The Search for Plueripotent Adult Stem Cells
The thorny ethical issues raised by human therapeutic cloning could be avoided if the stem cells didnt have to be harvested from an embryo. The Holy Grail of stem cell research is to find cells able to become any other kind of cell — technically known as pleutipotent stem cell — somewhere in the body of an adult human.
In 2001 researchers reported that they had found just such cells in the bone marrow of mice. They transplanted single stem cells from mouse bone marrow into the marrow of individuals whose marrow had been destroyed. After 11 months, the one stem cell had given rise to deswcendant cells that had migrated thropughout the body, forming new bone, blood, lung, esophagus, stomach, intestine, liver, and skin cells (figure 48.8). The bone marrow stem cells appear to have the properties of the long-sought pleuripotent adult stem cells. Many labs are trying to repeat this exciting preliminary result.
Many of the ethical issues concerning the use of human embryonic stem cells for research can be avoided if cells with similar developmental potential can be found in adult tissues. Some results suggest this may be possible.
Grappling With the Ethics of Stem Cell Research
Few advances in science have proven as controversial as embryonic stem cell research and the possibility of using therapeutic cloning to generate them. The relevant facts are straightforward. Human embryonic stem cells retain the potential to become any tissue in the body, and thus have enormous promise for treating a wide range of diseases. Human embryonic stem cells are very difficult to isolate and establish in culture, but a few dozen lines have been successfully obtained from the inner cell mass of six-day blastocysts. It is important to isolate the cells at this early stage, before development begins the process of restricting what sorts of tissues the stem cells can become. The blastocysts are obtained from reproductive clinics, which routinely produce excess embryos in the process of helping infertile couples have children by in vitro fertilization.
However, obtaining embryonic stem cells destroys the early embryo in the process, and for this reason stem cell research raises profound ethical issues. The timeless question of when human life begins cannot be avoided when human embryos are being deliberately destroyed. What is the moral standing of a six-day human embryo? In resolving the tension between scientific knowledge and moral sensibilities, religious, philosophical, and cultural issues all come into play. Table 48.1 illustrates the range of issues being discussed.
It will come as no suprise that government, which funds much of modern biomedical research, has become embroiled in the controversy. In Britain, reproductive cloning is banned, but stem cell research and therapeutic cloning to obtain clinically useful stem cells are both permitted. Because the research is funded by the government, there is careful ethical supervision of all research by a variety of governmental oversight committees. Britains HFEA, for example, is a panel of scientists and ethicists accountable to parliament, which oversees government-funded stem cell research. Similar arrangements are being established in Japan and France. Germany, by contrast, discourages all stem cell research.
In the United States, the situation is ambivalent. American stem cell research is chiefly carried out in private research labs using no government funds, and thus subject to no ethical oversight. This leaves American scientists pretty much free to do what they want, so long as they use private money. Federal funds were made available in the summer of 2001 for research on the small number of existing human embryonic stem cell lines. In what has become a very political contest between those favoring increased stem cell research and those opposing all such research on ethical grounds, it seems certain that federal government policies with regard to stem cell research will fluctuate for some time to come.
The Ethics of Stem Cell Research
1. Destruction of Human Embryos
opponents A human life begins at the moment egg and sperm are united, so destroying an embryo to harvest embryonic stem cells is simply murder, and morally wrong. Benefits to others, however great, cannot justify the destruction of a human life.
proponents While human embryos should be treated with respect, the potential for saving lives that embryonic stem cell research offers is also a strong moral imperative. The blastocysts being used to obtain stem cells were created to help infertile couples conceive, and would have been destroyed in any event. Besides, it is not clear that an individual life begins at fertilization. An early embryo can split, leading to the birth of identical twins, so it can be argued that individuality begins some days after fertilization.
2. Possibility of Future Abuse
opponents Permitting embryonic stem cell research may open the door to further ethically-objectionable research. Certainly the development of embryonic stem cell therapies will lead to a cry for therapeutic cloning, so that the therapies can actually be employed in clinical situations. This creation of embryos specifically for the production of embryonic stem cells is morally wrong. In addition, therapeutic cloning to obtain clinical embryonic stem cells is unnatural, as it involves producing a viable human embryo without fertilization. If therapeutic use of the results of stem cell research is unacceptable, then there is little use in carrying out the research in the first place. It only delays and excalibrates the morally difficult choice posed by therapeutic cloning. Even more disturbing, it opens the door to reproductive cloning — the production of human babies from cloned embryos. This moral nightmare will always be a threat if an absolute line is not drawn preventing all cloning.
proponents In practice, only a few hundred cell lines are likely to be required to carry out embryonic stem cell research. The derivation of human cell lines, which is difficult and expensive to do, would be limited to these few lines. Continuous destruction of embryos would be neither desirable nor likely. Therapeutic cloning presents a separate and more complex ethical issue. Because fertilization is not involved, the blastocyst might better be thought of as an activated egg rather than as an embryo. Such a distinction has biological merit, and avoids the ethical issues posed by human reproductive cloning, which should be banned.
3. Alternative Sources of Stem Cells
opponents Why not use stem cells derived from adult tissues? These stem cells raise no difficult ethical issues, and can lead to the same medical benefits.
proponents Adult stem cells simply cannot do the job. By the time embryonic stem cells ahve developed into adult stem cells, they have lost much of their developmental versatility, and so lack the range of medical capabilities necessary for regenerative medicine. Also, adult stem cells are not very prolific, and have proven to be difficult to use in therapeutic procedures on experimental laboratory animals.