Stem Cell Research
Stem Cell Research
Few topics in science and religion have been as hotly contested in recent years as stem cell research, largely because it involves the fate of, disposition of, and research on the human embryo. There are two basic types of stem cell research—that involving adult cells (AS cells) and that involving human embryonic cells (ESCs or hES cells); only the latter is a source of controversy. In both cases, research is still at the early stages regarding the programming and uses of these cells, and there is comparatively little data about the efficacy of AS and hES cells for human therapies. That is why most scientists agree that, in the United States, government funding should be widely available for research on both types of stem cells, an issue that has been contested in the U.S. Congress.
Stem cells are unspecialized and so are able to renew indefinitely; they also have the capacity to differentiate into specialized cells. In humans, these cells are found in some adult organs, in blood, and in bone marrow (Mezey et al. 2000; Bjornson et al.1999); in the inner cell mass of the human embryo at the blastocyst stage (five to six days after fertilization) (Thomson et al. 1998); on the gonadal ridge of aborted or miscarried fetuses (Shamblott et al. 1998); and in the placenta and umbilical cord (hematopoetic stem cells).
Because stem cells have the capacity to regenerate, particularly ESCs, they have ushered in the era of "regenerative medicine," signaling that, in theory, these cells can be used to regenerate human tissues and cells, and ultimately increase quality of life and the human life span. Embryonic stem cells are the progenitor cells for the human body and at their earliest stage (the blastocyst stage) they are completely undifferentiated and can give rise to any cell type in the human body (totipotent, pluripotent, and multipotent are all terms that have been used to describe this phenomenon). At this stage the cells have not yet received their "marching orders" for what they will become; therefore, scientists have been experimenting with controlling the programming of ESCs in culture in order to direct their ends (controlled differentiation) to specialized cells such as blood, skin, and nerve cells.
In order to extract these embryonic stem cells, scientists must collapse the trophectoderm that surrounds the blastocyst in order to get the stem cells from the inner cell mass (ICM) where they reside within the blastocyst or pre-embryo. Such a technique destroys the pre-embryo and renders it incapable of implantation in the uteran wall. This is the crux of the ethical problem for those who oppose embryonic stem cell research.
Studies in 2001 and 2002 indicate the potential for primate parthenotes to form embryonic stem cells and to develop a variety of differentiated cell types in culture (Cibelli et al. 2001; Holden 2002). Parthenotes are embryos that grow from unfertilized eggs (chemically tricked into fertilizing and retaining the full choromosomal complement) that are, so far as is known, incapable of becoming viable fetuses in primates and humans. Thus, scientists hope that this may prove to be an ethically uncontroversial way to obtain stem cells, allowing researchers to avoid therapeutic cloning as means to this end.
The ethical and religious issues surrounding stem cell research concern not so much the therapeutic ends of the research (cures for Parkinson's disease, juvenile diabetes, Alzheimer's disease, heart disease, and a host of other degenerative diseases); rather, the controversy surrounds the status of the human embryo and points to larger issues about what it means to be human and when life begins.
The Roman Catholic Church and conservative Protestant churches have made the strongest opposition to embryonic stem cell research of all religious traditions in the United States. The Catholic position is that life begins at conception; thus the human embryo is accorded the full rights and dignity of a human person from the very moment that the sperm penetrates the egg. Therefore, it is a grave sin to destroy any human embryo since the act constitutes destruction of life itself, a responsibility belonging only to God. Moreover, the Catholic Church has opposed the creation of human embryos for research purposes (therapeutic cloning, for example) for two reasons: To do so would be to treat human life as a mere means to an end, which is a violation of human dignity and the sanctity of life; and embryos ought only to be created in conjunction with the conjugal act of love within the context of marriage (natural law). (Donum Vitae 1987). It is important to note, however, that there are a variety of dissenting Catholic positions on this issue.
Conservative Protestant churches such as the Southern Baptist Convention and fundamentalist independent Christian churches have tended to join the Catholic protest against ESC research and have emphasized prioritizing AS research as an acceptable means to the end of regenerative therapies. The rationale for such opposition does not emphasize a natural law approach to ethics and emphasizes instead a biblical approach. An argument that the Christian tradition has a mandate to protect the weakest and most vulnerable members of society (the embryo in this case) is advanced by Lutheran theologian Gilbert Meilaender in his essay "Some Protestant Reflections" (2001).
