Posted on 10/21/2006 9:39:20 PM PDT by Coleus
Are Human Embryos Human Beings?
Silver claims that while many people believe that human embryos are human beings, what sets us apart is that we contend that this is a matter of scientific fact, rather than a proposition of faith. But we are scarcely set apart by this claim. On the contrary, our claim that the human being that is, say, you the reader, is the same determinate and enduring being the same individual member of the species Homo sapiens who at an earlier stage of his or her life was an adolescent, a child, an infant, a fetus, and an embryo, is a fact confirmed by contemporary embryology and attested to by the standard works in the field. (We mentioned several of these works in the essay to which Silver responds; he cites none in support of his position.) Terms such as human adolescent, infant, and embryo do not refer to beings of distinct kinds; rather, they refer to the same kind of being a human being, an individual member of the species Homo sapiens at different developmental stages. You were once an embryo, just as surely as you were once an adolescent, and before that a child. So that readers will be left in no doubt, let us quote a couple of the standard texts. Keith Moore and T.V.N. Persaud, in The Developing Human: Clinically Oriented Embryology, perhaps the most widely used of the embryology texts, make the following unambiguous statement about the beginning of a new and distinct human individual: Human development begins at fertilization when a male gamete or sperm (spermatozoon) unites with a female gamete or oocyte (ovum) to form a single cell a zygote. This highly specialized, totipotent cell marked the beginning of each of us as a unique individual (p.16, emphasis supplied). Ronan ORahilly and Fabiola Mueller, in their book Human Embryology and Teratology, say this: Although life is a continuous process, fertilization (which, incidentally, is not a moment) is a critical landmark because, under ordinary circumstances, a new, genetically distinct human organism is formed when the chromosomes of the male and female pronuclei blend in the oocyte (p. 8, emphasis supplied).
An even weaker point advanced by Silver is his complaint that we have not budged in our rejection of Ronald Baileys claim that embryos are analogous to skin cells or other somatic cells that can be used (it is assumed) in human cloning. People who recall our exchanges with Bailey on NRO know that our rejection of his proposed analogy has nothing to do with stubbornness. Rather, we showed by careful argumentation and the presentation of the relevant biological facts that somatic cells are analogous not to embryos which are complete self-integrating organisms that are, as the standard scientific texts attest, new individuals of their species but rather to gametes (sperm and egg) whose union can bring into existence a new and distinct individual in the embryonic stage of its development.
Parts and Wholes
Let us turn now to two key propositions in our argument that human embryos are human beings. Silver disputes these propositions. We said that human embryos are (embryonic) human beings because they are complete human organisms that have an internal active disposition toward the mature stage of a human being. Silver disputes this proposition. He claims that we know that a single cell can separate from a 4- cell embryo and develop into a separate human baby (on rare natural occasions). According to him, Lee and George would argue that this cell has the internal active disposition that makes it a human being. But this would be inconsistent with our position that ES cells are not human beings because they lack such an active disposition to develop to maturity.
Silver here commits the error of confusing a part with a whole. When the cell is part of the whole multi-cellular organism, it lacks any disposition to develop on its own: for, if it is not split off, then it will continue to act in coordination with other cells along a trajectory toward the mature stage of development of the organism of which it is a part. If the splitting of the cell from the whole embryo results in a new embryo, this is a form of asexual reproduction, and an example of what evidently occurs in normal monozygotic twinning. So the fact that a single cell could be separated from a four-celled embryo and produce a second embryo is completely consistent with our position that embryonic-stem (ES) cells are not human beings because they lack this internal active disposition.
Developmental Trajectories
Silver then claims that an internal active disposition is not a term that has any meaning in the context of cellular or molecular biology. But this is mere bluster. The concept of a disposition or tendency is scarcely foreign to scientific contexts, where one speaks, for example, of the solubility of salt (sodium chloride) or of sugar (sucrose) and their variance in relation to temperature; for, solubility is a disposition, a tendency to do something in a given context. We added internal and active to distinguish it clearly from the (merely passive) capacity of something to be changed by being acted upon by an extrinsic agent. Thus, when biology books refer to the capacity for metabolism as being one of the marks of a living system, capacity refers to the same sort of thing we were referring to by the term internal active disposition. We could also have used the term developmental trajectory or developmental program to make the same point. An organism with a single developmental trajectory toward the mature stage of a human being is already a human being.
