The Grail Searchers
Despite endless efforts to prove the contrary, science shows that an embryo is a human being.


Those searchers are William Neaves and Gerard Magill. Their candidate for the Holy Grail is an argument purporting to show that human embryos are the ontological and moral equivalent of induced pluripotent stem cells (iPSCs). Again the form of the argument is reductio ad absurdum. No one supposes that iPSCs are human organisms, so if it can be shown that iPSCs are the equivalent of embryos, then embryos cannot be human organisms either.

What are iPSCs? They are a widely touted class of cells that have been produced by Shinya Yamanaka in Japan and James Thomson in the United States. They are made by reprogramming (or “de-differentiating”) ordinary somatic cells to the pluripotent state — the state from which they can be coaxed to become virtually any type of tissue: heart, liver, nerve, you name it. What is exciting about these cells is that they have the medically useful features of embryonic stem cells, yet they can be produced without killing (or even using) embryos.

According to Neaves and Magill, however, iPSCs are not merely the biological equivalent of embryonic stem cells; they are the equivalent of the embryos that are destroyed to produce embryonic stem cells. On what basis do they advance this idiosyncratic view? (It is, incidentally, a view plainly rejected by Yamanaka and Thomson, who have explained that the significance of iPSCs is that they do not raise the ethical problems associated with killing human embryos.) Neaves and Magill claim that an iPS cell, though obviously not an organism, is the biological equivalent of an embryo because it can develop into a human organism.

How is that? Well, when joined to a tetraploid “embryo” (an entity created by fusing the first two cells of a normal embryo into a single cell that contains twice the normal amount of DNA), an iPS cell will contribute to the development of a mature organism of its species (as has been shown with mice). In this procedure, according to Magill and Neaves, the iPS cells are merely being provided with a placenta — which they view as a “component of a supportive environment for development,” like a uterus, rather than as an integral part of the developing organism. Just as a human embryo needs only a suitable environment to develop itself to a mature stage, so does an iPS cell need only the suitable environment supplied by a tetraploid “embryo.”

Replying to Neaves and Magill, we demonstrated that their claim that iPS cells develop into a complete embryo is scientifically inaccurate. Rather, they become part of a distinct, developing organism, consisting of structures derived from both the iPS cells and the tetraploid cells. The key question is whether a placenta is just a “component of a supportive environment” or an actual organ of the embryo, and anyone familiar with human embryology will recognize that it is a mistake to classify a placenta as a mere “environmental requirement.”

Here’s why: The developing organism requires information provided by both inner cell mass cells and trophectoderm cells (the precursors to the placenta). So in the prenatal stage of development, the placenta is an integral part of the developing human, functioning as a vital organ and even sharing a common blood circulation with the rest of the developing embryonic body. In contrast, the uterus of the mother is clearly part of a distinct organism that is not integral to the embryo itself, but merely provides a supportive environment and a source of nutrition. Hence a placenta is an organ of the embryo, albeit a transitory one. This means that together, the iPS cells and the tetraploid cells constitute an entire, integrated whole — an organism. In stark contrast, at no point does an iPS cell function as a whole organism by itself. So any argument resting on the claim that an iPS cell is the biological equivalent of an embryo simply won’t fly. The Grail searchers will have to search on.

But Ronald Bailey just doesn’t want to give this one up. In “Do Skin Cells Have Souls?” he deploys the tedious and shopworn strategy of writing as if only religious zealots think human embryos are human organisms — thus his reference to souls in the title and a reference to angels on the head of a pin in the concluding sentence of his essay. But as he well knows, biology and human-embryology texts have for a long time been quite clear that a new human organism comes into existence at fertilization. That is, in human reproduction, when sperm joins ovum, these two individual cells cease to be, and their union generates a new and distinct organism. This organism is a whole, though in the beginning developmentally immature, member of the human species. Readers need not take our word for this: They can consult any of the standard human-embryology texts, such as Moore and Persaud’s The Developing Human, Larsen’s Human Embryology, Carlson’s Human Embryology & Developmental Biology, and O’Rahilly and Mueller’s Human Embryology & Teratology.


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