One advantage of competitive fields is that results are replicated quickly. In 2006, Shinya Yamanaka of Kyoto University in Japan launched the field of induced pluripotent stem cells by demonstrating that genetically engineered mouse cells had striking similarities to embryonic stem cells1. In the summer of 2007, three groups reported that the cells could, when mixed with a regular mouse embryo, contribute to every tissue in the resulting mice, including gametes, a trait known as germline transmission2,3,4. Now, within ten days of each other, three more groups have reported that these cells pass another, more rigorous test of pluripotency: when injected into a special embryo that contributes only to the placenta and extra-embryonic tissues, induced pluripotent stem cells can make an entire mouse.

Earlier work with embryonic stem (ES) cells established a test for pluripotency known as tetraploid complementation: if ES cells were injected into tetraploid blastocysts (derived by fusing cells in a two-cell embryo), the pups that resulted after the blastocysts were placed in a surrogate mother would contain only cells descended from the ES cells. Until now, this same feat had not been accomplished using induced pluripotent stem (iPS) cells in place of ES cells.

Researchers led by Qi Zhou at the Chinese Academy of Sciences in Beijing created and assessed several iPS cells lines5. They made iPS cell lines from colonies that formed 14, 20 and 36 days after their exposure to retroviruses containing the reprogramming genes. Then they tested several lines capable of germline chimerism in tetraploid embryos. Although all produced embryos that developed for at least 10.5 days, the only cell lines that produced live-born pups were those made from colonies of cells produced 14 days after reprogramming. Of 848 tetraploid blastocysts injected with 1 of 3 iPS cell lines, 27 live pups were produced, which is just over 3%. For comparison, the researchers repeated the experiment with ES cells. Injections of 100 tetraploid blastocysts produced 3 live pups. Though some iPS mice died shortly after birth, all twelve iPS mice that have survived into adulthood and been mated have been fertile.

Researchers led by Shaorong Gao at the National Institute of Biological Sciences in Beijing also used inducible lentiviruses6. They began with one iPS cell line capable of a high degree of chimerism. They then injected iPS cells into 387 tetraploid embryos to produce 4 live pups, 1 of which grew to adulthood.

These papers were covered in a Nature news story by David Cyranoski, see Mice made from induced stem cells

Most recently, Kristin Baldwin of the Scripps Research Institute in San Diego, California, reprogrammed cells using inducible genes carried by lentiviruses. These genes are active only in the presence of a small-molecule drug, which researchers can introduce into culture media or into an animal's diet. Using inducible genes, Baldwin reasoned, should prevent their erroneous activation during embryo development. After assessing several cell lines for their ability to produce chimeric mice and other assays of transmission, the researchers injected nearly 1,400 tetraploid embryos with 1 of 6 iPS cell lines, 4 of which generated live pups with efficiencies of 0.3%–13%. Two mice that have grown to adulthood so far have been fertile7.

These mice derived entirely from iPS cells do more than help establish iPS cells as being similar to ES cells. They suggest different levels of pluripotency, which could not be previously assessed by germline transmission. For instance, Baldwin says, the four iPS cell lines that produced iPS mice came from the same reprogramming experiment, whereas the two lines that did not had been produced using a slightly different method of passaging the cells. Another interesting question is figuring out whether iPS cells made from starting material besides mouse embryonic fibroblasts can yield entire mice.

Also, for the first time, researchers can study mice and organs entirely descended from reprogrammed cells, says Baldwin. This will allow scientists to better evaluate in situ whether such cells seem prone to tumourigenicity, senescence or dedifferentiation.

But ultimately, these kinds of tests might yield an easier way to assess iPS stem cells. Tetraploid complementation is incredibly labour intensive, says Baldwin. "We could find molecular correlates and that would help us to compare iPS cells across labs," she says. "But we don't yet know whether there is a molecular signature of that state."

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Mice made from induced stem cells

See Nature Reports Stem Cells' blog, The Niche, for a reading list of articles on induced pluripotent stem cells.