For months, congressional efforts to ban the cloning of humans have been smoldering in the Senate.
Now, research results from two teams of scientists may inflame the debate over whether to ban all forms of human cloning or allow cloning human embryos for stem-cell research.
Embryonic stem cells form within the first few days of an embryo's development. Many researchers say they hold great potential for treating a range of degenerative diseases, because with the right cues, stem cells give rise to the full range of major cell types found in the human body.
That trait, however, no longer appears to be confined to embryonic stem cells. A team led by Catherine Verfaillie, who heads the University of Minnesota's Stem Cell Institute in Minneapolis, says it has demonstrated that adult "progenitor" cells act like their youthful counterparts in several critical aspects once thought unique to embryonic stem cells, including differentiation into the body's three broad cell lines.
Proponents of a ban on all forms of cloning say such advances in research in adult stem cells bolster their case. They argue that cloning, then destroying, embryos for science not only is morally repugnant, it is increasingly unnecessary. They say scientists could be tempted to allow a cloned embryo to come to term.
The second team, led by Ron McKay of the National Institute of Neurological Disorders and Stroke in Bethesda, Md., says it has used embryonic stems cells from mice to generate properly functioning new nerve cells in the brains of rats bred to model Parkinson's disease. The team says the treated rats showed promising signs of recovery during behavioral tests.
Dr. McKay's results dovetail with the hope of those who believe that Congress should ban any cloning that would result in a fully-grown person, but not the cloning of embryos for research and therapeutic, purposes.
Both sets of results were released online today by the journal Nature.
"While the two papers will no doubt rekindle the debate on the relative merits of embryonic versus adult stem cells, together they emphasize the outstanding potential of stem cells, and the need for continued research in all areas of stem-cell biology," says Natalie DeWitt, a senior biological-sciences editor at Nature.
Verfaillie agrees. She acknowledges that the use of embryonic stem cells is ethically contentious. But "it is far too early to say which cells, embryo or adult, will provide the best benefit to a particular patient or for a particular disease."
Indeed, her research center recently hired a specialist in embryonic stem cells in preparation for those types of experiments.
In their research, Verfaillie and her colleagues essentially found that adult stem cells can yield a wide variety of cell types. The team took their cue from a rare cell type they isolated from human bone marrow. They found that in petri dishes, these cells which they term multipotent adult progenitor cells, or MPACs multiplied at a prodigious rate, doubling their populations more than 80 times. They also differentiated into the three main human cell lines.
The team isolated the mouse and rat counterparts to human MPACs, injected them into mouse embryos, then brought the mice to term. The scientists found that a single progenitor cell could generate up to 40 percent of the cells in a fully developed mouse. No obvious abnormalities resulted from the procedure.
They also injected the progenitor cells into fully grown mice and found that the progenitor cells took their cues from the organs they entered and began to form that organ's cells.
In McKay's experiment, embryonic stem cells from a mouse were used to generate new neurons in the brain of another mouse bred to model Parkinson's disease. The newly generated cells exhibited the physical and behavioral traits of normally generated neurons, and the "patients" showed signs of recovery.
The results come at a time when international research on embryonic stem cells is picking up speed. Three months ago, the British government granted the first licenses to scientists seeking to extract stem cells from embryos donated as leftovers from in-vitro fertilization clinics, an approach closed to US researchers.
The results also come as the Senate reviews three bills that would ban cloning to various degrees.
One, introduced by Sen. Sam Brownback (R) of Kansas, mirrors a bill the House passed last year that would ban all forms of cloning, as well as prohibit the importation of products derived from cloning. Two others, introduced by Sen. Dianne Feinstein (D) of California and Sen. Arlen Spector (R) of Pennsylvania, would ban reproductive cloning but permit research cloning.
At this stage, "there's no set schedule for bringing these to the floor for a debate and vote," says one congressional aide.
At the end of last week, efforts between Senator Brownback and the Democratic leadership to negotiate a timetable broke down over procedural issues. The Senator's attempts to tack cloning-related amendments onto other legislation has been unsuccessful.
For their part, Feinstein supporters express confidence that they will be able to assemble the 60 votes needed to pass their measure "very soon."
While the near-term trajectory of the bills is unclear, several observers expect to see action before the session ends for the fall election.
"The United States has no law against cloning at all," says one congressional source. "The issue is too important to be put on the backburner."