Cloning Sheep

Cloning Sheep

For more than a decade, scientists have been using genetic technology to produce biologically identical copies, or clones, of animals. In theory, cloning can be used to improve sheep and cattle breeds by ensuring that the animals' most desirable genetic characteristics are passed on. But in practice, cloning has often proved disappointing: scientists have been limited in the number of clones they could produce, and the young animals frequently have a low survival rate. Now, scientists at the Roslin Institute near Edinburgh have demonstrated a dramatically different kind of cloning technology. Starting with cells from a sheep embryo, they grew thousands of copies in a culture. Technicians then fused the cells to unfertilized eggs and implanted the eggs in female sheep. In the end, only a handful of cloned Welsh Mountain lambs were born. But members of the Roslin team said that when the new technique is perfected, it should be possible to create thousands of identical sheep and cattle at a time. "This is very exciting," said Prof. Allan King, an embryologist and geneticist at Ontario's University of Guelph. "It has big implications for livestock breeding and production."

The experiment, described in the March 7 issue of the British scientific journal Nature, suggested that the new technology could be used someday to create cattle with leaner meat and cows that produce low-fat milk. Keith Campbell, the cell biologist in charge of the experiment, said that kind of genetic fine-tuning could become possible because, unlike existing methods, the new cloning system would enable scientists "to make much more precise genetic changes in the cells used to produce cloned animals."

The Roslin scientists scored an unexpected triumph when they achieved a type of cloning that had defeated past attempts by American and European scientists. The method differs from existing cloning technology in several important ways. In conventional sheep cloning, technicians usually remove embryos, consisting of between 50 and 60 cells, from artificially inseminated ewes, divide the cells into two clusters and re-insert these into recipient ewes.

The Roslin team started with slightly more mature embryonic cells, which were then grown in a culture where they multiplied rapidly - providing a far higher number of potential clones than usual. According to Campbell, the cells' high rate of growth may have been induced when scientists withdrew some of the nutrients in the culture. "This put the cells into a quiescent state," said Campbell, "which may have made them more suitable for controlling development into a fetus."

The use of a culture for growing cells should also make finer genetic tuning possible. In the past, scientists have tried to inject new genes into an embryo before cloning - an approach, said Roslin team member Ian Wilmut, that "is very primitive, like firing a shotgun." Using a culture, added Campbell, "we should be able to make much more precise genetic changes, and then use only the altered cells to produce new animals."

Inevitably, advances in animal cloning raise the prospect of scientists applying the same techniques to humans. Doctors at George Washington Medical Center in Washington did just that in 1993 when they produced 48 short-lived clones of human embryos. The controversial experiment, which was made public at a scientific conference held in Montreal, triggered a fierce controversy. Since then, some industrialized nations, including Canada, have issued guidelines against the cloning of human embryos. Meanwhile, the survival rate among cloned animals remains low. Of the five Roslin lambs born in Scotland last July, only two survived infancy and were still living last week as their story was told to the world.

Maclean's March 18, 1996