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Tiny functioning human livers have been grown from stem cells in the laboratory by scientists in Japan.

They said they were "gobsmacked" when liver buds, the earliest stage of the organ's development, formed spontaneously.

The team, , hope that transplanting thousands of liver buds could reverse liver failure.

Experts welcomed the findings, describing them as "exciting".

Scientists around the world are trying to grow organs in the lab to overcome a shortage of organ donors.

Some patients already have bladders made from their own cells, but dense solid organs such as the liver and kidneys are much harder to produce.

The team at the Yokohama City University were reproducing the earliest stages of liver development - similar to that in an embryo.

They had mixed three types of cells - two types of stem cells and material taken from the umbilical cord.

Unexpectedly, the cells began to organise themselves and appeared to curl up to form a liver bud.

These buds were transplanted into mice, where they hooked themselves up with the blood supply and began to function as little livers.

The transplants increased the lifespan of mice with liver failure.

Prof Takanori Takebe said: "We just simply mixed three cell types and found that they unexpectedly self-organise to form a three-dimensional liver bud - this is a rudimentary liver.

"And finally we proved that liver bud transplantation could offer therapeutic potential against liver failure."

He told the 91�ȱ� that he was "completely gobsmacked" and "absolutely surprised" when he first witnessed the buds forming.

It is thought that other organs such as the pancreas, kidneys and even the lungs could be developed in the same way. However, turning this into a treatment is still a distant prospect.

The buds are 4-5mm in length but the researchers say they would need to develop buds that are much smaller and could be injected into the blood.

The buds would not grow to be a whole new liver, but would embed themselves in the failing one and restore it.

Dr Varuna Aluvihare, a liver transplant physician at King's College Hospital in London, told 91�ȱ� News: "This a great piece of work and as a proof of concept, very interesting.

"The real highlight is that such simple mixtures of cells can differentiate and organise themselves into highly complex tissue structures that function well in an animal model."

He said the liver was very damaged in chronic liver disease so there were still questions about where the buds were transplanted and how they would function.

The risk of a tumour developing after the transplant would also need to be assessed.

Dr Dusko Ilic, a stem cell scientist at King's College London, said: "The strategy is very promising, and represents a huge step forward.

"Although the promise of an off-the-shelf-liver seems much closer than one could hope even a year ago, the paper is only a proof of concept. There is much unknown and it will take years before it could be applied in regenerative medicine."

Prof Chris Mason, the chair of regenerative medicine at University College London, said there might be more immediate benefits for drug testing.

New medicines can be toxic to the human liver in a way which does not show up in animal tests. He said using liver buds might be a better way to test for toxicity.