This week scientists reported experiments that may lead to the ability to restore damaged brain and spine tissue, results that have electrified the scientific community. The great excitement among biologists about the isolation of embryonic stem cells arises because of the potential for regenerating human tissue. In adult humans nerve tissues do not divide often, if at all. That is why spinal chord injuries that cut the spine cannot be treated once dead, the spinal neurons do not regenerate. Similarly, brain tissue lost in Alzheimer’s patients cannot be replaced. The exciting promise of embryonic stem cells is that they may provide a way around these difficult situations, allowing a patient with injuries that affect the brain and spine to restore lost function.
By act of Congress, no federal research funds may be used to conduct research on embryonic stem cells, as embryonic stem cells come from discarded embryos or aborted fetuses. Such experiments can, however, be done lawfully if funded privately. Here is how such an embryonic stem cell experiment might proceed:
Immunological Matching. First, the embryonic stem cells would be altered, using the techniques of gene engineering, to protect them from the intended patient’s immune system. To do this, genes for the embryonic stem cells’ “dog tag” surface identification proteins are replaced with copies of the patient’s ones.
Tissue-Directed Differentiation. Then, the immunologically protected embryonic stem cells would be injected into the patient’s own body where new tissue was desired, say into a damaged brain. There the stem cells would respond to local developmental signals, chemicals from the surrounding tissue that would direct the stem cells to cconvert themselves into whatever type of brain cell surrounds them, and so replacing the damaged brain tissue.
Would such an experiment actually work? Similar experiments have already been tried successfully in mice. Heart muscle cells have been grown from mouse embryonic stem cells and successfully integrated with the heart tissue of a living mouse. This suggests that the damaged heart muscle of heart attack victims might be repairable with stem cells, an exciting possibility for future research.
In pathfinding laboratory experiments by Dr. Evan Snyder of Harvard Medical School, reported in Wednesday’s Proceedings of the National Academy of Sciences, stem cells were able to restore lost brain tissue. He and his coworkers injected neural stem cells (immediate descendents of embryonic stem cells able to become any kind of neural cells) into the brains of newborn mice with a disease resembling multiple sclerosis. These mice lacked the cells that maintain the layers of myelin insulation around signal-conducting nerves. The stem cells migrated all over the brain, and were able to convert themselves into the missing type of cell. The new cells then proceeded to repair the ravages of the disease by replacing the lost insulation of signal-conducting cells. Many of the treated mice fully recovered. Thus it seems that stem cells may provide a treatment for MS. The excitement among researchers is palpable.
But a man is not a mouse. Can lost or damaged human tissue be replaced with stem cells? Only clinical trials can answer this question. The increasing pressure for human stem cell research raises important ethical issues. If you can grow a mouse from a single embryonic stem cell, you should in principle be able to grow a human from a single human embryonic stem cell. So, if you can do that, is a single embryonic stem cell the moral equivelent of an entire human embryo? This distinction is not merely academic. Many people supporting a pro-life view of abortion hold that a human embryo is a fully protected human being. It is because of this that the Federal Government has banned the use of federal money for research involving tissues isolated from human embryos. In May, the National Bioethics Advisory Commission appointed by President Clinton recommended rescinding the ban on most stem cell research. They argue that the moral claims of suffering individuals outweigh the moral claims of potential life. The next round of the abortion debate in Congress is likely to become heated indeed. The ethical line is getting very difficult to draw.