created by privately funded scientists.
Americans are told stem-cell research holds great promise for the curing of such debilitating and life-threatening diseases as Parkinson’s and Alzheimer’s. But how? Just what are stem cells? Where do they come from, and how do scientists think they can help improve medical treatments?
In basic terms, a stem cell is one that can be formed into virtually any human cell found in the body. To understand the origin of the cells, a review of human development is necessary.
Human life is created when an ovum – the egg from a female – is fertilized by a sperm. When joined, the two “gametes” create a single-cell organism that eventually develops into a baby. The fertilized egg is “totipotent,” meaning its potential is total. The single cell splits within hours of fertilization, forming two identical totipotent cells. Either of these cells has the potential to become a fully developed baby. Indeed, identical twins are formed from two totipotent cells that develop separately into genetically identical individuals. Click here to see a diagram of the basic development of a human, courtesy of the National Institutes of Health.
After several days of continued cell division, the totipotent cells begin to “differentiate,” or specialize. An outer, hollow sphere of cells encompasses a cluster of cells inside, known as the “inner-cell mass.” Together, the cells are known as a “blastocyst.” The blastocyst’s outer layer of cells will go on to become the placenta and other tissues necessary in nurturing the growth of a human baby. The cells comprising the inner-cell mass can form virtually every type of cell found in the human body, but they are unable to fully develop into a baby without the outer layer. If pluripotent cells alone were implanted into a woman’s uterus, they would not form into a baby. Because of this, the inner-cell mass cells are not totipotent, but rather are called “pluripotent.”
Over time and more cell division, pluripotent cells continue to differentiate, forming into cells that perform specific functions. These specialized cells are called “multipotent.” While the inner-cell mass cells of a blastocyst go on to become various organs, nerves and tissues, some of the multipotent cells always remain as they are to create replacement cells for their specific functions. They can be found in children and adults. For example, blood multipotent cells continually replenish the body’s blood supply. While multipotent cells have been found in various areas of adults, they have not yet been found for all human tissues, but progress is being made, according to the National Institutes of Health. This diagram from NIH shows the progress of totipotent cells to multipotent cells.
Pluripotent and multipotent cells are both refereed to as “stem cells” and are the subject of controversial research making headlines.
Because pluripotent stem cells – the precursors to multipotent stem cells – eventually develop into nearly all the cells required by humans, scientists say they have great medical-treatment applications. Through various experiments with animals, scientists have shown they can direct the specialization of pluripotent stem cells. In other words, the cells can be manipulated into virtually any specialized cell.
The process of manipulating stem cells into more specialized cells has many potential medical applications. According to the NIH, simply studying the cells’ development can aid in understanding certain genetic diseases.
“A primary goal of this work would be the identification of the factors involved in the cellular decision-making process that results in cell specialization,” the NIH explains. “We know that turning genes on and off is central to this process, but we do not know much about these ‘decision-making’ genes or what turns them on or off. Some of our most serious medical conditions, such as cancer and birth defects, are due to abnormal cell specialization and cell division. A better understanding of normal cell processes will allow us to further delineate the fundamental errors that cause these often deadly illnesses.”
But the most hoped-for application of stem-cell research by advocates of the practice is the creation of cures to various diseases. By directing the specialization of pluripotent stem cells, scientists hope to create “cell therapies.”
“Many diseases and disorders result from disruption of cellular function or destruction of tissues of the body. Today, donated organs and tissues are often used to replace ailing or destroyed tissue. Unfortunately, the number of people suffering from these disorders far outstrips the number of organs available for transplantation. Pluripotent stem cells, stimulated to develop into specialized cells, offer the possibility of a renewable source of replacement cells and tissue to treat a myriad of diseases, conditions, and disabilities including Parkinson’s and Alzheimer’s diseases, spinal cord injury, stroke, burns, heart disease, diabetes, osteoarthritis and rheumatoid arthritis. There is almost no realm of medicine that might not be touched by this innovation,” the NIH states.
But how are stem cells obtained? As stated earlier, multipotent stem cells can be found in children and adults. The NIH says multipotent stem cells have not been found for all types of adult tissue, but more discoveries are being made. Others claim adult stem cells are not the best specimens from which to create stem-cell lines because the cells have aged and may have suffered damage.
Such scientists advocate pluripotent stem-cell research. But those cells can only be found in a developing embryo, which is where the cells are created. To obtain the cells, left-over embryos from in-vitro fertilization clinics are used. A blastocyst’s outer- and inner-cell layers are separated, destroying the embryo.
Many arguments exist on both sides of the embryonic stem-cell research debate. Because the process of obtaining the cells destroys a human embryo, many oppose the research. Others counter that the embryos were destined to be destroyed anyway, since they are “leftovers” from a couple’s efforts to become pregnant.
At a recent congressional hearing on stem-cell research, two couples asked members of the joint committee to consider alternative fates for leftover IVF embryos. The couples brought their children, who were adopted as embryos and implanted into the women’s uteruses, enabling infertile women to carry and give birth to their adopted children.
But proponents of embryonic stem-cell research say adoption of that sort is a rarity and that the potential benefits from the research outweighs any moral objections to destroying leftover embryos. Besides, some argue, the embryo is only the potential for human life and is not actual human yet.
To those who believe life begins at conception, destruction of a fertilized human egg at any stage of the growth process is immoral. And that immoral action cannot be mitigated by any potential good that may come from the research.
There is a third method of obtaining stem cells: from aborted fetuses. Cells from an aborted fetus’s reproductive tissues have been manipulated by scientists, producing the same result as experiments on pluripotent stem cells. But this method of obtaining research material is objectionable to abortion opponents.
True, the fetus is already dead when the cell extraction takes place, but the method of the baby’s death is still immoral – even evil. Participating in research that uses aborted fetal tissue is, by association, participating in evil, they argue.
Questionable ethics is exactly what research proponents say they are trying to avoid through federal funding and regulation of the experiments. By funding the research, the U.S. government will be in a position to control the way the research is conducted. However, privately-funded scientists will not be affected by government restrictions, unless the restrictions are made law for the general public.
In its recommendations to former President Bill Clinton, the National Bioethics Advisory Commission outlined what it believed should be the future of embryonic stem-cell research. The commission expressed its belief that government should prohibit the creation of embryos specifically for research purposes. Established by a Clinton-signed executive order in 1995, the panel made a total of 13 recommendations in its 1999 report.
As stem-cell research progresses – now with federal funding – more is being learned about adult stem cells. Scientists in Seattle, Wash., and Milan, Italy, discovered in 1999 that adult neural stem cells in mice had the ability to de-differentiate and could then be manipulated into other cells. While results with human cells has been more limited, according to the NIH, successful development of the research could eliminate the need for fetal cells in stem-cell experiments.
