Dawn of a new age: A personal repair kit grown in a test-tube

We stand on the edge of a medical revolution that promises to cure the incurable with a personalised body-repair kit made from the cells of cloned embryos.

We stand on the edge of a medical revolution that promises to cure the incurable with a personalised body-repair kit made from the cells of cloned embryos.

The limited cloning of human embryos which the Government agreed to yesterday could eventually prove to be as important for 21st-century medicine as the development of antibiotics and organ transplantation was in the 20th.

At the centre of the scientific excitement are key cells of the embryo that have achieved almost mythical status in the eyes of researchers."Stem cells" are capable of two things: multiplying continuously in the test-tube and developing into any of the 200 to 300 specialised tissues of the body.

The aim is to capture stem cells in the very earliest stage of development, when an embryo is about five or six days old, and grow them for transplantation into patients suffering from any one of the many illnesses caused by diseased or defective tissue, from heart disorders to brain defects.

"Stem-cell research opens up a new medical frontier," said Liam Donaldson, the Chief Medical Officer of England, who chaired the expert committee that was charged with looking into the use of stem cells derived from human embryos. "It offers enormous potential for new treatments of chronic diseases and injuries and the relief of human suffering," Professor Donaldson said.

According to the committee's report, published yesterday, there appears to be no limit to the diseases that stem-cell technology can fight. It says: "Repairing cells lost in Parkinson's and Alzheimer's diseases, replacing insulin-producing cells in diabetes, changing the outcome of spinal-cord injury and multiple sclerosis, replacing heart muscle cells in cases of congestive heart failure, bone cells in osteoporosis and liver cells in hepatitis or cirrhosis all seemed realistic prospects if the research fulfilled its potential."

Stem cells gathered later in human development, either from foetal tissue or from blood extracted from the umbilical cordduring birth, have remarkable potential for treating otherwise intractable diseases.

Babies born without an immune system, for example, have recovered after having transplants of stem cells obtained from the umbilical cords of donor babies. These stem cells, however, seem destined to produce only the specialised blood cells of the immune system.

It is generally assumed that the earlier that stem cells can be captured in human development, the more capable they are of specialising in a wider range of tissues. Work on animals has already established that stem cells derived from early embryos are capable of being used successfully in the repair of cardiac muscle. The implanted cells even beat in time with the heart.

Other animal work has shown that embryonic stem cells can even be used to repair damaged nerves. "These findings in particular suggest future possibilities for the treatment of people with brain, spinal cord or peripheral nerve damage," yesterday's report says.

Using stem cells from a human embryo created by the normal fusion of egg and spermwould still not overcome the problem of tissue rejection, a bane of transplant operations. The Donaldson committee suggests that a way around this is to create cloned embryos by "cell nuclear replacement", where a nucleus from an adult cell is transplanted into an unfertilised egg which has had its own nucleus removed.

Using the same technique that created Dolly, the cloned sheep, this fusion of unfertilised egg with an adult-cell nucleus could be made to divide like an embryo. By day six it will have developed stem cells within the "inner cell mass" of the blastocyst, the hollow ball of embryonic cells.

The Donaldson committee accepts that the use of "therapeutic cloning" raises new ethical concerns: "Even those who accept the current research uses of embryos might express concern about the research use of embryos created in this way. Such embryos can be seen as being created simply as a means to an end and for use as a product source."

The committee acknowledged that such limited cloning could be seen as the "slippery slope" to the full cloning of a human being. However, Professor Donaldson said that "reproductive cloning" - the implantation of a cloned embryo into a womb - would remain a criminal offence.

By recommending the go-ahead for therapeutic cloning and the use of stem cells derived from embryos less than 14 days old, the committee says it has taken the "middle ground" between those who believe an embryo should be given full status as a human being and those who say it is merely a bunch of cells. "The middle ground, on which the current research uses are based, recognises the special status of an embryo as a potential human being but accepts that it is justified to use early embryos for serious research purposes which may benefit others," the report says.

The Donaldson recommendations include permission to use nuclear replacement to avoid the passing on of genetic defects in human eggs' mitochondrial DNA - special genes which exist outside the cell nucleus. This involves extracting the nucleus of an unfertilised egg and transferring it to the egg of a healthy woman which has had its nucleus removed. After fertilising with sperm, the egg then develops into a normal embryo which will not suffer the mother's defects in her mitochondrial DNA.

The committee accepts this breaks new ground in that it will technically result in the manipulation of the human genome and be illegal in terms of changes to the DNA of the nucleus. "While treatments developed from such research could be seen technically as constituting a modification of the human genome which would be passed on to the next generation, this modification was likely to be of a modest nature," the committee says.

A huge unknown at present is whether there will be enough spare human eggs to allow cell nuclear replacement to go ahead. It took 277 attempts to create Dolly the sheep and the most optimistic assessment is that it will take at least a dozen human eggs to create one viable clone for stem-cell research. The committee hopesit may one day be possible to circumvent the need for human eggs entirely by working out how to reprogramme adult cells so that they revert to their embryonic form.

"This is the real Holy Grail of stem-cell research," said Professor Donaldson.

Unfortunately, science is much further away from this goal than it is from the possibility of generating an unlimited supply of stem cells from cloned human embryos.