They hope their method could someday provide a way to create tailor-made medical treatments without having to start from scratch using cloning technology.

That would mean generating the valuable cells without using a human egg, and without creating a human embryo, which some people, including President George Bush, find objectionable.

But the team, led by stem cell expert Douglas Melton, Kevin Eggan and others at Harvard Medical School, stress in a report to be published in next Friday's issue of the journal Science that their method is not yet perfect.

Stem cells are the body's master cells, used to continually regenerate tissues, organs and blood. Those taken from days-old embryos are considered the most versatile. They can produce any kind of tissue in the body.

Treating diseases

"These findings show that human embryonic stem cells have the capacity to reprogram adult somatic cell chromosomes after cell fusion"

Research team statement, Harvard Medical School

Doctors hope to use embryonic stem cells, someday, as a source of perfectly matched transplants to treat diseases such as cancer, Parkinson and some injuries.

But because some people object to the destruction of or experimentation on a human embryo, US law restricts the use of federal funds for this kind of research.

It is a hot debate in Congress, and several bills have been offered for consideration when the Senate comes back next month that would relax or tighten the federal restrictions.

Melton has complained about the restraints and, like other experts, has used private funding to pursue stem cell work.

Programming ordinary cells

He and other stem cell experts say they only want to understand how to re-programme an ordinary cell, and hope the use of human embryos would only be a short-term and interim step to learning how to manufacture these cells.

The Harvard team says it has taken a big step in this direction. Currently, embryonic stem cells are taken from embryos left over from fertility clinics or generated using a cloning technology called nuclear transfer.

This requires taking the nucleus out of an egg cell and replacing it with the nucleus of an adult cell, called a somatic cell, from the person to be treated. Done right, this reprogrammes the egg, which starts dividing as if it had been fertilized by a sperm.

"On the basis of previous experiments with (mouse embryonic stem cells) we reasoned that human embryonic stem cells might provide an alternative source of material for the reprogramming of human somatic nuclei," the Harvard team wrote.

New cells

The Bush administration has 
approved of partial stem research

So, they fused embryonic stem cells to human adult skin cells, and managed to reprogramme them back to an embryonic state. The new cells acted like stem cells, forming tumors called teratomas when injected into mice - a classic test for a true embryonic stem cell- and with hope for marker genes.

The cells also appeared to be very long-lived, another test of a true embryonic stem cell. And when cultured in lab dishes, the cells differentiated, or matured, into the three major basic types of cell.

"In conclusion, these findings show that human embryonic stem cells have the capacity to reprogram adult somatic cell chromosomes after cell fusion," the researchers wrote.

But there is "a substantial technical barrier" they warned. The newly fused cell contains chromosomes from the original embryonic stem cell. Therefore, it would not be a perfect genetic match to the patient.

The researchers hope if they could get around this problem, they would have found a way to generate the valuable cells.