Stem cells and cloning: What's the difference and why the fuss?

As a writer of embryology textbooks, I find myself regularly wishing that my latest book hadn’t gone to press just a month before the most recent major breakthrough in embryotechnology. This year, the cause celebre was the twin announcements of the production of human embryonic stem cells derived from the inner cell mass of the blastocyst6 or from primordial germ cells taken from 5-to-9-week-old human embryos.4 These reports, coupled with the news account of an announcement that a private biotech company in Massachusetts (Advanced Cell Technology) claims to have produced embryonic stem cells by fusing a buccal epithelial cell of a human with the enucleated oocyte of a cow,7 have spawned numerous articles and editorials that simultaneously extol the technological leaps and decry our collective inability to provide an ethical basis for such work. For at least the next few years, the public image of cloning will continue to be the indelible imprint of Dolly, the impassive sheep that was generated by fusing a mammary epithelial cell with an enucleated egg. Dolly, by the way, has by now produced a normal offspring of her own in the old-fashioned way. In the popular press, the distinction between stem cell technology and cloning is often blurred. Yet, despite superficial similarities in certain aspects of technique, the goals and processes are quite different. A fundamental biological unifying factor, however, is the fact that the nuclei of most cells in both embryos and adults contain a full complement of genetic information. As normally understood, cloning consists of fusing the nucleus of a donor cell (or the whole cell in the case of Dolly) with an enucleated oocyte. The donor cell can come from either an embryo or an adult. In the case of mammals, the fused cell is allowed to divide a few times in vitro before being implanted into the specially primed uterus of a surrogate mother. One of the most difficult aspects of cloning has been matching the condition of the donor nucleus with that of the egg, so that cell division (embryonic cleavage) will occur. What is often forgotten is that the first cloning was done 40 years ago on plants, when F.C. Steward of Cornell5 was able to produce entire carrot plants from single somatic cells (the kind that we eat). Now in the agricultural world, cloning from somatic cells