Published: Wednesday, July 3, 2013, 6:42p.m. Updated: Wednesday, July 3, 2013

An international stem cell research team reported on Wednesday that they have grown functioning human liver tissues in mice.

The human liver buds implanted in the mice represent a first experimental step in growing replacement organs from stem cells for transplants, such as liver, pancreas and kidneys, says the research team headed by Japan's Takanori Takebe of the Yokohama City University Graduate School of Medicine. The team relied on a cocktail of so-called induced stem cells grown to resemble the nascent liver bud cells used in the experiment.

The liver bud is formed at the very early stage of development normally in humans, maybe around five or six weeks, Takebe said. We basically mimicked this very early transition process of the liver-bud-forming process.

Discovered in 2006, induced stem cells are grown from mature tissues, typically skin cells, into the unspecialized stem cell state that allows for their cultivation into a wide variety of cell types, from brain to blood to liver cells.

Implanted into mice, the liver buds released human liver enzymes much more effectively than more copious amounts of liver precursor cells implanted alone in mice. The buds also developed blood vessels and grew to resemble normal liver tissues within about two days of implantation. As a final test, the researchers induced a kind of chemically induced liver failure that resembles the disease in people in 12 of the mice, and they report that implanted liver buds helped the mice survive.

Despite the effort's success, Takebe warned that implants of such tissues in human patients are at least a decade away, after tests of their long-term growth and safety.

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Stem-cell cocktail produces human liver tissue in mice