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IT HAS been a roller-coaster year for one of the most exciting areas in biology. Can stem cells really replace tissues lost to age or disease? There have been both ground-breaking discoveries and some major disappointments.

First the good news. The assumption was that no stem cell found in adults could be as versatile as embryonic stem cells (ESCs), which can give rise to every tissue in the body. But in January, ¿ìè¶ÌÊÓƵ revealed that Catherine Verfaillie’s group at the University of Minnesota had found stem cells with properties strikingly similar to those of ESCs in the bone marrow of both adult mice and humans.

And it looks as if there is an entire subset of adult stem cells with incredible plasticity, which the body relies on to repair injuries. Experts think they will identify more kinds in the future, thanks to advances in cell isolation and growth techniques.

The implications are huge, since turning our own cells directly into matched tissues for implantation would be far easier and cheaper than cloning them to obtain ESCs and then growing tissues from the ESCs. It would also be unnecessary to destroy an embryo to get stem cells.

But a few earlier studies claiming that even normal adult stem cells are surprisingly versatile (such as blood stem cells giving rise to muscle tissue, for example) could not be repeated. That setback may come as a much-needed wake-up call, experts believe, encouraging more rigorous work and thorough scrutiny of claims.

With a question mark hanging over adult stem cells, work on ESCs continued apace. The year saw some of the first successful examples of ESCs being turned into specific cell types in the lab. One group showed they could reliably generate large numbers of dopamine-producing neurons from ESCs – the kind needed to treat Parkinson’s disease.

Progress might have been even faster were it not for the restrictions on research in the US. Federally funded work is only allowed on embryonic stem cell lines derived before August 2001. ¿ìè¶ÌÊÓƵs complained to Congress that the most of these cell lines are poorly characterised or very difficult to obtain.

Two experiments this year also proved that therapeutic cloning can work. Adult cells from mice and cows were cloned, and ESCs were harvested from the resulting embryos. The ESCs were turned into the desired type of specialised tissue in the lab and then successfully put back into the animals.

The technique should work for people too – but cloning requires eggs, and human eggs are expensive and hard to come by. Except, it seems, in China, where several teams have reportedly managed to derive human ESCs from cloned embryos.

What hit the headlines, though, were rumours that the first human clones were on the way. The field was left wide open for such mavericks after UNtalks on a worldwide ban on human reproductive cloning were suspended. The sticking point was the demand by the US, the Vatican and some Islamic states that the ban include therapeutic cloning.

The US itself is still debating similar legislation. Patients’ groups, supported by celebrities such as Christopher Reeve (pictured) and Nancy Reagan, oppose a therapeutic cloning ban.

But there is an even greater threat than legislation to the progress of stem cell research. Many of the pioneering companies are struggling to stay afloat. Stem cell giant Geron of California laid off a third of its workforce. Advanced Cell Technology of Massachusetts is also said to be strapped for cash, and lost a star researcher, Jose Cibelli, to a university.

Meanwhile, as ¿ìè¶ÌÊÓƵ went to press, the Australian government had yet to release the US$26 million earmarked to set up its National Stem Cell Centre, after its leading researcher, Alan Trounson, was accused of misleading Parliament about stem cells’ potential.