Scientists in Japan said they had grown human liver tissue from stem cells in a first that holds promise for alleviating the critical shortage of donor organs.

Creating lab-grown tissue to replenish organs damaged by accident or disease is a Holy Grail for the pioneering field of research into the premature cells known as stem cells.

Now Takanori Takebe of the Yokohama City University Graduate School of Medicine and a team reported Wednesday in the journal Nature that they grew tissue "resembling the (human) adult liver" in a lab mouse.

They first created induced pluripotent stem (iPS) cells which they mixed with other cell types and coaxed into "liver buds" -- the precursor clusters that develop into a liver. The buds, each about five millimeters (0.2 inches) big, were then transplanted onto a mouse brain, where they were observed transforming into a "functional human liver" complete with blood vessels, the scientists wrote.

"To our knowledge, this is the first report demonstrating the generation of a functional human organ from pluripotent stem cells," said the report.

The technique has yet to be tested in humans, but serves as an important proof of concept, it added.

Stem cells are infant cells that can develop into any part of the body. Until a few years ago, when iPS cells were created, the only way to obtain stem cells was to harvest them from human embryos. This is controversial because it requires the destruction of the embryo, a process to which religious conservatives and others object.

But iPS cells are easily obtainable mature cells that are "reprogrammed" into a versatile, primitive state from where they can develop into any kind of cell in the body. Takebe told a press conference ahead of the report's release that the man-made liver was observed through a replacement glass skull that was fitted around the mouse's brain.

The liver developed blood vessels which fused with those of the animal. It also performed certain human-specific liver functions -- producing proteins and processing specific drugs. "We have concluded that this liver is functioning," the scientist said. "We think this is enough for improving the survival after liver failure."

Scientists commenting on the research described it as promising.

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A First: Human Liver Created from Stem Cells