Because they can divide to become almost every other kind of body cell, human embryonic stem cells may hold the key to seemingly miraculous cures. They may be able to create new, healthy pancreatic islets which can once again produce natural insulin for diabetics. They may be able to regrow damaged nerves to allow a paralysed person to walk again. The hope is that if human embryonic stem cells can create healthy cells to replace diseased ones, any part of the body which no longer works correctly can be regrown into a healthy and fully functioning replacement.
Yet obtaining human embryonic stem cells is ethically problematic. At our current level of medicine, obtaining embryonic stem cells for research requires killing an embryo. The current main source for embryonic stem cells are the embryos left over after successful IVF treatment which has resulted in the desired number of children, and then only when those embryos are released for research by the parents. The only other options for these embryos is to be donated to other infertile couples within a short period of time or to be discarded entirely.
Countries are divided on whether it is ethical to conduct such research. Some countries have few or no regulations governing human embryo research. Some countries do not permit any research involving human embryos. The United States falls somewhere in the middle. Between 2001 and 2009, there was a complete moratorium on publicly funded research. This moratorium was overturned in March 2009. It is entirely possible that it or something like it might be reinstated in future, perhaps even as legislation, depending on which side of 50% the majority of voting Americans currently fall. Even during the moratorium, however, nothing restricted privately funded human cell stem research which was not dependent on government research grants.
Other options exist. Previously donated embryos have been coaxed into dividing and growing in the laboratory, creating pluripotent stem cell lines. The potential lifespan of such cultures is not known. Even though it does not kill the line, this kind of research still diverts at least one human embryo from ever becoming a human being. It also still experiments on cells which individually have the potential to become a human being.
Another option is to draw multipotent cells from older foetuses, potentially as a standard part of in utero surgery to correct a congenital condition before birth. These cells no longer have the full potential to become a human being, which also makes them slightly less valuable in stem cell research. This is the catch-22 of stem cell research: the closer to the 8- or 16-cell embryonic state, the more the cell is capable of differentiating, and thus the more valuable it is to stem cell research. Any cells taken as part of in utero surgery will also carry the congenital condition, limiting their use in therapeutic medicine.
With the development of preimplantation genetic diagnosis (PGD), another option has opened up. In PGD, a single cell is removed from an 8-cell embryo to examine it for the presence or absence of a particular severe genetic disease such as Tay-Sachs syndrome. If the cell is found to be healthy, the now 7-cell embryo is allowed to develop normally. It is possible that in future, we could similarly remove a single cell from one or more embryos and coax it into indefinite pluripotential division in the lab.
Even with the PGD option, we would still be experimenting on cells which have the potential to become a human being. If we choose to pursue human embryo stem cell research, however, this may be as close as we can come to an ethical solution.