Moreover, dispositions or developmental trajectories in organisms have structural bases; that is, present in the organism are constituents and arrangements (organization) responsible for the active disposition or developmental trajectory. So, when Silver next claims, If George and Lee want to claim otherwise [namely, that internal active disposition does have a meaning in the context of cellular and molecular biology] Id like to know their perception of the molecular attributes that distinguish human being-cells from non-human-being, yet still fully viable and human cells we are happy to inform him. At the one-cell stage, comparing a zygote to, say, a stem cell, the one-celled embryo differs epigenetically from the stem cell (that is, while each has the complete genome there are differences in the activation or silencing of various genes).
But that is not all, for there are also differences with respect to several cytoplasmic factors. The embryo is such that, unlike a stem cell or any other somatic cell, its whole developmental program, both the epigenetic state of its DNA and various proteins and RNA factors in its cytoplasm (differences that are fully discernable at the molecular level) enable it to develop along a trajectory towards developmental maturity. At later stages of development, each of the cells of the embryo will differ from a separated stem cell or from other somatic cells among other ways in being modified by interaction with other cells within the embryo in such a manner as to contribute to homeostasis and the development of the organism as a whole. Therefore, in the context of cellular and molecular biology, a single-cell human being (i.e. a zygote) is distinct from any other viable human cell in terms of 1) its epigenetic state, 2) its pattern of gene activation, 3) its molecular composition, and 4) its subsequent pattern of development all of which can be (and indeed are) easily determined by scientists without recourse to crystal balls, sacred texts, or articles of faith.
Are Embryonic Stem Cells Human Beings?
In an astonishingly misleading paragraph, Silver quotes from a quick summary given by a reporter from Science magazine about a meeting of scientists working on gene knockouts (a procedure in which, by a complicated process, a gene is deactivated knocked out in a mouse in order to discover what that genes normal function would be). Suspecting something fishy, we took the trouble to look. The scientists were examining how they could cooperate more effectively on an international level. In the past they had shared research data and materials in part by shipping mice that had been engineered with a certain gene knockout. Silver quotes the reporter summarizing their discussion as follows: the participants agreed that it would be most economical to avoid trafficking in live mice and instead decided to maintain the knockouts as embryonic stem (ES) cells: clumps of tissue that can be frozen down and later grown up into full-fledged mice. Professor Silver takes this as evidence that [t]hose
who have worked only with early-stage mouse embryos are more honest than scientists who work on human stem cells when speaking about the relationship of embryos to stem cells. In other words, Silver presents this quote as proof from authority that stem cells really are the moral equivalent of embryos (because they can somehow by themselves grow into embryos). However, first, it is clear that the reporter is merely trying to summarize quickly the conclusions of a long meeting of stem-cell scientists; he is not attempting to select his words with exactitude, and much less is he addressing the issue in dispute between Professor Silver and ourselves. Silver is making much out of what is manifestly nothing more than a reporters shorthand manner of speaking. The article from Science is available online. We urge readers who are evaluating Silvers arguments against our own to examine this article for themselves.
Moreover and much more significantly Silver fails to mention that the knockout procedure involves first knocking out a specific gene in mouse stem cells, then injecting those stem cells into an early mouse embryo, where the manipulated stem cells will be randomly incorporated into the body of the developing embryo. In some of the chimeric mice formed in this manner, the stem cells will contribute to the gonads, and therefore the germ cells produced by the mature animal will also lack the gene that was eliminated from the stem cells. In this case, by back crossing such germ line deficient mice (i.e., inter-breeding the male and female offspring of the mouse carrying the gene deletion), ultimately a mouse is produced that lacks the knocked out gene in all the cells of its body, and the phenotype that results from the loss of that genes action can be determined. The point the scientists were making was simply that it would be more economical and efficient to ship to other sites the manipulated stem cells rather than the mice with the gene knockouts, largely because many such knock-out mice are quite unhealthy, and therefore they are difficult to breed and expensive to maintain. When the scientists described the ES cells (or, more precisely, when the reporter recounted them as describing the ES cells as) clumps of tissue that can be frozen down and later grown up into full-fledged mice, it was understood as part of the knockout procedure that this later growing up process would involve injecting those ES cells into mouse embryos. Silvers selective editing makes it appear that these scientists are asserting something they did not assert, and are speaking to an issue they did not address.
