The achievement represents a new direction in the use of human pluripotent stem cells, which have the potential to develop into any of the tissues of the human body. So-called induced pluripotent stem (iPS) cells, which are derived from adult human tissue, have the added advantage of producing tissues and organs genetically matched to a recipient, avoiding the problem of immune system rejection. But creating fully mature human tissue in a petri dish (in vitro) has proved a daunting challenge, especially when it comes to producing 3D organs.
Rather than do it all in a dish, the group decided to try starting the process in vitro but completing it in an animal (in vivo). They tried hundreds of different recipes; eventually they discovered that if they mixed liver precursor cells (derived from iPS cells) with two other types of standard human cell lines known to be important for embryonic liver development, then the cells would spontaneously form a 4 to 5-millimeter 3D structure called a liver bud. "This worked beyond our expectations, though the mechanism is not entirely clear," says group leader and stem cell scientist Takanori Takebe of Yokohama City University in Japan.
Next, they implanted these buds in mice with disabled immune systems to see if they would engraft, or attach to the blood vessels of the animal, and continue to mature. They did: The transplanted liver buds developed into what Takebe calls "miniature livers." The team then confirmed that these tiny organs produced proteins typically made by human livers, and they properly processed certain drugs that mouse livers cannot handle. This "proof of concept demonstration" provides a promising new approach to regenerative medicine, the team writes in a paper published online by Nature Wednesday.
The experiment shows that precursor cells can develop into functional organs when placed within the body of an adult mammal, says Takebe, who hopes to use the technique to grow organs in nonhuman primates and eventually in humans. He believes it could be applied to other organs, such as kidneys or pancreases, as well.
Stem cell scientist Martin Pera of the University of Melbourne in Australia, calls it an "exciting study" that demonstrates that the environment within an adult body can help immature stem cells develop to an adult stage, something that has proven difficult to do in vitro. The report "provides hope that even primitive tissues made from stem cells will one day restore the function of dead or diseased organs in patients," he adds.
But there are significant hurdles to clear before the technique reaches the clinic. In addition to passing all the safety checks needed to gain approval for human use, there is a practical problem. Takebe says they will need to transplant huge numbers of liver buds, comprising billions or trillions of the human iPS-derived precursor cells to even partially replace a human liver. Producing cells in those numbers will require a breakthrough in automated cell proliferation. Because of that, Takebe says use in humans is at least a decade away.
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This is adapted from ScienceNOW, the online daily news service of the journal Science. http://news.sciencemag.org
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