Human kidneys grown in mice

The technique could offer a more practical way of helping patients who need organ transplants.

Thousands in the UK alone are on the waiting list for a new kidney.

Even if one is found, the patient will have to take anti-rejection drugs forever.

However, just a small number of specially selected cells taken from the foetus and transplanted into a mouse were able to carry on developing and create an entire organ with all its different parts.

No human source

And while the age of the foetal cells required means it would be impossible to establish a supply from a human source, researchers achieved the same effect with pig embryo cells.

This might allow replacement organs to grow from pig tissue without some of the normal problems this might create, such as rejection of the new organ by the human immune system.

The advance, by doctors at the Weizmann Institute of Science in Israel, was reported in the latest edition of Nature Medicine.

The technique works because cells were taken from the aborted foetuses aged between seven and eight weeks, and from pig foetuses which are only four weeks old.

Early cell

At that time, the foetus is just preparing to develop its kidneys, and a type of cell called a "kidney precursor".

This is a stem cell, a "master" cell that has the ability to divide and become all the different types of cell required to form a fully grown kidney.

Both porcine (pig) and human versions of these cells were taken and transplanted into mice.

Both types developed into perfect organs at an appropriate size for a mouse.

They produced urine and were supplied with blood by vessels from the host.

" Retroviruses are not a huge risk, but it presents a problem, and it's better to be wise before the event "
Professor Robin Weiss, University College London

In addition, there was no dangerous immune response to the new organs - scientists believe because the cells were taken early in foetal development, cells which would normally trigger the immune system were not present.

The researchers involved said that a possible treatment might be developed within the next few years.

However, while there is growing support for the use of pig organs in transplantation, some experts still believe there is a tiny risk that previously unknown viruses could pass into humans and cause illness.

Professor Robin Weiss, from University College London, UK, said the potential threat from retroviruses that are actually coded into the genetic makeup of every pig cell remained even if organs from adult pigs were not used.

He said: "It's exactly the same, no better or worse.

"Retroviruses are not a huge risk, but it presents a problem, and it's better to be wise before the event."