A US team have been able to engineer miniature kidneys and small "patches" of heart tissue from cloned cells and successfully test them in cows.

They said that the fact that the sophisticated immune system of the cow did not reject the tissue provides hope therapeutic cloning could also be carried out on humans without rejection.

But the technique is unlikely to be used in humans because legislation prevents organs being removed from mature foetuses.

The research, led by scientists at Advanced Cell Technology (ACT), in Worcester, Massachusetts, was published in the journal Nature Biotechnology.

Cloning could theoretically provide a limitless supply of cells and organs for any type of regenerative therapy

Robert Lanza, ACT

The aim of therapeutic cloning is to take a cell from a patient and grow nerve, muscles or cartilage to replace their own tissue.

It is hoped it will be possible to create tissue which is a perfect match for the patient.

Scientists hope it could be used to treat patients who have heart, lung, liver or kidney disease - and even conditions such as diabetes, arthritis, Aids, strokes or cancer.

The problem in studies so far has been that animals produced by cloning inherit some DNA from the egg rather than the donor cell, and these 'foreign genes' create a risk of rejection after transplantation.

Functioning kidneys

In the ACT study, cows were cloned and the resulting embryos grown in the cow's womb.

When, after a few weeks, the organs had begun to form, the researchers removed some kidney and muscle cells and grew them into mini-organs.

These were then grafted back under the skins of the cows from whom they were cloned.

The scientists found the organs were not rejected, despite having some foreign DNA, and also began working.

I would be quite surprised if this would be permitted in human embryos in any state of the US - or anywhere with the facilities to carry it out

Baroness O'Neill, House of Lords select committee on stem cell research

As a cow's immune system is relatively complex; the researchers say their success indicates human applications may be possible.

Robert Lanza, vice president of medical and scientific development at ACT, who led the study, said: "This study furnishes the first scientific evidence that cloned tissues can be transplanted back into animals without being destroyed by the body's immune system.

"The use in medicine to generate immune-compatible cells using cloning would overcome one of the major scientific challenges in transplantation medicine - namely, the problem of organ and tissue rejection."

"These results bode well for the future of human therapeutic cloning.

"Cloning could theoretically provide a limitless supply of cells and organs for any type of regenerative therapy."

He added: "Before now, therapeutic cloning as a means of preventing rejection was criticised by some as being purely theoretical - just an idea."

Research restrictions

The researchers used cloned bovine foetuses to generate the necessary cells, but say in humans, technology should only be used to clone human embryonic stem cells and not an actual pregnancy.

The method used in their research could not be used in the UK because legislation forbids the removal of organs from a human embryos more than 14 days old.

Baroness O'Neill, a member of the House of Lords select committee on stem cell research, said: "This is when the 'primitive streak' can first be seen in the embryo - up to that point the embryo may split and become twins.

"It is when the embryo starts becoming an individual.

"I would be quite surprised if this would be permitted in human embryos in any state of the US - or anywhere with the facilities to carry it out."