Using stem cells from human skin, Japanese scientists have grown a small human liver inside the skull of a mouse.

Hideki Taniguchi and Takanori Takebe from Yokohama City University used stem cells generated from human skin cells and developed them into percussor liver cells, the New Scientist reports.

Then they added other cells from umbilical cord blood vessels. The combination of cells then "guided itself" to form a small structure similar to liver tissue, Takebe said.

"We mixed and graded the cells onto the culture dish and they moved to form a cluster," he said. "It was a surprising outcome from what was, to be honest, an accident."

They implanted the structure into the head of a mouse, which was suffering from a severe genetic immune system disorder that prevented it from having an immune reaction to the foreign tissues.

The increased blood flow in the mouse's skull allowed the tissue to keep growing.

Within 48 hours, human blood vessels and human proteins formed. Glycogen and amino acids levels were the same as those of a human liver.

"It's not yet a perfect liver," Takebe said. "Improvements need to be made, such as the reconstruction of a bile duct."

The study could be significant for the field of regenerative medicine, but the researchers aren't yet sure whether the organ is a fully functioning liver, or whether they will be able to scale it to human size.

The findings were presented at the at the International Society for Stem Cell Research's annual meeting in Yokohama.

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Scientists grow tiny liver in mouse's head