On the other hand, mainline Protestant denominations (United Church of Christ, Episcopal, Presbyterian, Methodist) tend to be supportive of all stem cell research so long as the human embryo is treated with respect. In 2001, the General Convention of the Presbyterian Church voted to endorse embryonic stem cell research. Mainline Protestantism has focused on the great amount of good that can come of this research and on concerns of distributive justice to ensure that the poor will receive the benefits of stem cell research equally with the rich. Moreover, most mainline Protestants (and many Catholics) support using excess embryos for stem cell research. These embryos have been frozen in fertility clinics and would be thawed and discarded eventually if they were not put to what many believe is a good end—human healing. One Lutheran theologian who supports ESC research, in contrast to Meilaender's argument, is Ted Peters (Peters 2001).
Although there are three main branches of Judaism (Orthodox, Conservative, and Reform), and it is sometimes difficult to find agreement on bioethical issues, in this case most Jewish scholars are supportive of all stem cell research. This is due, primarily, to the fact that Judaism professes a strong mandate from God to heal and to reduce human suffering. Moreover, in Jewish law the embryo has no moral standing outside the womb; a developing embryo in laboratory culture is morally neutral until implantation. Therefore, the ends of all stem cell research appear to be morally coherent with Jewish ethics (Dorff).
Islam is also a diverse religious tradition. However, in general, Islam would be in favor of all forms of stem cell research since there appear to be no "recent rulings in Islamic bioethics regarding the moral status of the blastocyst from which the stem cells are isolated" (Sachedina). Islamic scholars have found that the Qur'han's focus is primarily on the developing fetus in the womb. Islam shares with Judaism a concern with human healing; thus, if ESCs hold real (not just speculative) potential for therapeutic healing, there would be no objection to proceeding with such research.
See also Biotechnology; Christianity, Roman Catholic, Issues in Science and Religion; Cloning; DNA; Gene Patenting; Gene Therapy; Genetic Engineering; Genetics; Judaism; Islam
bjornson, christopher r. r., et al. "turning brain into blood: a hematopoietic fate adopted by adult neural stem cells in vivo." science 283 (1999): 534–7.
cibelli, jose b., et al. "parthenogenetic stem cells in nonhuman primates." science 295 (2002): 819.
chapman, audrey r.; frankel, mark s.; and garfinkel, michele s. "stem cell research and applications: monitoring the frontiers of biomedical research." in aaas science and technology policy yearbook, eds. albert h. teich, stephen d. nelson, ceilia mcenaney, and stephen j. lita. washington, d.c.: american association for the advancement of science, 2000. available from http://www.aaas.org/spp/yearbook/2000.
doerflinger, richard. "destructive stem-cell research on human embryos." origins 28 (1999): 769–773.
congregation for the doctrine of the faith. "donum vitae (gift of life): instruction on respect for human life in its origins and on the dignity of procreation, replies to certain questions of the day." washington, d.c.: united states catholic conference, 1987.
dorff, elliot n. "stem cell research: a jewish perspective." in the human embryonic stem cell debate: science, ethics and public policy, ed. suzanne holland, karen lebacqz, and laurie zoloth. cambridge, mass.: the mit press, 2001.
farley, margaret a. "roman catholic views on research involving human embryonic stem cells." in the human embryonic stem cell debate: science, ethics and public policy, ed. suzanne holland, karen lebacqz, and laurie zoloth. cambridge, mass.: the mit press, 2001.
green, ronald m. "the stem cell conundrum." religion in the news 4 (2001): 18–20, 25.
holland, suzanne; lebacqz, karen; and zoloth, laurie, eds. the human embryonic stem cell debate: science, ethics, and public policy. cambridge, mass.: mit press, 2001.
meilaender, gilbert. "some protestant reflections." in the human embryonic stem cell debate: science, ethics, and public policy, eds. suzanne holland, karen lebacqz, and laurie zoloth. cambridge, mass.: mit press, 2001.
mezey, Éva, et al. "turning blood into brain: cells bearing neuronal antigens generated in vivo from bone marrow." science 290 (2000): 1779–82.
national bioethics advisory commission. ethical issues in human stem cell research, 3 vols. rockville, md.: nbac, 1999–2000.
national research council committee on biological and biomedical applications of stem cell research; united states institute of medicine board on neuroscience and behavioral health; and national research council board on biology. stem cells and the future of regenerative medicine. washington, d.c.: national academy press, 2002.
