Opponents of research on embryonic stem cells have insisted that adult stem cells are a therapeutically superior and ethically more acceptable alternative. However, a new report in Nature finds that adult stem cells may not be the panacea that the bioconservative movement had hoped.
What's the difference between the two types of stem cells? Both have some ability to transform themselves into many different types of tissue. The idea is that stem cells would be transplanted to regenerate tissues and organs damaged by such maladies as heart attacks, spinal cord injuries, and diabetes. As their name suggests, adult stem cells are isolated from adult humans, often from their bone marrow. Ideally, a patient would be able to donate stem cells to herself, thus avoiding the problem of immune rejection that plagues transplants of tissues and organs taken from other donors.
Embryonic stem cells are derived from seven-day old blastocysts (microscopic balls of 150 or so cells). Immune rejection might be handled either of two ways: First, researchers might derive and preserve many lines of stem cells that would genetically match the immune systems of a wide number of patients. Or second, embryonic stem cells might be created to order by means of somatic cell nuclear transfer; that is, by taking a cell nucleus from a patient's cells and inserting it into an enucleated egg to produce a blastocyst from which stem cells of nearly identical genetics could be derived and used for transplant. Since obtaining human eggs is uncomfortable and expensive, researchers hope eventually to decipher the biochemical signals that human eggs use to reprogram mature nuclei into embryonic stem cells. Once this is achieved, physicians would dose a patient's adult cells with the right chemicals, transforming them directly into embryonic stem cells. In the meantime, embryonic stem cells are opposed by pro-life activists, and the House of Representatives has twice passed a bill that would outlaw medical treatments using them.
So why not avoid the contentious area of embryonic stem cell research and just pursue the use of adult stem cells to treat people? After all, initial studies using adult stem cells, especially research by Catherine Verfaillie at the University of Minnesota, had apparently shown considerable promise for transplants of these cells to repair or replace damaged tissues. Verfaillie's adult stem cells, derived from bone marrow, seemed to be able to transform themselves into a wide variety of tissues, including heart, brain, lung, liver, and muscle cells. Unfortunately, new research at Stanford University, the Howard Hughes Medical Institute, and the University of California at San Francisco casts doubt on this finding.
The researchers modified bone marrow stem cells in mice to produce a variety of easily detected proteins, including the glow-in-the-dark green fluorescent protein (GFP). These cells were then injected into other mice. Several months later, the rodents were sacrificed and their tissues minutely inspected. The researchers found that the bone marrow stem cells had not turned into heart, liver or brain cells, but had instead fused with those cell types. The fused cells expressed the proteins from the modified bone marrow cells, and could also be identified because they contained two nuclei, the original one plus one from the bone marrow cells.
This finding offers some therapeutic promise, perhaps as a way to smuggle fresh undamaged genes into aging cells as a way to revitalize old cells. However, such fused cells do not divide to produce new cells, so they appear to be of limited use in regenerating and replacing tissues damaged by heart attacks, diabetes, or Parkinson's disease.
The continuing struggle over stem cell research highlights the dangers of politicizing biomedical science. Various lines of research should be pursued simultaneously in order to have the best chance of discovering effective future treatments. It may well turn out that adult stem cells are good treatments for certain diseases, and embryonic stem cells are better at curing other maladies. Contrary to the claims of bioconservatives, it has never been either adult stem cells or embryonic ones; for the sake of millions of suffering patients, it's both.
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