Robert George, co-author of the forthcoming book, Embryo: A Defense of Human Life, which we'll be offering in the Club in January, took time out of what sounds like an overwhelming schedule to give this preliminary answer -- and to offer more detail later. I very much hope he'll be able to respond to my continuing doubts!

The key thing to see is that an embryonic stem cell is not an embryo. It is a cell that is removed from the inner cell mass of a blastocyst-stage (5-6 days) embryo (ordinarily causing its death) and then cultured in the lab to induce pluripotency. Even when made pluripotent, it is not an embryo. Although it can be used to produce many (perhaps all) the various cell types in the body, it lacks totipotency, that is, the active capacity and disposition (by virtue of its epigenetic constitution), to organize itself and by an internally directed process develop along the species-specific trajectory towards full maturity. Unlike an embryo (human or otherwise), a stem cell -- even an embryonic or other pluripotent stem cell—is not an organism. It is a part, rather than a whole. An embryo, unlike a stem cell, is a whole living individual of a particular species (human, mouse, what have you) at the earliest stage of its natural development.

What is exciting about the research published by Yamanaka and Thomson is that it shows that ordinary somatic cells can be "reprogrammed" to the pluripotent (not totipoent) state. These pluripotent cells are not produced by removing cells from embryos, and they are not embryos themselves. It is possible that they could be used in the production of embryos (just as ordinary somatic cells are used in the production of embryos by cloning), but that is a separate matter. Needless to say, I am opposed to producing embryos by cloning or any method for use in research in which they are destroyed. (As a matter of fact, I am opposed to the extracorporeal creation of embryos, even for reproductive purposes—though I do not view this as being on the same moral plane with creating embryos for research.)

All of which suggests a couple of points.

First of all, again, why are we so sure that pluripotent stem cells aren't human beings? What makes us certain that the capacity that they lack -- the "totipotency" that allows any embryo, whether from reproduction or from cloning, to go on organizing itself properly as it grows -- is the sine qua non of human being-hood? Or of organism-hood, I guess, which is the necessary condition for human being-hood and then person-hood?

Compare what the scientists do when they take pluripotent stem cells from embryos (which we know are human beings) to what some evil scientist might do to a fully grown human being. If some new Dr. Frankenstein -- or maybe a new Medea is more to the point -- took a living man, chopped him to bits and threw away some of them, stuck the undiscarded parts (including his brain) into a nutrient brew that kept them alive and even allowed them to multiply themselves unnaturally, would we be sure that the orginal man was gone?

It would be hard to argue that he was killed; something or other that used to be that man would still be alive. Would the original man certainly have been destroyed? I'd say, at least arguably, he would have been "destroyed" only in the sense of "maimed beyond repair," not in the sense of "obliterated" or "no longer existing."

Professor George defines the "totipotency" that organisms have and organs don't as "the active capacity and disposition (by virtue of its epigenetic constitution), to organize itself and by an internally directed process develop along the species-specific trajectory towards full maturity."

But there are obvious human beings who don't have the capacity to develop to full maturity -- infants with Tay-Sachs disease, for example.

How can we be sure that pluripotent stem cells aren't horribly maimed organisms, rather than things that aren't organisms at all?

Pluripotent stem cells do, after all, have some capacity to organize themselves and their development -- a capacity that the scientists who work with them are perpetually needing to thwart, in order to keep them pluripotent, or to steer them in the developmental direction the scientists prefer. One of the first articles I read about pluripotent stem cells, in 1999, included quotations from researchers complaining that the cells "behave as though they have a mind of their own" and that if you left them to their own devices, they form "little masses of cardiac cells that soon begin to beat in unison like a tiny heart."* I wondered whether under other conditions they might not form 'little masses of neurons that soon begin to think in unison like a tiny brain'!

Even if pluripotent stem cells never spontaneously produce a human brain, would a human brain that was deliberately grown from pluripotent stem cells by scientists certainly be so different from an ordinary brain that we can be sure it wouldn't have human thoughts?

Secondly, notice that the gold standard, the ultimate test of whether you've created pluripotent stem cells, is whether you can get them to grow into a whole organism. That's what Dr. Yamanaka did with his pluripotent mouse cells. He grew them into mice by some means short of cloning -- because we already know that ordinary somatic cells can be cloned, so turning induced pluripotent mouse cells into mice by cloning would not prove that the cells were in fact pluripotent.

If we’re sure pluripotent stem cells are not organisms already, then we should be able to isolate the precise point in the growing-organisms-from-pluripotent-stem-cells process at which the pluripotent cells, which are not an organism, become an organism. Just as in reproduction we can point to fertilization, and in cloning we point to the moment at which the egg’s cytoplasm reprograms the somatic cell’s DNA to act like a one-cell embryo.

In other words, on the theory that pluripotent stem cells are not organisms, there must now be a THIRD way, besides natural reproduction and cloning, to produce human beings (or at least mice). What is it?

*Here are a couple of similar articles.