Once thought of as beyond the pale, the cloning of human embryos is now being encouraged by the government. Jenny Bryan looks at what's in store

When the government decided to back the Donaldson report on stem cell research and cloning this summer, it effectively said 'yes' to a type of technology that has the potential to revolutionise the way medicine is practised in the third millennium.

Unlimited supplies of organs for transplantation, cures for Alzheimer's and Parkinson's disease, and repair kits for brain damage and spinal cord injuries are just a few of the items on the shopping lists of scientists working in the fledgling field of cloning.

The breakthroughs have been coming thick and fast since the birth of Dolly the sheep on 5 July 1995. So will business plans for novel, cloning-based treatments soon be landing on commissioners' desks? Or will the scientific and ethical hurdles facing scientists mean that cloning budgets can be safely left to future generations of purchasers?

Potentially, the entire population could benefit from whole-animal cloning. First would be those who are missing essential enzymes, blood-clotting factors and other proteins. Some of these, such as alpha-1antitrypsin, an enzyme used to treat emphysema and cystic fibrosis, can be isolated from human blood, but only in small amounts and at considerable cost. Now, animals are being genetically modified to carry genes which allow them to produce such proteins in large quantities in their milk and then be cloned to produce herds suitable for mass production.

Next could come cloned pigs, genetically modified to make their organs suitable for transplant into humans.

Scientists have already bred pigs that carry human proteins on their cells which help reduce their risk of rejection by the human immune system.

But natural reproduction is a pretty hit-and-miss affair. Cloning an animal with highly specific changes to its genetic make-up would ensure that each of its offspring inherited the same desirable characteristic.

Once such herds were established - whether for protein production or organ harvesting - they could be allowed to revert to simpler, cheaper natural reproduction, since all the animals would have the same key genes.

Humanised pig hearts, kidneys and livers might help relieve the chronic shortage of organs for transplantation in the short term. But they are unlikely to provide a permanent solution.

Cloning human embryos in order to grow human organs and tissues is likely to produce better long-term outcomes.

When the concept made headlines in 1978 with David Rorvik's fictitious account of human cloning, it was almost universally condemned. Nearly 20 years later when Dolly was born, there was a further outcry against human cloning. But, in just five years, there has been such a substantial change in thinking that the government felt able to support therapeutic cloning of human embryos, as outlined in the Donaldson report, and has promised to bring forward legislation to implement such proposals as 1The Donaldson committee made its recommendations on human cloning only in the context of stem cell research and rejected completely the idea of cloning for reproductive purposes.

Under the proposals, only cloned embryos under 14 days old will be used for research. At this very early stage in their development, embryos are excellent sources of totipotent and pluripotent stem cells - cells that have the potential to turn into any part of the body, from liver to lung, brain to bowel, heart to hip.

Later in gestation, stem cells become more committed to the part of the body which they will be so that, after birth, through infancy and adulthood, it is much harder to influence what they will become.

In the past few years, scientists have learned a lot about how stem cells differentiate into organs and tissues and are starting to use them to repair damaged parts of the body (see box, right) and even to grow blood vessels, bone and cartilage.

Growing clumps of brain and nerve cells is proving hard enough but, assuming the scientific and ethical issues can be resolved, the next goal for human therapeutic cloning will be to harvest stem cells for spare-part surgery. The picture of an ear growing on the back of a mouse, published a few years ago in some newspapers, may not have seemed very appealing. But the fact that scientists can not only grow cells in the laboratory but shape them into recognisable body parts with synthetic matrices shows just how close they are getting to growing whole tissues and organs.

Using stem cells taken from an individual's personal cloned embryo has the advantage of being a perfect tissue match with no risk of rejection. But the idea of making a clone for its stem cells - even if it is only 14 days old and unrecognisably human - is unacceptable to many people.

The good news is that, ultimately, there may be no need to clone embryos for their stem cells. As scientists become more familiar with the control switches for stem cell manufacture, they hope to be able to re-programme an individual's own adult stem cells to go back to their embryonic origins and become totipotent once more. That way, they could be used to grow replacement organs and tissues in the laboratory without the need to create new life.

No longer science fiction, such plans are nonetheless highly futuristic and unlikely to feature in any NHS contracts for many years. If successful, they could see the end of hip replacements, cataract operations, angioplasties and transplants, and mean extra decades of active life for people entering their 60s and 70s.

How far they can extend life expectancy is another matter.

Cloning and stem cell re-programming are essentially repair and replacement techniques, but they won't prevent our bodies from ageing.

That will probably need a scientific breakthrough even more dramatic than the birth of Dolly the sheep.

REFERENCES

1 Chief Medical Officer's expert advisory group. Stem Cell Research: medical progress with responsibility. Department of Health, 2000.

2 Wilmut I, Campbell K, Tudge C. The Second Creation. Headline, 2000.

3 Onishi A, Iwamoto M, Akita T et al. Pig cloning by microinjection of foetal fibroblast nuclei. Science 2000; 289: 1188-1190.

4 Polejaeva I, Chen S-H, Vaught T et al. Cloned pigs produced by nuclear transfer from adult somatic cells. Nature 2000; 407: 86-90.