peters, ted. "embryonic stem cells and the theology of dignity." in the human embryonic stem cell debate: science, ethics and public policy, eds. suzanne holland, karen lebacqz, and laurie zoloth. cambridge, mass.: mit press, 2001.
sachedina, abdulaziz. "islamic perspectives on research with human embryonic stem cells." in ethical issues in human stem cell research, vol. 3. rockville, md.: national bioethics advisory commission, 2000.
shamblott, michael j., et al. "derivation of pluripotent stem cells from cultured human primordial germ cells." proceedings of the national academy of sciences (usa) 95 (1998): 13726–31.
thomson, james a., et al. "embryonic stem cell lines derived from human blastocysts." science 282 (1998): 1145–7.
Stem Cell Research
Stem Cell Research
In 2001 President George W. Bush (1946–; served 2001–) authorized limited federal funding of research on existing human embryonic stem cell lines. His act allowed funding only when the process of obtaining stem cells had been initiated prior to the bill, and the embryo from which the stem cell line was derived no longer had the possibility of
development as a human being. This marked a partial reversal of his position in the 2000 presidential campaign, when, with the strong backing of pro-life (anti-abortion) groups, he opposed any use of stem cells obtained from human embryos. The 2001 act infuriated some religious and pro-life groups. On the other hand, Bush's bill so limited embryonic stem cell research that it raised equally strong opposition from advocates of stem cell research.
What is a stem cell?
Human stem cells are cells that are capable of developing into the different varieties of tissue present in the body of a fully formed human. There are three kinds of stem cells: embryonic, adult, and cord blood stem cells. An embryo is an unborn organism at the stage immediately after conception and before it develops into a fetus around the eighth week of pregnancy —embryonic stem cells are formed in the very early stages of the embryo. Adult stem cells can be taken from adult tissues. Cord blood stem cells can be found in the human umbilical cord, the cord that carries blood, oxygen and nutrients to the fetus from the placenta during pregnancy.
While adult and cord blood stem cells can only develop into a limited number of varieties of human tissue, embryonic stem cells are capable of developing into any one of the 210 or more varieties of tissue present in the body of a fully formed human. This is because a blood cell, for example, once formed, cannot become a brain cell, or vice versa; but at the embryonic stage, all cells are of a single type.
Stem cells have generally been obtained from surplus, or extra, embryos that were not used during in-vitro fertilization, a procedure that helps infertile women have babies. When a woman undergoes in-vitro fertilization, she is given medication that causes her to produce multiple eggs. The eggs are removed from the uterus and fertilized by sperm in a lab, creating embryos. In three to five days, the embryos reach what is called the blastocyst stage and are ready to be implanted in the woman's womb. Those that are not used are either frozen in liquid nitrogen or thrown away. Most of these surplus embryos will eventually be destroyed, but they are the basis of a huge national debate. If scientists could obtain the stem cells from them, they could almost certainly use them to save human lives and suffering. On the other hand, opponents point out that obtaining cells from any embryos destroys their potential for life.
Stem cell research has the potential to provide almost limitless benefits to medicine and human health. Among other things, the cells could make it possible to cultivate spare organs and other body parts; to produce human tissue for use in the treatment of diseases, such as cancer, that involve the degeneration of human cells; or for testing potentially dangerous drugs on human cells without actually testing them on human beings. There is reason to hope that in the not too distant future, stem cell research can provide cures for such diseases as Alzheimer's and Parkinson's diseases, juvenile diabetes, and even for spinal cord injuries. Enormous suffering would be eliminated with these cures. Beyond that, the knowledge scientists can derive from stem cells is expected to result in stunning health advances across the board.
But stem cell research also raises important moral and political considerations. Most opponents of embryonic stem cell research do not oppose adult stem cell research, but they connect embryonic stem cell research with abortion. They believe that the destruction of the embryo violates moral law and dehumanizes the unborn. There are also fears that if embryonic stem cell research is not prohibited, people will be encouraged to create surplus embryos for profit.
The short history
The issue of stem cell research came to the fore during the administration of President Bill Clinton (1948–; served 1993–2001). At that time, the proposal under consideration permitted federal funding of embryonic stem cell research as long as the researcher had not been involved in actually obtaining the cells. By the time Bush limited research with his act of 2001, the National Institutes of Health (NIH) had established a registry listing the 78 human embryonic stem cell lines that were eligible for use in federally funded research, but only 22 cell lines were then available. Scientists were concerned about the quality and longevity of these 22 stem cell lines and hoped to be funded in their attempts to obtain new lines.
