Carter Snead responds to Silver on Stem Cells

Carter Snead had this to say in response to biologist Lee Silver's argument that ipc cells are potential embryos:

Silver has made this argument in the past, and it has been decisively rebutted by Robby George and Pat Lee ( http://article.nationalreview.com/?q=OTNiYWM2ZjJiYWVlN2IyMzFjOWYwMDZmMTc4MzU2MGU =).  In short, Silver is arguing that ES cells can become human beings because a process called "tetraploid complementation" (in which ES cells are fused with a mouse embryos that has 4 sets of chromosomes rather than the usual two) produces a new chimeric organism.  But as George and Lee show below, tetraploid complementation is simply a kind of conception (or inception) in which ES cells play a role.  Silver's argument is exactly analogous to the claim that a human skin cell can become an organism because when its nucleus is placed in an ennucleated egg, this process (cloning or SCNT) gives rise to a new organism, genetically virtually identical to the skin cell donor.  Therefore, this argument would conclude, a skin cell is the same as an embryo.  This is obviously fallacious, and ignores the key difference between organ and organism, parts and wholes.

George & Lee's fuller explanation follows below:

. . .

Silver bases his claim that “embryonic stem cells are equivalent to embryos” on the fact that mouse embryos can be generated from embryonic mouse stem cells and have all of their genetic makeup, and cell lineage, derived from those initial stem cells. A tetraploid embryo-like entity known, though controversially, as a tetraploid “embryo” (“tetraploid” meaning that the entity has four sets of chromosomes rather than the normal two sets) is developmentally defective, so it can give rise only to trophoblastic cells (precursors of the placenta and associated tissues) and not to the cells of the “embryo proper.” When combined with mouse ES cells (ones that have a normal number of chromosomes), these can produce a chimeric mouse in which the cell lineage of its placenta and associated tissues is derived from the tetraploid entity (or “embryo”), and the cell lineage of the mature embryo (the “embryo proper”) is derived from the ES cells. From this, Silver infers that ES cells can by themselves develop into the mature stage of the animal (see his book, p. 140) ­ “by themselves” in the sense that the DNA in all of the mature embryo’s cells is identical to that in the ES cells.

Since it is often argued that human embryos are human beings because they can “by themselves” develop into mature humans, it follows ­ on Silver’s argument ­ that embryos and stem cells are (ontologically and morally) equivalent. But since it is absurd to think that ES cells are human beings, it also is absurd (Silver’s argument continues) to think human embryos are human beings.

This argument is a descendent of an earlier, similar argument that embryos are morally equivalent to somatic cells (such as skin cells) because somatic cells can produce mature human beings by way of cloning via somatic cell nuclear transfer (SCNT). In SCNT the nucleus of a somatic cell is inserted into an enucleated ovum and they are caused to fuse by an electrical stimulus, the result (if all goes as planned) being a cloned embryo. It was argued (by Ronald Bailey, Peter Singer, and others) that since somatic cells are converted into embryos, and these grow into mature members of the relevant species, human embryos have no more exalted a status than ordinary somatic cells, such as skin cells. Some critics of this argument replied that the somatic cells cannot by themselves develop into mature humans, which is certainly true. The important point, though, is that the SCNT process makes use of the somatic cells to create entities of a different nature ­ using parts of human organisms (somatic cells) to make new complete human organisms at the embryonic stage of development.

Thus, in the context of SCNT cloning, somatic cells are analogous to sperm and oocytes (parts of whole organisms) rather than to embryos (which indeed are whole organisms). But Silver thinks production of a mature mouse by tetraploid complementation answers that reply: In this process, the more mature organism is derived directly from the ES cells, so the ES cells do (according to Silver) in some sense by themselves become whole embryos. He thinks this process shows that, just like embryos, ES cells also can develop into mature members of their species if they are just given a suitable environment.