Professor Silver had argued in his book that aggregating mouse stem cells with mouse tetraploid embryos allowed the stem cells to develop by themselves into mature mice. We pointed out that the aggregation of the stem cells with tetraploid embryos (or embryo-like entities) does more than release an inner capacity in the stem cells. In fact, it generates a new and distinct organism, just as combining the nucleus of a somatic cell with an enucleated ovum (as in cloning) generates a new organism in both cases, an embryonic member of the mouse (or other) species in question. In his reply, Silver now says the following:
This is supposed to show that stem cells really are equivalent to embryos because they can by themselves, or all by their lonesome, develop into mature members of their species. However, this argument is dubious at best, given that there is no evidence that an embryo could be generated by recombining ES-cells that have been differentiated into placenta with an undifferentiated ES cell, and there is sound scientific evidence to suggest embryos cannot be generated in this manner. In fact, the second article Silver cites in his book on this subject is much more cautious in its conclusions than Silver himself is. The authors write:Furthermore, as I wrote in my book, human ES cells have already been differentiated into placenta. This means that, in theory, the requirement for a second source of cells to reconstitute an embryo may be nullified, and ES cells all by their lonesome could develop into a fetus and human baby. This discussion was conveniently left out of Lee and George's review.
Moreover, as partly recounted in Silvers book, and more fully in the scholarly articles to which Silver refers, the stem cells (in the experiment Silver cites) by no means develop into embryos and fetuses without extrinsic causes in the form of external manipulations. No one denies or has denied that one can manipulate factors found in early stem cells and combined those factors with stem cells in such a way as to generate a new embryo. But just as combining factors from an ovum and a somatic cell to generate a new individual member of a species in the embryonic stage does not show that an ovum or a somatic cell used in the process was somehow already an embryo or its equivalent, the combining and manipulating of factors found in early stem cells does not show that a stem cell is an embryo or its equivalent. The fact that one can combine A (a stem cell) with B (another cell that is produced by a significant manipulation of a stem cell) to produce C, does not in the least prove that A or B were identical with C all along.The human equivalent to trophoblast stem cells has not yet been derived, and it is likely that different growth factors will be required for their propagation.[note omitted] Although, in our current studies, BMP4 efficiently induced differentiation of human ES cells to trophoblast, these trophoblast cells propagated poorly, even in the continued presence of bFGF and fibroblast feeder layers (data not shown), suggesting that additional growth factors are required for their long-term proliferation. (Ren-He Xu, et al., BMP4 initiates human embryonic stem cell differentiation to trophoblast, Nature Bioetechnology 20 (December 2002), 1261-1264, at 1263)
But this argument is fallacious. Darwin proposes at least a partial explanation of the emergence of new species (whether it is a sufficient or complete explanation is another question, which we need not settle here). The idea is that some species evolved from other species, and that the human species evolved from lower animal species, at least with respect to the organic aspect of the human being (whether that is all there is to human nature is another question, which, again, we need not settle in the context of the current debate). But this does not show that there must have been some creatures which were in between humans and non-humans creatures that were neither human nor non-human. It only shows that certain non-human animals evolved gradually (or perhaps not so gradually the point is disputed among evolutionary biologists) to become more and more like humans until, at least partly through genetic mutations, the human species (as a distinct and identifiable species) emerged. Nothing in the Darwinian account suggests that there is or was a species that was neither human nor non-human a species that was in between human and non-human. In other words, if species B evolved from species A though an accumulation of minute genetic changes, that does nothing to show that the last entity just before the last genetic change which (wholly or in part) produced the new species B, must have been something that was partially B. Rather, that last entity prior to B was a non-B that was in a great many respects like a B.The most serious challenge, of course, came from Darwin, whose theory of natural selection suggests that in the evolution of pre-human apes into human beings, there was no first human being. Instead, there appears to have been a continuum of evolutionary forms in a process during which no child was significantly different from its parents. The scientific implication is that some "things" might be in-between non-human and human.
So our point has always been only that in some instances of defective fertilizations what results will lack the program for or developmental trajectory toward the mature stage of a human being, and so is not a human embryo. An embryonic human may certainly have the basic program or disposition orienting it toward the mature stage, but also have defects that prevent the full actualization of that program, as is the case with trisomy 21 and anencephalic infants.To be a complete human organism (a human being), the entity must have the epigenetic primordia for a functioning brain and nervous system, though a chromosomal defect might only prevent development to maximum functioning (in which case it would be a human being, though handicapped) (emphasis added). If fertilization is not complete, then what is developing is not an organism with the active capacity to develop itself to the mature (even if handicapped) state of a human.
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