Attempts to continue the research
Embryonic stem cell research has had bipartisan (both political parties) backing from the start. This was especially evident in 2004 when former first lady Nancy Reagan (1921–) made a public plea to support stem cell research. Her husband, President Ronald Reagan (1911–2004; served 1981–89) was in the late stages of Alzheimer's disease, a degeneration of the brain functions that causes memory loss and eventually death, and Nancy Reagan wanted to waste no more time in finding a cure so that others would be spared his suffering. Since stem cell research is going on in other countries, many Americans fear a time will come when they will be excluded from cures that are available elsewhere. Polls have repeatedly shown that a majority of Americans approve of stem cell research.
Congress was not deaf to the pleas. On May 24, 2005, the House passed an act that would allow federal support of research that utilized embryonic stem cells regardless of the date they were taken from a human embryo. The act would have negated Bush's 2001 bill limiting research to the existing stem cell lines, but President Bush vetoed the bill in July 2006.
Some state governments have also taken action either for or against embryonic stem cell research. By 2007, seven states had provided money for stem cell research in their states: California, Connecticut, Illinois, Maryland, New Jersey, New York , and Wisconsin . Massachusetts was in the process. Iowa and Missouri , though they have not funded research, have made it clear that embryonic stem cell research is legal. On the other hand, Arkansas, Indiana, Louisiana, North Dakota , and South Dakota have prohibited embryonic stem cell research.
An alternative: Therapeutic cloning
Several alternatives to obtaining stem cells from embryos were studied in the early 2000s, particularly therapeutic cloning (also termed “embryo cloning”). Cloning is the scientific process of obtaining a group of genetically identical cells from a single cell to create a genetic copy. In August 2003 a Chinese research team reported that it had made human embryonic stem cells by combining human skin cells with rabbit eggs. The researchers removed the rabbit eggs' DNA and injected human skin cells inside them. The eggs then grew to form embryos containing human genetic material. After several days the embryos were dissected to extract their stem cells. There is mounting evidence to suggest that stem cells from cloned embryos have even greater potential as medical treatments than stem cells harvested from unused embryos at fertility clinics.
Unfortunately, therapeutic cloning is sometimes categorized with human cloning, the creation of a genetically identical copy of an existing human. Because of this connection, therapeutic cloning of stem cells has also met with strong opposition. While nearly all lawmakers concur that Congress should ban reproductive cloning, many agree with scientists that stem cells derived from cloned human embryos have great medical value. They assert that the aim of allowing research is to relieve suffering. Like the issue of abortion, the debate is likely to continue for a long time.
A newly fertilized egg is the ultimate stem cell. It is totipotent – capable of generating all the different types of cells found in the body, and also the fetal part of the placenta and supporting tissues. The fertilized egg splits into two, and those into four, and so on. For the first few divisions, up to at least the 8-cell stage, all the cells of the tiny embryo are totipotent stem cells. Indeed, if these early cells separate, they can each continue to develop, making identical twins, triplets, quadruplets, etc.
About four days after fertilization, the route to commitment starts. Some cells form an outer layer, which becomes part of the placenta, while others make the inner mass, which is the beginning of the true embryo. Initially this consists entirely of pluripotent stem cells, which cannot give rise to placental tissue but can make any component of the fetus itself. As the embryo grows, and the parts of the body start to emerge, the individual stem cells within each future organ or tissue become further specialized so as to be capable of producing only a certain range of possible final cell types. These stem cells are then called multipotent. At a certain stage in the development of each ‘family tree’ of cells, one or both of the daughter cells produced by the division of a stem cell becomes ‘committed’, that is, incapable of further division. These committed daughters continue to differentiate and become the normal functional cells of the heart, skin, brain, kidney, and other organs.
Adult animals still have some multipotent stem cells, especially in tissues such as skin and blood, in which cells last only a short time and have to be replaced. Indeed, even in the adult brain, previously thought to be incapable of making new nerve cells, there are populations of stem cells, which are constantly producing relatively small numbers of new neurons.