Confusion over Tetraploid Complementation
However, this fundamentally misinterprets the results of tetraploid complementation. In addition, it repeats the mistake in the earlier argument. Tetraploid complementation does not show that the “entire embryo” can be derived from ES cells. The embryo (or embryo-like entity) generated from tetraploid complementation is a chimera, with parts derived from the original embryo that was induced to become tetraploid and parts derived from the ES cells. The placenta and other organs generated from the tetraploid cells are parts of the embryo. They are bodily organs that function only during embryonic life, but they are bodily organs none the less ­ analogous to baby teeth, which also function only during a portion of the animal-organism’s life cycle. The placenta is a vital organ of the embryo, and it is not directly made by the ES cells.

Moreover, in tetraploid complementation, the ES cells do not by themselves generate a mouse since they do not by internal self-direction develop into a mouse. So the ES cells are not mouse embryos or their equivalent, and never were. True, the mature mouse’s cell lineage is completely derived from the ES cells, but an analogous point is also true in SCNT cloning ­ the DNA of the mature mouse or sheep or other animal is identical to that of the donor somatic cell (although the cytoplasm from the enucleated ovum also has a determinative effect). The central point is that, just as in SCNT cloning, so here: the manipulation (in this case, the combining of the ES cells with the tetraploid entity or “embryo”) generates a new type of biological entity. This is demonstrated by the profoundly new type of behavior observed in the entity produced by the process or processes of tetraploid complementation. The manipulations involved do much more than merely release an inner capacity of the stem cells. The combining of the stem cells with the tetraploid embryo does not merely place these cells in an environment hospitable to the process of organismal development. Rather, it transforms them from functional parts to components of an actively developing whole organism. Or, more precisely expressed: the combining of the stem cells with the cells of the tetraploid embryo generates a new organism, an organism that is not a stem cell.

The tetraploid complementation procedure is simply a type of cloning. In the most common form of cloning ­ SCNT ­ a new organism is generated; it comes into being as an embryo which immediately begins actively developing itself into the more mature form of the whole organism it now is. Completely analogous to what occurs in tetraploid complementation, the new embryo in SCNT cloning has the same genetic code as the somatic cell. The combining of stem cells with a tetraploid embryo does to the mouse stem cells what fusing an enucleated ovum does to a somatic cell in SCNT, the procedure that generated Dolly the sheep and many other cloned mammals ­ namely, it produces a distinct, whole organism.

So a stem cell does not “become” an embryo (the way an embryo truly does become a fetus, an infant, a child, an adolescent, and an adult). Rather, many stem cells are used in a cloning process that, if successful, results in the production of a new and distinct organism. The proof that this results in an entirely new and distinct organism is that it has a radically different trajectory of growth.

Thus stem cells are not equivalent to embryos. They lack the defining feature of embryos, namely, the internal active disposition to develop themselves to the mature stage of the organism of the relevant species. If placed within an environment suitable for the development of human embryos, human stem cells do not do what embryos do. (The crucial fact that by themselves ­ that is, when not introduced into a pre-existing, albeit defective, tetraploid embryo ­ ES cells produce only disorganized masses of tissue, either embryoid bodies or teratomas, is conveniently ignored by Silver.) Only if human stem cells are joined with other factors so as to generate a distinct and whole organism in SCNT, or in the combining of stem cells with tetraploid embryo-like entities, does an embryo come to be.

Several passages in Silver’s book indicate that he regularly fails to see the significance of the distinction between a whole organism, on the one hand, and a tissue or part of an organism, on the other hand. Thus, he believes our argument is easily refuted by pointing out that a single human skin cell or a teratoma has the same genetic code as other (whole) members of the relevant species. But our argument has never been simply that human embryos are human beings because they have the full genetic code. Rather, we have always argued that their full genetic code, plus (and more importantly) their internal active disposition for self-directed development toward the mature stage of a human, show that they are what the standard embryology texts say they are, namely, distinct and whole (though immature) individuals of the human species.