At this moment, the full promise of stem cell research remains unknown, and it should not be overstated. But scientists believe these tiny cells may have the potential to help us understand, and possibly cure, some of our most devastating diseases and conditions: to regenerate a severed spinal cord and lift someone from a wheelchair; to spur insulin production and spare a child from a lifetime of needles; to treat Parkinson's, cancer, heart disease and others that affect millions of Americans and the people who love them.It's good that the president wishes to avoid overstating the "promise of stem cell research," but it's hard not to be impressed by that list of maladies. It looks like the kind of list you see in an advertisement for organic bovine fecal extract (or similar remedies). How did stems cells, and specifically embryonic stem cells, get this reputation?
Embryonic stem cell (ES cells) have two distinguishing properties. First and most notably, they have unlimited developmental potential. In other words, an embryonic stem cell is capable of developing into any cell type: nerve cell, muscle cell, skin cell, blood cell. Unlike every other known cell type, and unlike so-called adult stem cells, an ES cell hasn't burned any bridges by becoming committed to a particular lineage of descendants. The stem cells in the bone marrow, for example, are adult stem cells which are quite useful for making new blood cells but are unable to make muscle cells by virtue of their commitment to the blood-cell lineage. The bone marrow stem cells can make several different types of cells, so they are referred to as multipotent. The ES cells, lacking any constraint on their developmental trajectory, are called pluripotent. It is their pluripotency that makes them so interesting and so potentially useful.
The second property is self-renewal. A stem cell, by definition, has the capacity to regenerate itself every time it divides. So when a stem cell divides, it makes two distinct cells: one is a cell that can go on to develop into a particular cell type (nerve, muscle, skin, blood) and the other is a cell that retains the properties of the original stem cell. This is important, because when cells make their final developmental decision they often forfeit their ability to divide anymore. If stem cells merely turned into such cells, the body would quickly run out of stem cells. Now, note that this second property is not specific to ES cells: all stem cells of every kind have the property of self-renewal. That's what makes them stem cells.
Putting these two properties together, we see that ES cells are cells that can self-renew continuously, so that a small population (in a culture dish) can quickly be expanded into a very large population which can be easily subdivided and shared with multiple labs and/or frozen for future use. And these cells can be coaxed into becoming any cell type. Add a little of this and a little of that, and you can get ES cells to turn into dopaminergic neurons, which are the very cells that are destroyed in Parkinson's disease. Skin cells? Done. Heart muscle cells? Done. Sperm? Eggs? Believe it or not, yes, that's been done. (Think about that one. We'll come back to it in the future.)
And that means that ES cells represent a practically unlimited source of human cells of every type. This is why the cells are thought to have the potential to help us understand and address maladies of the kind the president listed: they provide an opportunity to study the ins and outs of development, but much more importantly they provide an opportunity to create the cells and tissues that can be used to fix damaged bodies.
The potential is real. It shouldn't be overstated, but it's real. And yet we all know why the creation of ES cells is profoundly controversial. It seems to me that this is a serious ethical conflict, a real problem. Defining it as a "false choice" – as the president unwisely did in his speech – doesn't solve the problem. Is there another way?