[Part 1 here. Part 3 here]
Getting back to the science, stem cells are not all created equal. There are several different kinds.
Embryonic stem cells are naturally capable of differentiating into any tissue (or organ, if we knew how to trigger the right signals). We have no way, yet, of getting them to do their thing inside the body where they're needed, so they have to be grown in tissue cultures. Tissue culture techniques require the stem cells to grow in association with feeder cells. It's hard enough getting human stem cells, so in the early research the feeder cells came from mice. The reason that's important is that DNA can sometimes move around, and especially viral DNA can move around. So the early cell lines potentially have bits of mouse DNA or mouse viruses incorporated into the human DNA of the stem cells. For medical use, this is Not Good.
When Bush turned off federal funding for stem cell research in 2001, almost all the cell lines were still at the early research phase and used mouse feeder cells. So, almost all of them are useless for medical applications (although okay for most medical research).
Furthermore, normal cells can't grow forever. Only cancer cells can do that. So embryonic stem cell lines can be maintained for many cell divisions, but certainly after a few years, it's time to start over. And mutations accumulate with each cell division, which is also medically Not Good.
It's essential to have a high diversity of stem cell lines to use in research. Otherwise there's no way to be sure that results are generally applicable. A somewhat analogous situation might be making only kidney transplants legal. There would be no heart, lung, liver or corneal transplants. Also, for some applications like pharmaceutical testing, results can't be obtained unless specific lines are available.
For all these reasons, limiting the embryonic stem cell lines to be used in research does not show "balance" or "compromise." It shows the ignorance we've come to expect from this Administration. It's also very bizarre to believe that using such cells is murder, and then saying that some murder is okay. I'd say it shows that even they don't believe the things they spout.
Recently, a new kind of embryonic stem cell has been generated. Scientists found a way to trigger cell division in egg precursor cells. (Cloning and Stem Cells Journal, 2007, vol 9, no 3 (pdf), or a simpler summary.) These are egg precursor cells that still have the full complement of 46 chromosomes. They're genetically matched to the donor, like any other cell in the body, and could be used for therapies in that woman without requiring anti-rejection drugs.
Ethics ... again
The cry has gone up from people who don't know much about it that it's not just the traditional egg+sperm combination we need to be concerned about. A dividing egg by itself could develop into a complete human being, someday, when we have the technology worked out. The problem with this scenario, of course, is that the dividing egg only has half the chromosomes. Given the complexities of mammalian development (about which there'll be a bit more in Part 3), the dividing egg isn't going anywhere all by itself. What these people are really worrying about is egg precursor cells, and those, as we just saw, are already being coaxed toward complete development. This is not a problem coming in the future. It's here right now.
The problem in the future is much worse than that. Once we have the technology worked out, we'll be able to take any cell in the body, de-differentiate back to the equivalent of that egg precursor cell, and then have it grow into a human (who would be an identical twin, or clone, of the cell donor).
Let me say it more clearly: soon many cells in the body will have the potential to turn into a complete human being.
So what are the people urging us to "Think of the cells!" going to do? Start holding funerals for each cell left on the sidewalk when anyone scrapes their knee after a fall?
That's the problem with muddled thinking that can't distinguish potential human life from actual human life. Before technology, muddled thinking didn't matter because the two were functionally equivalent. Even a few weeks of prematurity was a death sentence. Then technology kept pushing back the date of viability. Then it occurred to some people that potential humanity goes right back to the fertilized egg. Now science has pushed the potential beyond that, to the proto-egg alone. Some day, it'll be every single cell.
Science has taken away the easy conflation of potential and actual. The two were never equal, any more than the fact that I'm a potential Nobel Prize winner makes me an actual one. But science is making that fact of logic painfully obvious. It's forcing people to come to grips with the difficult question of what they mean by "human." If I were them, I'd think it through consistently enough to avoid absurd conclusions. Such as having to find a name for every one of the millions of intestinal lining cells sloughed off with every bowel movement.
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