DAVID GREENE, HOST:

We're learning this morning about an event that's being hailed as a milestone in medicine. Scientists in Japan have created human livers in a laboratory. This marks the first time researchers have created a complex organ using stem cells. Here's more from NPR's Rob Stein.

ROB STEIN, BYLINE: Ever since scientists discovered stem cells, the ultimate goal has been to use them to make tissues, and even whole organs to treat diseases. Harvard stem cell researcher George Daley says scientists have made some progress.

GEORGE DALEY: There have been groups that have attempted to generate liver cells, and that's been promising. There's been other groups that have seeded scaffolds, like from a donor trachea with cells, and created a trachea, which is kind of like an organ.

STEIN: But no one had been able to make a whole organ, or anything really close, until now. In this week's issue of the journal Nature, researchers at the Yokohoma City University in Japan report they have produced structures that are similar to primitive human livers. The researchers described their work during an international telephone briefing.

UNIDENTIFIED WOMAN: First, we're going to hear from the lead author of the paper, Professor Takenori Takebe.

TAKENORI TAKEBE: OK, so, with this study, we succeeded in generating the human liver precursor. We call this liver buds.

STEIN: Liver buds, because they're very tiny, and they don't have all the parts a regular liver has. But when Takebe's team transplanted them into mice, they did the most important things livers do. They attached themselves to blood vessels. They produced key proteins. They broke down chemicals in the animals' blood. And they can even keep the animals alive after their own livers had failed.

TAKEBE: Taken together, we concluded that this liver is functioning.

STEIN: So how did they succeed, where so many others failed? They mixed together three types of cells: stem cells known as induced pluripotent stem cells, umbilical cord cells and cells from bone marrow. Interpreter Matthew Salter stepped in during the briefing to help Takebe explain what happened next.

MATTHEW SALTER: They noticed that during that process, that some of the plates they were using had allowed them to grow these three-dimensional structures. As a result of that, they realized this extraordinary result, which gave them a functioning, three-dimensional liver bud.

STEIN: At first, Takebe and his colleagues couldn't quite believe what they had done.

SALTER: It's a little difficult to translate directly, but he was completely, I think gobsmacked is probably the word in English, absolutely surprised and thought, you know, this is something which he really couldn't predict.

STEIN: But after confirming his results with other researchers, Takebe was convinced, and other experts are hailing the results. Stephen Forbes is a liver specialist who's been doing similar research at the University of Edinburgh.

STEPHEN FORBES: Well, it's very exciting.

STEIN: Forbes says laboratory-made livers could someday help thousands of patients suffering from liver failure.

FORBES: Given that there's insufficient donor livers available for transplantation, that would be a very exciting future use.

STEIN: Now, everyone agrees that much more work is needed before the stem cell livers will be ready to help any patients. For one thing, the liver buds the Japanese scientists created don't have bile ducts, which are needed to drain away toxins, and according to Harvard's George Daley, so far, they're too small to keep a human alive.

DALEY: This is very early. It's a very different matter to make enough liver buds to save a mouse. The scale-up alone for treating a human is a Promethean task. But this is the first step.

STEIN: Takebe predicts it will take 10 years before anyone is ready to start testing his stem cell livers in humans. But he has already started to use the same approach to try to make other organs, including kidneys and pancreases. And so far, that work looks promising, too. Rob Stein, NPR News.

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