by John Newell
former Science Editor, BBC World Service
UK scientists have succeeded in cloning sheep. They have artificially produced several lambs with identical genomes. This is the first time that large mammals have been successfully duplicated this way - opening the gates to mass-producing farm animals with specially desirable characteristics, such as high meat or milk production.
The scientists, led by Dr. Ian Wilmut of the Roslin Institute near Edinburgh, Scotland, carried out the experiment by taking sheep embryos that were a few days old and multiplying them in laboratory culture over a period of several months.
The cells produced in this way were then fused, using a brief electric pulse, with unfertilised eggs that had earlier had their own nuclei removed. This produced eggs that went on to develop as though they had been fertilised normally, although all the genetic material in every egg came from just one sheep.
The eggs were transferred to the uteri of sheep and some developed normally. Several lambs were born and grew normally. Scientists now anticipate using cloning, eventually combined with genetic engineering, to speed up the adding of new desirable characteristics to farm animals.
The technique is still at an early stage of development. Although large numbers of early embryos have been produced, only a few have developed to term. Of five lambs born in the first experiments, only two survived. The next step is to increase the success rate of the process.
Earlier experiments succeeded in cloning mice, but this is the first time that large mammals have been successfully cloned. Work is alread under way in various laboratories to try to clone cattle and pigs.
The use of cells that had been grown and had divided in culture over a long period opens up exciting possibilities, of becoming able to improve farm animals genetically without the limitations of conventional animal breeding programmes.
Well-proven techniques for introducing genes into animal cells already exist. These techniques could be used to introduce genes for wanted characteristics into the embryo cells grown in culture and used to provide donor nuclei.
In this way wanted genes for any required characteristics could be introduced into cloned animals, with the need to introduce many other, often unwanted genes as well, as must happen in conventional animal breeding.
Cloning could also be used to speed up the production of animals genetically engineered for special purposes, such as the production of human body substances for use as medicines, or the provision of organs for transplant.
Other British research teams have already raised sheep that produce alpha antitrypsin, required for the treatment of emphysema, in their milk. A team at the Imutran company and Cambridge University has developed pigs whose organs are not rejected by the human immune system. These are called transgenic organs.
Animal Donor Parts
These developments will, it is hoped, allow diseases that are difficult or risky to treat by conventional means to be treated safely and effectively. They could also put an end to unnecessary loss of human life as a consequence of the growing shortage of human donors for organs.
As the technique needed to create the transgenic animals required for these purposes become established, and their products are shown to be effective, the demand for the animals will rise rapidly. Cloning could offer a means of multiplying them much more rapidly than conventional means.
The factors that contribute to successful cloning are now becoming clear, thanks to the work at Edinburgh. Ovulated oocytes - egg cells - are clearly much better recipeients for donor genetic material than are fertilised eggs.
This may be because cytoplasmic factors necessary for chromosomal remodelling and gene activation are phased out of the egg after fertilisation (possibly by time-dependent degradation).
The fact that the ability to clone large mammals have been demonstrated has inevitably led to speculation about the same thing being attempted with humans. Dr. Wilmut and his colleagues are vehement in their opposition to the idea. "I cannot imaging a clinical reason why one would want to do it and I think it would be totally unacceptable and probably illegal," he says.
Lord Robert Winston, Professor of Fertility Studies at Hammersmith Hospital in west London, says: "1990 legislation would forbid this being used for humans in the UK and there are no plans to use it anywhere in the world as far as I know."
The Reverend John Polkinghorne, a former scientist, now president of Queens' College, Cambridge, and chairman of the Anglican Church's science and medicine committee, said it would be "totally unethical" to clone humans.
However, he and spokesmen for other religious persuasions generally see no objections to the cloning of animals, as long as no suffering is involved.
For additional information contact:
Roslin Institute (Edinburgh)
Roslin, Midlothian, United Kingdom EH25 9PS
Telephone +44 1331 4402726
FAX +44 131440 0434