As the political fortunes of embryonic stem cell research hang in the balance, scientists increasingly are looking for alternatives in case federal funding for work on human embryos falls through.
The ethical dilemma of embryonic research grew in the last week when two groups revealed the first projects to create human embryos solely for their stem cells, using in vitro fertilization and cloning. Those announcements came as President Bush, who has said he opposes using embryos for research, met with bioethicists for advice on whether to allow federal funding of embryonic stem cell experiments.
Opponents of such research argue that some alternatives are at least as promising as embryonic stem cells, which may be able to regenerate tissue to cure ailments such as diabetes and Parkinson's disease. The desperate search for other research options has led to possible stem cell sources in adult bone marrow, cells made by mimicking the process of cloning and even fat tissue.
Yet some of the alternatives, such as fat, almost certainly are dead ends, according to stem cell experts. Adult stem cells from bone marrow show more promise and have reached human trials for some diseases. Even so, experts say, such cells have lacked the power of embryonic stem cells to form virtually any type of tissue.
For those reasons even the most prominent adult stem cell researchers say they support federal funding of work on cells from embryos.
"The vanguard of the field has really been the embryonic work," said Dr. Ira Black, a neuroscientist from the University of Medicine and Dentistry of New Jersey. Last year, Black presented the first report showing adult stem cells from blood could be transformed into brain cells.
Research on all kinds of stem cells is still at an early stage, and no one knows which approach might prove best. But so far, the remarkable versatility of embryonic stem cells has set them apart from adult stem cells, said Dr. John Gearhart, a Johns Hopkins University researcher who co-discovered human embryonic stem cells in 1998.
"At this time, this source is superior. There's no question about it," Gearhart said, referring to stem cells from embryos. "It's renewable, and it can form anything."
The hallmark of stem cells--whether the source is embryos or adults--is their ability to form many types of tissue. Adult stem cells tend to be more specialized--stem cells from bone marrow or umbilical cord blood, for example, are most adept at making the cells found in blood or bone.
In contrast, embryonic stem cells, which typically come from embryos that have grown for five days, give rise to all of the body's tissues during embryonic and fetal development.
When grown in a laboratory, embryonic stem cells have the ability to divide virtually without limit, up to "swimming-pool volume," in the words of one researcher.
Yet researchers have been less successful at growing adult stem cells into quantities that would be useful for treatment, experts said. In most cases researchers have not even isolated the individual adult stem cells that give rise to other tissue.
Among the most promising aspects of embryonic stem cells is their ability to form tissue that appears to function. For example, pancreas cells made in this way have been shown to produce insulin. Several researchers said they have coaxed embryonic stem cells into forming heart muscle cells that pulsate in unison in a laboratory dish.
Using adult stem cells
Although adult stem cells can become several different tissue types, there is little hard evidence that they can replicate essential functions, said Dr. Irv Weissman, a Stanford University scientist and pioneer of adult stem cell research.
"We have a small number of intriguing experiments in mice where we could show, for example, that a blood-forming stem cell could turn into liver," Weissman said. "But almost all of the other experiments that were done, even with mice, are not rigorous experiments--they're sort of flashes out of the box."
Most adult stem cell research has focused on cells from bone marrow. A potential advantage of adult stem cells is that unlike embryonic stem cells, which have never been transplanted into humans, doctors have decades of experience with bone marrow transplants for cancer patients.
One of the first attempts to use adult stem cell therapies for uses other than replacing bone marrow involves children with brittle bone disease at St. Jude Children's Research Hospital in Memphis.
The five patients, all younger than age 5, have a condition called osteogenesis imperfecta, which prevents the healthy growth of bones and can lead to dangerous respiratory problems and infections.
"A great many of these patients die in childhood," said Dr. Edwin Horwitz, a stem cell researcher at St. Jude who is leading the study.