The announcement that researchers at a US biotech company had cloned human embryos from adult cells made headlines around the world earlier this week — and heightened the controversy surrounding 'therapeutic' cloning. But other researchers in the field question the claimed significance of the reported findings.

On 25 November, Advanced Cell Technology (ACT) of Worcester, Massachusetts, announced that it had created a cloned human embryo. It claimed this as an important step towards the goal of therapeutic cloning — in which cloned embryos would be used to harvest embryonic stem (ES) cells to grow replacement tissue perfectly matched to individual patients.

The ACT team fused adult cumulus cells — ovarian cells that surround eggs after ovulation — with human eggs that had been stripped of their own chromosomes. Three out of eight reconstructed eggs developed by dividing to form bundles of cells, with the best making it to six cells before stopping. Similar attempts to create embryos using skin cells met with failure.

The cloned embryos.

Other groups have previously claimed to have cloned human embryos, but the ACT team is the first to publish its results (J. B. Cibelli et al. J. Regen. Med. 2, 25–31; 2001).

In a press release, the company said that the paper provides “the first proof that reprogrammed human cells can supply tissue”. But other experts reject this claim — and some even warn that ACT's results indicate that human therapeutic cloning will be much harder than animal studies suggest.

Before ES cells can be isolated from human embryos, they must form a hollow, fluid-filled ball of cells called a blastocyst. The ACT clones are “nowhere near” that stage, says Alan Colman, research director of PPL Therapeutics near Edinburgh, and a member of the team that cloned Dolly the sheep.

Colman also notes that the development of ACT's embryos compares unfavourably with experiments in animals. “Clearly, just extrapolating from the cow system into the human has not worked very well,” he says. “With cows you would expect over 30% of the reconstructed eggs to go to the blastocyst stage.”

Jose Cibelli, ACT's vice-president of research and the paper's lead author, agrees the work is at an early stage but argues that it is still of interest: “We understand that these are early and preliminary results, but given the importance of this emerging field of medicine we decided to publish our results now.”

While scientists debate the significance of ACT's findings, political opponents of cloning are attempting to outlaw the research. The US Senate is considering a bill, already passed by the House of Representatives, that would ban all forms of human cloning, both reproductive and therapeutic. Observers now expect this to be passed, and to be signed into law by President George W. Bush.

Meanwhile, emergency legislation intended to outlaw human reproductive cloning will be debated in the British parliament this week — closing a legal loophole identified by the High Court (see Nature 414, 381; 2001). The court ruling effectively leaves therapeutic cloning unregulated in Britain, although the government is appealing the decision.

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