The first cloning experiment in mammals has succeeded in producing three healthy mice by taking genetic material from one embryo and implanting it into a foreign one, according to reports of scientists familiar with the work.
A paper reporting the success experiment -- conducted by Drs. Peter Hoppe of Jackson Laboratory in Bar Harbor, Maine, and Karl Illmensee of the University of Geneva -- is expected to be published in the next issue of Cell magazine.
The work, which was essentially completed more than 18 months ago but is only now being fully reported in a scientific journal, has taken a major step past an array of technical difficulties and toward the cloning of higher animals, scientists say. And that step, say some scientists who fear it will lead to experimentation in the cloning of humans, should not necessarily be greeted with enthusiasm.
A spokesman for Hoppe said he would be unavailable for comment for several days, but said he denies the work should be called cloning. "He calls it nuclear transplantation," the spokesman said.
The mice that resulted from the experiment were not identical, or "clones" in the technical sense. But other scientists said that cloning was the appropriate word for the experiment, because the same kind of work in frogs was called cloning. In fact, there was a statistical chance that some identical mice would be produced in this experiment as well.
"This came along a lot faster than many expected," said Dr. Frank Ruddle, chairman of the biology department of Yale University and a professor of biology and human genetics. "The whole tempo of biological research is accelerating."
Ruddle, echoing the concerns of some of his colleagues, said, "This work crosses a technical barrier and brings cloning a lot closer to home -- that is worrisome. Now someone might try the same kind of experiment in primates, though it would be expensive and difficult. In principle, we are a lot closer to cloning higher mammals."
Previous successes in cloning were with frogs, which are far enough down the ladder of evolution from mice to be a far less challenging problem.
Dr. Fotis Kafatos, chairman of cell and developmental biology at Harvard University, said, "The work is a tour de force and will be useful in research and scientific analysis. But I do not think this work should be received with enthusiasm. I think there ought to be a ban on cloning in humans, and this work does make one worry about the next steps which might be taken.
"If one decided to do this kind of cloning experiment in primates, one ought to have a very good reason to do it."
In the Hoppe-Illmensee experiment, the nuclei were taken from the cells of four-day-old mouse embryos, all of them the embryos of gray or dun-colored mice. The nucleus of a cell contains the chromosomes that carry the genetic information for a living being, including its coloration.
The nuclei were injected into newly fertilized mouse eggs, from which the resident nuclei were then removed. The eggs were from black mice. Of the several hundred "nuclear transplantations" thus performed, about 140 of the eggs survived and then were grown in cultures.
Fewer than 50 of those survived. Sixteen of the resultant embryos were implanted in the uteruses of five white mice. In order to produce litters of normal size, these mice also received implants of 44 embryos from white mice that were not subject to nuclear transplant.
The implants were successful in all five white mice, who then gave birth to 35 mice. Thirty-two of those were white. Two -- a male and a female -- were gray, and one female was dun-colored. None of the mice had the color characteristics of the black mice who supplied the eggs for the nuclear transplantation experiment.