In the contentious political debate over embryo stem (ES) cell research, both proponents and opponents begin with the premise that "embryos are destroyed for their cells," as President Bush claims and non-partisan journalists repeat frequently in stem cell news stories.

It's only when partisans get beyond this premise that opinions diverge. Proponents of ES cell research argue that embryo destruction is justified based on the promise of extraordinary medical advances. In contrast, the President and his supporters describe ES cell derivation as "the taking of innocent human life," which is "always immoral." And yet, in strictly biological terms, the conventional wisdom is wrong. No life is being "taken" or destroyed when embryos are transformed into ES cells.

To understand the relationship between embryos and ES cells, it is critical to understand the process of development that is initiated by fertilization. The single-cell embryo undergoes multiple rounds of division giving rise to about 100 cells after five to six days. At this stage, the cells along the surface undergo biochemical changes that eliminate their potential to differentiate into anything other than the placenta. And in the center of the embryo, only about two dozen cells retain the ability to develop into every tissue and organ that makes up the human body proper.

These central cells are, by definition, embryonic stem cells. They -- and they alone -- will divide and morph into the entire fetus and child. These cells are also the ones that can be retrieved and grown in a Petri dish.

If you and I and every other human being emerged entirely from a group of ES cells in the embryo, and ES cells in a Petri dish are biologically equivalent to ES cells in an embryo, then what is destroyed in the process of growing ES cells? According to conventional wisdom, ES cells do not have the potential to become a mature organism because they can't produce the placenta, which is required to nourish the fetus to term. (In fact, human ES cells -- unlike mouse ES cells -- have been tweaked into producing placental tissue.) And yet, over a decade ago, the Canadian embryologists Andras Nagy and Janet Rossant invented an experimental trick to overcome this deficiency.

The trick is to sandwich a dozen or so ES cells in-between two freshly created embryos manipulated so that each of their cells contains double the normal diploid (2X) number of chromosomes. The tetraploid (4X) embryonic cells are entirely incompetent in regards to fetal tissue development, but they retain the capacity to form a normal placenta. In the presence of this placenta, ES cells take over full responsibility for developing all the tissues of the fetus. With this protocol, Nagy and Rossant succeeded in creating viable, fertile animals that were -- in their words -- "completely ES-cell derived."

Based on the standard definition of an embryo as "a group of cells arising from the egg that has the potential to develop into a complete organism" (according to the U.S. National Academy of Sciences), a bunch of ES cells could be referred to as an embryo. But this logic could complicate the current political battle to allow federal funding of the research. Simply put, if one believes that a human embryo is sacred, one must hold ES cells to be sacred as well.

Still, isn't it true that human embryos must be ripped apart to obtain their internal ES cells? The answer is "no." Two years ago, in a feat hailed by Nature magazine, the Chicago embryologist Yury Verlinsky coaxed whole four-day old human embryos into becoming ES cells under a specified set of culture conditions. No biological life was destroyed in the process. Instead, the embryos were transformed directly into a living stage defined by the capacity to divide indefinitely without differentiating.

If neither biological life nor developmental potential need be "destroyed" in the process of deriving ES cells, why do so many people insist otherwise? The answer comes from the common non-scientific interpretation of the word "life" as a vitalistic entity rather than a biological one. In the view of many fundamentalist Christians and Catholics, a single-cell embryo is a human being, while human ES cells are not even forms of human life.

The disconnect between public and scientific spheres of thought is aptly illustrated by open discussions occurring among scientists who work with non-contentious mouse ES cells. This past summer, the National Institutes of Health (NIH) initiated a $50 million program to create a centralized resource of 10,000 mouse strains, each containing a mutation in a different disease-causing gene. NIH wants to make these special animals freely available to researchers, but as Science magazine explained, "it would be most economical to avoid trafficking in live mice."

The proposed alternative is "to maintain the [mutant animals] as embryonic stem (ES) cells: clumps of tissue that can be frozen down and later grown up into full-fledged mice." The feasibility of this goal increased dramatically with the December 2006 Nature Biotechnology report of a highly efficient method for generating "mice fully derived from gene-targeted embryonic stem

cells" that eliminates the requirement for tetraploidization.

To mouse geneticists, ES cells are a convenient method for "maintaining" embryos, not destroying them. But scientists who work with human cells have been less than forthcoming about these pesky details of biology. It's not that they think ES cells per se are deserving of greater respect. On the contrary, most cell biologists think that embryo progenitors are deserving of no more respect than the ES cells they grow into. But voicing such an opinion is politically unwise in America where the meaning of life -- from beginning to end -- is still defined by religion, not biology.

Nevertheless, in today's "flat world" (to use Thomas Freidman's term), the research won't be held back. It will simply blossom in societies more enlightened than our own.