We now stand at the threshold of a potential revolution in medical treatment for diseases and disorders in which organs stop working properly. At present, some such conditions, such as heart, kidney and liver disease, can be treated by transplantation of a replacement organ from another person. But demand for donor organs is far outstripping supply, and the failure rate of such surgery is quite high, mainly because of the problem of rejection. Many other disorders, such as stroke, diabetes and Alzheimer's disease, cannot presently be treated by transplantation. The great hope is that suitable stem cells, produced in large quantities through cell culture methods and injected into failing tissues and organs, will produce fresh, replacement cells to take over from lost or damaged ones.
Stem cells for such replacement therapy could be produced in a number of different ways. Ultimately, it might be possible to make them with the kind of methods used to produce the first cloned mammal, Dolly the sheep. An ordinary specialized adult cell from the patient could be used to produce a totipotent stem cell by removing the nucleus (with the DNA-containing chromosomes), and inserting it into a human egg from which the nucleus has been removed. But there are many problems with this approach, not least the fact that adult cells may have accumulated genetic errors, which will be transmitted to the stem cells produced. Everyone agrees that formidable technical obstacles must be overcome before the cloning of stem cells from adult cells becomes safe. There is also concern that the development of methods for therapeutic cloning would inevitably lead to the production of whole human beings, who, like Dolly, are genetic replicas of an adult. At present, the vast majority of scientists and clinicians, not to mention ethicists and politicians, are opposed to such reproductive cloning, but it must be said that resistance may decrease if the techniques involved can be made more reliable.
In principle, some of the patient's own stem cells could be harvested (most likely from bone marrow or certain parts of the brain), multi-plied in culture and injected into a diseased or damaged region to produce new cells. Stem cells derived from the patient's own body would have the great advantage that they would not be rejected. This approach has already been successful in experimental animals, with stem cells from bone marrow used to replace damaged heart muscle. It may soon be used in humans to treat heart disease, diabetes, and other such diseases. However, it would not be appropriate for the replacement of tissues that are diseased because of a genetic disorder (such as Huntington's disease or cystic fibrosis), since stem cells from the patient would have the same genetic mistake in their DNA. This strategy would also be inappropriate in acute conditions, demanding immediate treatment, because of the time needed for stem cells to multiply in culture.
The most immediately promising strategy is to isolate pluripotent stem cells from human embryos just a few days after fertilization, to culture them, and to inject them into the patient's diseased or damaged organ. Since such cells carry different DNA from that of the patient, they could be used to treat genetic disorders. On the other hand, this means that precautions would have to be taken to avoid rejection.
Transplantation of immature nerve cells and stem cells from the brains of aborted human embryos has been used for several years to treat the degenerative brain condition, Parkinson's disease, with reasonably encouraging results. Such treatment has not greatly alleviated the characteristic tremor of the hands, and some patients have developed disturbing unintended movements. But most have regained the ability to initiate and control their actions. It is probable that embryonic stem cell injection will soon be used in efforts to treat the degenerative diseases Huntington's disease and Alzheimer's, and even stroke, in which parts of the brain are destroyed becomes of interruption of the blood supply.
There is wide agreement among medical scientists that research on human embryonic stem cells is an important first step towards stem cell therapy, even though it may eventually be possible to use adult stem cells. Yet the prospect of harvesting cells from living human embryos smacks of Frankenstein or Brave New World, and ‘pro-life’ religious groups have mounted stout moral opposition. However, it would not be necessary to fertilize additional human eggs specifically for such research. Present methods for the production of ‘test-tube babies’ involve the production and storage (by freezing) of several fertilized eggs, the unwanted ones simply being destroyed or permanently stored. These surplus eggs could, with parental agreement, provide a ready source of embryos for stem cell collection. Moreover, as long as there are strict limits on the time for which the embryo is allowed to develop, it will have no nervous system or other organs, no possibility of feelings, and nothing approaching an independent life. Also, the indubitable suffering of the many people who might be helped by stem cell therapy ought to weigh heavily in the complex moral equation.
In 2001, the British government authorized stem cell research on human embryos up to 14 days post-conceptual age. Given the huge potential benefits of stem cell therapy, it is likely that other nations will follow suit.
Further reading: Thomson, J. et al. (1998) Embryonic stem cell lines derived from human blastocysts. Science 282: 1145–1147.
US National Institutes of Health website. Stem cells: a primer. http://www.nih.gov/news/stemcell/primer.htm
See also: antenatal development; assisted reproduction; cloning; disease; gene therapy; genetics, human; organ donation; pregnancy; transplantation.
