Blastocyst Brouhaha.

Which human cells count as people?

Are 5-day-old human embryos people? That's the fundamental question at issue in the ongoing and heated discussion regarding embryonic stem cell research. The debate was sparked when privately funded researchers announced their discovery nearly three years ago. They found that stem cells, which have the potential to develop into any of the body's 220 different types of specialized cells, hold great therapeutic promise. The hope is that stem cells will someday be able to help physicians fix spinal cord injuries, cure diseases such as Parkinson's, Alzheimer's, and diabetes, and regenerate malfunctioning organs such as livers and hearts.

Embryonic stem cells are derived from 5-day-old human embryos, technically called blastocysts at that stage of development. These consist of about 150 cells. While they are not the only source of usable stem cells, embryonic stem cells are regarded as the best source for possible therapeutic applications. They are also the most controversial: Opponents claim that the destruction of human embryos, a byproduct of extracting the stem cells from the blastocysts, is equivalent to destroying a human being.

So, are the embryos from which stem cells are derived people? The answer: Only if every cell in your body is also a person.

Why? Because it is logically (if not quite logistically) possible for each of your body's cells to become your twin. Each skin cell, each neuron, each liver cell is potentially a person. All that's lacking is the will and the application of the appropriate technology. Cloning technology at this point in time is clunky. In the future, though, researchers will likely be able to skip cloning, and simply flip a few genetic switches to regress any of your cells to earlier stages of development, says Harold Varmus, former director of the National Institutes of Health. Ultimately, researchers could take your cells all the way back to the embryonic stage, then implant them into a womb where they could eventually develop into complete human beings.

"What happens when a skin cell turns into a totipotent stem cell [a cell capable of developing into a complete organism] is that a few of its genetic switches are turned on and others turned off," writes bioethicist Julian Savulescu in the April 1999 issue of the Journal of Medical Ethics. "To say it doesn't have the potential to be a human being until its nucleus is placed in the egg cytoplasm [i.e., cloning] is like saying my car does not have the potential to get me from Melbourne to Sydney unless the key is turned in the ignition."

Like turning the key in the ignition to begin a journey, starting a human egg on a particular path, either through fertilization or cloning, is a necessary condition for developing a human being. But it isn't sufficient. A range of other conditions must also be present. Those include the availability of a suitable environment, such as a woman's womb. Two well-known pro-life politicians understand that "certain conditions" make a difference.

Sen. Orrin Hatch (R-Utah) argues that "a frozen embryo stored in a refrigerator in a clinic" isn't the same as "a fetus developing in a mother's womb." Former Sen. Connie Mack (R-Fla.) declares, "For me, as long as that fertilized egg is not destined to be placed in a uterus, it cannot become life." In other words, for Hatch and Mack, location in a petri dish makes a lot of difference.

"I cannot see any intrinsic, morally significant difference between a mature skin cell, the totipotent stem cell derived from it, and a fertilised egg," writes Savulescu. "They are all cells which could give rise to a person if certain conditions obtained.... If all our cells could be persons, then we cannot appeal to the fact that an embryo could be a person to justify the special treatment we give it."

The DNA content of a skin cell, a stem cell, and a fertilized egg are exactly the same. The difference between what they are and what they could become is the environment in which their DNA is found. Thus, the mere existence of human DNA in a cell cannot be the source of a relevant moral difference. The differences among these cells are a result of how the genes in each are expressed, and that expression depends largely on which proteins suppress or promote which genes.

So people who oppose stem cell research must logically be committed to the notion that the only difference between your skin cell and your twin are the proteins that decorate their DNA strands. But can moral relevance really be reduced to the presence or absence of certain proteins in a cell?

Ronald Bailey ([e-mail]) is REASON's science correspondent.