(St. Louis Post-Dispatch) -- While politicians, ethicists and religious leaders argue over the social and moral implications of funding research on stem cells taken from human
embryos, researchers say the scientific debate on the topic is far from over.
Stem cells are primordial cells capable of generating many different types of body parts. The question confronting scientists involved in stem cell
research is whether stem cells taken from adults are as flexible as stem cells derived from embryos.
For years, scientists thought that only embryonic stem cells were able to produce all of the cells in the human body. Stem cells taken from adult tissues
such as brain, bone marrow or fat were thought to be less capable and only able to make a subset of body tissues. New research is challenging that view
and leading many scientists to call for federal support for both types of research.
Adult tissues may actually be more promising sources than embryos of stem cells used for transplants, said David Prentice, a cell biologist at Indiana
State University in Terre Haute. Prentice is one of the founding members of Do No Harm, a group of scientists and others who challenge embryonic stem
cell research on ethical grounds.
Prentice says that work with adult stem cells has shown progress in treating lupus, Parkinson's disease and damaged corneas and hearts, while
embryonic stem cells have failed to live up to their potential. He admits that his is a minority opinion.
Many other scientists and professional societies contend that embryonic stem cells may hold greater promise in treating a wide range of human diseases
and disorders including heart disease, diabetes, brain-degenerating diseases such as Alzheimer's disease and Huntington's disease, arthritis and paralysis.
They say 20 years of research on embryonic stem cells from mice has shown that embryo cells have the potential to become any of the more than 200
types of cells that make up the human body. But no one is yet sure how many tissues adult stem cells can make. That's good enough reason to fund both
types of research, these scientists say.
Embryonic stem cells are harvested from a mass of cells inside the early embryo. Scientists say that those cells are pluripotent - that is, they are able to
divide and replicate almost indefinitely, and they are capable of making many different types of cells. Adult stem cells stop dividing after about 50
generations.
As development progresses, the cells are led into one of three developmental pathways. Adult stem cells are already committed to follow one of those
pathways. Scientists used to think that adult cells were incapable of reversing or changing their programming to go down a different developmental path. But
recent work with mouse cells indicates that adult stem cells may be more flexible than previously thought.
In mice, blood-forming stem cells have been shown to make liver cells; separately, liver stem cells can form heart cells. Those cells seemed to have
crossed developmental boundaries - something scientists didn't belie ve was possible before. The results are still preliminary and need to be replicated
before scientists will completely accept them, said Ida Chow, executive officer for the Society for Developmental Biology, headquartered in Bethesda, Md.
No one knows how the cells are able to accomplish the identity switch, and the experiments have not been duplicated outside an animal. Scientists say
conducting the experiments in a petri dish is necessary to fully understand and control the cells.
Embryonic stem cells can be coaxed into making heart cells that beat in a petri dish, or nerve cells that behave like nerve cells, Chow said.
It is still much too early to say whether adult stem cells or embryonic stem cells are better for developing practical treatments for diseases, said James
Huettner, a cell biologist at Washington University School of Medicine. The fact that new research reports on the capabilities of each type of stem cell are
published nearly every week is evidence that scientists still have too little information to answer the question, Huettner said.
The two approaches complement each other, he said. Research on embryonic stem cells in mice has made the recent advances with adult mouse stem
cells possible, Huettner said. The same is likely to be true for human cells, but no one will know for sure unless research on human embryos is allowed.
"Even if no one was ever cured by embryonic cell therapy, it would still further the work that is done with adult cells," Huettner said.
Most of the attention on stem cell research has centered on transplanting the cells in people in hopes of repairing damaged or diseased tissue. But some
scientists say that only embryonic stem cells can answer fundamental questions about early human development.
Knowing how stem cells behave could help scientists understand what causes birth defects and develop strategies to prevent or correct those defects,
Chow said. Miscarriages may also be the result of problems very early in development. Understanding what goes wrong in some pregnancies could one day
help prevent miscarriages, Chow said.
Extracting that knowledge is something that cannot be done with adult stem cells, because those cells have already progressed through the earliest
phases of development, she said.
Research on embryonic stem cells taken from primates closely related to humans can approximate the events in early human development - without the
ethical problems associated with human embryos, Prentice said.
But each species has subtle differences in its developmental programs that could be significant, Chow said. Although animal work should be considered
a necessary precursor to work on human embryonic cells, the only way to know how closely primate development parallels humans is to conduct research
on both types of cells, she said.
Prentice, however, says moral considerations outweigh scientific precision.
"If it's a question of close enough or destroying more embryos," Prentice said, "I'll take close enough."
