One small step for a sheep...
Dolly may look like other lambs, but she is the most remarkable animal ever born. She is not the result of mating between a ewe and a ram but was cloned from a single cell taken from the udder of a six-year-old ewe. She is the first mammal ever created from the non-reproductive tissue of an adult animal. If the same could be achieved in humans, it would mean that each of us could have clones of ourselves made from our own tissue.
"Three years ago we didn't think it could be done," says Ian Wilmut, one of the team from the Roslin Institute and pharmaceuticals company PPL Therapeutics-both based in Edinburgh-which describe their breakthrough in this week's Nature.
Last year they used the same reproductive technology to create the world's first cloned lambs (Nature, vol 380, p 64). They produced identical lambs called Megan and Morag, which originated from different cells of the same embryo. The difference with Dolly is that all her DNA originated in a cell from the udder of an adult sheep.
The latest experiments have also produced three lambs from the cells of a sheep fetus aborted after 26 days, and four from a nine-day-old embryo.
The researchers wanted to see whether "mature" cells that have differentiated to fulfil a specialised role-such as that of an udder cell or a fetal cell-could be returned to a primitive state from which they could grow into entire organisms.
"A year ago, we showed that you could do it with cells from embryos," says Wilmut. "This time, we thought we'd take them from a fetus and an adult as well. As you can see, it worked with all three."
For each clone, the Roslin researchers combine material from two sources. First, they extract immature, unfertilised egg cells called oocytes from the ovaries of ewes. They then remove the original chromosomes from these to leave DNA-free cells.
Next, the researchers take cells containing donor genetic material. Dolly's DNA came from a cell in the udder of a six-year-old Finn Dorset ewe. Wilmut and his colleagues fuse the empty oocyte with the donor cell by bringing them together and subjecting them to an electric current.
"They fuse as one cell," says Wilmut. After growing and dividing for a week or so in a laboratory culture dish, the fused cell forms an early embryo called a blastocyst, which Wilmut's team implants into a surrogate mother. If all goes to plan, the surrogate gives birth five months later.
The process is very hit and miss, however. Dolly was the only lamb born from 277 fusions of oocytes with udder cells. Wilmut says there were so many failures because it is difficult to ensure that the empty oocyst and the donor cell are at the same stage of the cell division cycle.
The high failure rate concerns other researchers. "There were an alarming number of miscarriages and abnormalities with the technique," says Roger Gosden, a reproductive biologist at the Leeds General Infirmary.
And Jonathan Slack, an embryologist at the University of Bath, says that it is dangerous to base big ideas on a single case of a lamb raised from an adult ewe cell. He points out that it is easy to get a "false positive" when attempting to transplant a nucleus, because researchers might inadvertently fail to remove all the original DNA from the egg.
In addition, Dolly's DNA may have come from a stem cell that had not yet matured into an udder cell. Mammary glands are rich in these cells, which are more adaptable than other tissue.
But if, as seems likely, the Roslin team has succeeded in making an entire animal from adult tissue, it might be possible to do the same for humans. Wilmut remains cautious: "We've no idea if this would work, and everyone involved thinks it would be unethical to try it in people. It's illegal anyway, even though it makes a good science-fiction story." While it is banned in Britain, however, human cloning is legal elsewhere, including the US.
Instead, the Roslin team's research is aimed at agricultural applications. Particularly valuable animals could be cloned from adult cells without the uncertainties of crossing them with other animals or tinkering with embryos.
More importantly, biotechnologists will for the first time be able to manipulate the genes of cells from farm animals directly before growing them into embryos. At present, this is only possible with mice, using so-called embryonic stem cells. Researchers can insert new genes into these or knock out existing genes with great precision. But scientists have not managed to isolate such cells from farm animals, and must rely instead on injecting genes randomly into early embryos. The new technique means they will not need embryonic stem cells.
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