Researchers in Oregon announced yesterday they had grown a live monkey from one-quarter of a monkey embryo, an unprecedented feat that suggests the possibility of creating up to four identical monkey clones from a single embryo.
The work provides strong evidence that scientists could split human embryos to produce identical twins, triplets or quadruplets, a form of human mass production not precluded by federal restrictions on human cloning. But the Oregon team emphasized that its technique for cutting embryos into quarters to produce identical foursomes had been developed as an aid to medical research, where it could eliminate the genetic variability of monkeys that has long plagued scientists studying human diseases in primates.
"By having defined genetics in laboratory monkeys we can finally tease apart a lot of interesting issues, including the relative contribution of genes and environment in various diseases," said Gerald Schatten, who with Anthony W.S. Chan led the work at the Oregon Regional Primate Research Center in Beaverton.
Previous efforts to clone monkeys by a more complicated technique--the method used to make Dolly the sheep--resulted in just two births in 1997 and more than 100 subsequent failures, inspiring scientists to try the simpler embryo splitting method. In theory, the Dolly technique can produce entire herds of genetically identical animals, while embryo splitting has the potential to make only four clones at most. But experts said yesterday that even small numbers of identical monkeys could be invaluable to medical researchers wanting to test new therapies on animals larger than mice, which are prized for their genetic homogeneity but are too different from people to be of use in some experiments.
The Oregon researchers took young, eight-celled macaque monkey embryos and divided them into subembryos of two, three or four cells each. They transferred 13 of those partial embryos to the wombs of surrogate mother monkeys, and four of those became established pregnancies.
Three of those pregnancies ended in miscarriage. Tests on one of the fetuses showed it was genetically identical to the surviving cloned monkey, which was born in September and is named Tetra, according to a report in today's issue of the journal Science. Schatten said four other monkeys from split embryos are due in May.
Even just twosomes of genetically identical monkeys would be very useful in AIDS vaccine research, said David Baltimore, president of the California Institute of Technology and chair of the federal AIDS Vaccine Research Committee. "It would allow us to do an experiment we've wanted to do for a very long time," he said, in which immune system cells from a monkey given a test vaccine could be transferred to an identical monkey to see if those cells convey protection from AIDS. Such an experiment could at last answer the question of what kind of immune system cell can protect against AIDS and allow researchers to focus on candidate vaccines that stimulate those cells.
Schatten suggested that by freezing embryo fractions, scientists could raise identical monkeys sequentially in the same mother and, without the confusion of variable genetics, investigate the effects of various environmental stimuli on developing monkeys. "You could have some listening to Mozart and others to hard rock and look for differences," he said.
The creation of human clones by embryo splitting raises fewer ethical concerns than Dolly-style human cloning, experts said. For one thing, embryo splitting would duplicate an embryo that was conceived normally by two parents, essentially mimicking the natural process by which identical twins are made, while Dolly-style cloning creates duplicates of a single adult.
Researchers at George Washington University caused a stir in 1993 by conducting human embryo splitting experiments, but no baby is known to have been produced by the method. The American Society for Reproductive Medicine (ASRM) has acknowledged the potential usefulness of embryo splitting as a treatment for infertility, because a woman who produces a single embryo might be able to turn it into four.