by Kenneth W. Krause.
Kenneth W. Krause is a contributing editor and “Science Watch” columnist for the Skeptical Inquirer. Formerly a contributing editor and books columnist for the Humanist, Kenneth contributes regularly to Skeptic as well. He may be contacted at email@example.com.
Parkinson’s disease results from the death of nigral neurons, the dopamine-producing brain cells that facilitate signal transmission between nerve cells. Neuroscientists have successfully grafted fetal neurons into the brains of Parkinson’s patients, dramatically relieving some sufferers’ symptoms. But six fetuses are needed to treat just one patient, in part because transplanted cells perish at a rate of 90 percent shortly after grafting. As such, there will never be enough fetal tissue to treat all of the people who need it, and, according to science writer Marcia Barinaga, that is precisely why researchers are “pinning their hopes on cultured stem cells” that could potentially produce an unlimited supply of nigral cells for more than one million American Parkinson’s patients.
But stem cells have raised many questions and stimulated much debate in recent years. How do different kinds of stem cells compare? What are the scientific challenges, ethical implications, and political realities that complicate proposed research? And what medical progress has already made? In The Stem Cell Controversy, a manageable but refreshingly balanced anthology, philosophers Michael Ruse and Christopher Pynes relay recent commentaries, official decrees, and most importantly, expert analyses addressing each of these crucial issues.
The limitations of adult stem cells (ASCs) have been well documented by many, including the National Institutes of Health. ASCs are difficult to isolate and, in fact, have never been isolated in many tissue types. When located, they are often present only in small quantities, and they quickly lose the ability to divide and differentiate in cultures. ASCs are also more likely than embryonic stem cells (ESCs) to contain genetic errors or blemishes resulting from disease or the unavoidable hazards of daily living. And although certain varieties of ASCs—bone marrow and cord blood stems in particular—might be more versatile than others, there is no evidence to suggest that any ASCs are pluripotent, or capable of producing almost any type of cell in the human body.
But some of these concerns are largely unwarranted, argues neurobiologist Maureen Condic. That ASCs are difficult to locate and grow are mere technical problems that science will likely solve, given sufficient time and effort. The numerosity issue is trivial because autologous transplants, involving stems extracted from one’s own tissue, require fewer cells than would be necessary to treat groups of patients. Diseased donor tissues, as a practical matter, affect ASCs and ESCs equally because injuries or foreign agents are responsible for most illnesses. On the other hand, genetically based diseases are not usually relevant because they typically occur only later in life.
And assuming the successful isolation, maintenance, and transplantation of sufficiently numerous ASCs, adds neurologist Sidney Houff, their proliferative limitations might actually offer clear practical advantages. After all, as stem cells or more advanced progenitor cells becomes further differentiated, fewer things can go wrong. Prior commitment is an especially attractive attribute in the context of central nervous system cell transplants where phenotype control is crucial. In addition, ASCs and progenitors are likely to be more responsive than ESCs to signals in niches, where stems typically reside. In fact, ESCs might require signals that are simply not available in adult tissues.
But ESCs are uniquely pluripotent, supplying scientists and patients with the greatest hope for wide-ranging success, everything else being equal. ESCs possess vast potential with respect to cell and tissue therapies, despite the very iniquitous menaces of immune rejection and teratoma formation. But they can also help us to better understand the processes by which the most perilous diseases—cancers and birth defects, for example—are created. In other words, a more sophisticated comprehension of cellular activity at the executive level, and therefore of life itself, may hang in the balance.
But at what moral cost? Most are familiar with the standard permissive argument—generally, that the barely visible, 150-cell blastocysts from which inner cell masses and ESCs are extracted, do not display the attributes of personhood. But philosopher Don Marquis finds this position ethically dubious, even from a purportedly objective and non-religious perspective. Moral agency, for example, cannot be the standard because neither infants nor the mentally retarded can be considered moral agents. Sentience and consciousness cannot be considered elemental to personhood either because every person drifts into unconsciousness, albeit temporary, on a regular basis. ESC research, Marquis concludes, is nothing more than age discrimination.
Bioethicists Glenn McGee and Arthur Caplan attempt to address the typically conservative theological argument—that installation of the human essence (or soul) occurs at conception, and that the destruction of a human embryo necessarily results in the destruction of its human essence. If only for the sake of argument, McGee and Caplan grant blastocyst personhood. They point out, however, that only the replaceable and relatively nondescript parts of the early embryo—the cytoplasm, the cell wall, and mitochondria—are destroyed in the process of stem cell production. The truly human feature of the blastocyst, the recombined DNA of each parent, is not destroyed. This is quite significant in the context of in vitro fertilization (IVF) procedures where embryos are slated for destruction from the very beginning, and it becomes even more significant at the point when we can harvest DNA from stem cell lines to create fresh, nuclear transfer-derived embryos.
Political scientist Simon Clark agrees, contending that contemporary German, Australian, and American stem cell policies, essentially confining the use of ESCs to those currently in existence, are inconsistent with these countries’ policies allowing infertile couples to take advantage of IVF programs in order to have children. If we permit embryos to be destroyed for the purpose of procreation, why not for the purpose of saving lives? As a compromise, Clark proposes a national laissez-faire position permitting the creation of human embryos for the sake of research alone, but including compelling incentives for the accomplishment of the same goals by other, less controversial, means, perhaps in the form of financial incentives or tax breaks.
The research has just begun, of course, but certain successes furnish us with adequate grounds for continued optimism. Katty Kay and Mark Henderson report an experiment involving 120 mice and rats infected with a virus causing a paralyzing and generally incurable spinal disease. Following an injection of human ESCs, every rodent regained at least some mobility. Kay and Henderson add that the damage originally diagnosed in the animals was very similar to that which afflicts motor neuron disease sufferers, including celebrated astrophysicist Stephen Hawking. And CBS news correspondent, Carol Marin, introduces Keone Penn, a young boy with a particularly severe and painful case of sickle cell. Ineligible for a bone marrow transplant, the standard treatment for someone in his condition, Keone was saved by an injection of stem cells derived from umbilical cord blood.
Parkinson’s patients are typically forced to endure frustrating stiffness and tremors. In advanced stages, they can suffer uncontrollable falls, incontinence, difficulty in swallowing, and even dementia. Patients might eventually require constant assistance and become bed-ridden. Experiments with mice, however, have proven that neural stem cells can effectively replace defective cells throughout the brain and, in some cases, correct the underlying problem. And now we have the ability to generate specific types of neural cells from ESCs. According to the National Bioethics Advisory Commission, these advances “hold much promise for the treatment of severe neurological disorders that today have no known cure.”
Stem cell reports continue to surface at an encouraging rate. Fully functional liver cells have been grown from stem cells located, again, in umbilical cord blood. One scientist has coaxed human ESCs into retinal stems, six percent of which have grown into photoreceptor cells in the lab—great news for victims of macular degeneration. Researchers in Boston believe they have discovered a means of generating ESCs without destroying embryos. A group in Japan claims that they can create these cells without using an embryo in the first place. Perhaps it’s only a matter of time. But no person, whether she considers herself a scientist, a politician, or a common citizen, should ever forget that, for many, time grows increasingly short.