A stem cell has two special qualities: the ability to produce offspring of itself indefinitely, and the ability to differentiate into different types of specialized cells. “Adult” stem cells are found in various organs of fully formed organisms. For example, umbilical cord blood and bone marrow contain stem cells capable of producing the various cells found in the blood, such as red blood cells, white cells, and platelets.
Public debate about ethical, social, religious, and legal issues involving stem cells has centered on a different kind of stem cell, so-called embryonic stem cells, usually obtained from excess embryos created by in vitro fertilization (IVF), but sometimes created specifically for research or therapeutic purposes. These human embryonic stem cells (hESCs) have the capacity to form any tissue in the body; that is, they are totipotential.
HESCs are of scientific and medical interest for three reasons: (1) they provide an opportunity to do laboratory research on normal and abnormal differentiation; (2) they provide an opportunity to test experimental therapies, including drugs and genes, at a cellular level, without exposing living animals or humans to risk; (3) they present an opportunity to develop and transplant cell lines that can replace vital molecules such as insulin (for patients with diabetes mellitus) or dopamine (for patients with Parkinson’s disease), or to replace damaged tissue in the heart, nervous system, or elsewhere.
HESCs from residual IVF embryos are unlikely to be sufficient for all research and therapeutic interests. If, for example, stem cells are to be useful in treating diabetes, it will be important to create a cell line that is genetically identical to the recipient, so that it will not be rejected after transplantation. This can be accomplished by removing the nucleus of an egg, replacing it with the nucleus from a cell obtained from the potential recipient, and allowing the egg to grow to a stage when stem cells can be removed. This is called “somatic cell nuclear transfer” (SCNT).
SCNT is also of interest for laboratory research on genetic disorders such as cystic fibrosis or Tay Sachs disease. An embryo is created using the nucleus from a somatic cell of a patient with the disorder being studied, and then stem cells with the abnormal gene are obtained from the early embryo. This is sometimes called “research cloning.” SCNT for the purpose of creating a cell line that would be used for treatment is sometimes called “therapeutic cloning.”
Objections to research involving hESCs involve several concerns. First, some believe that an embryo has the same moral status as a fully formed human and is entitled to the same protections. Destruction of an embryo, in this view, is morally equivalent to murder. Proponents of hESC research point out that residual embryos are used only when the parents intend to destroy them anyway, and are not destroyed because of the interest in stem cell research. They also point out that tens of thousands of residual IVF embryos are destroyed annually without similar objection.
Second, opponents also argue that there are alternative approaches to obtaining totipotential stem cells, such as using adult stem cells. Most experts believe adult stem cells are not totipotential and therefore should not divert research funds from the more promising embryonic stem cells.
Third, opponents object to SCNT combined with hESC research because of concerns that it is a critical technical step for reproductive cloning, the creation of genetically identical replicas of existing persons. Advocates of hESC research argue that reproductive cloning is nearly universally opposed at the present time, largely because of concerns about biologic safety, and that “slippery slope” arguments are insufficient to prohibit research that can help alleviate suffering, disability, and death from diseases affecting large numbers of existing persons.
Fourth, concerns have been raised that the transfer of human cells into the developing brain of laboratory animals could result in an animal capable of human experience and therefore with moral status comparable to a human. Although most neuroscientists consider this to be unlikely, some groups have proposed prohibiting full gestation of nonhuman primates if human stem cells have been implanted in their central nervous systems early in embryonic development.
Governmental policies reflect a range of approaches in different countries and states, and policies within any jurisdiction are often in flux, subject to the success of politicians with various views. Some prohibit human embryonic stem cell research; some permit it but have restrictions on use of public funds; some permit research using existing embryos but prohibit creation of embryos for research; and some restrict somatic nuclear cell transfer because of concerns that it may accelerate human reproductive cloning.
SEE ALSO Ethics in Experimentation; Medicine; Neuroscience; Public Policy; Reproductive Politics
McHugh, Paul R. 2004. Zygote and Clonote—The Ethical Use of Embryonic Stem Cells. New England Journal of Medicine 351 (3): 209–211.
Sandel, Michael J. 2004. Embryo Ethics: The Moral Logic of Stem-Cell Research. New England Journal of Medicine 351 (3): 207–209.
Walters, LeRoy. 2002. Human Embryonic Stem Cell Research: An Intercultural Perspective. Kennedy Institute of Ethics Journal 14 (1): 3–38.