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Canadian researchers say they have found a reliable way to direct the growth of stem cells from human embryos, taking an important step toward generating replacement parts for ailing patients.

Stem cells are the shape-shifters of biology, able to morph into any of the 200 tissue types in the human body. But while their chameleon abilities could be a potential boon for medicine, they have so far been the bane of scientists trying to control them.

Now, by adding a single gene into the DNA of a human embryonic stem cell, scientists have been able to grow pure batches of the early endoderm cells that form the essential organs of the digestive and respiratory systems. This includes the lung, liver and pancreas and raises the prospect of new treatments for diseases as diverse as diabetes and cystic fibrosis.

Researchers also found these progenitor cells proliferate endlessly, suggesting they could one day serve as an unlimited supply of starter material to grow healthy tissues as needed.

"It gives us a starting population [of cells] that we can tailor to grow the tissue type or the organ we want to replace," said Cheryle Séguin, a scientist at the Hospital for Sick Children in Toronto.

"It is early days, but that's the big end goal," added Ms. Séguin, lead author of the study published as the cover story this week in the journal Stem Cell.

Human embryonic stem cells have long been a paradox for scientists. Their ability to grow infinitely and become any tissue type makes them prized for medical research, but also dangerously unpredictable for medical treatments.

"The good news is they can make every cell type. The bad news is they can make every cell type," said co-author Janet Rossant, chief of research at Sick Kids and senior scientist at its developmental and stem cell biology program.

Scientists, trying to coax stem cells to become pure patches of bone cells, for example, might find that some become brain cells. Going for muscle cells, they might inadvertently grow the type that lines the gut.

For a decade, researchers have struggled to figure out an effective way to curb their versatility. "Our problem has been that we can't really control them," Dr. Séguin said.

Researchers have tried dousing the cells with growth-factor hormones that only encourage a particular cell type to grow, or seeding them in specialized cultures. None of these methods, however, has proved reliable.

But instead of external cues, Drs. Seguin and Rossant, working with Jonathan Draper at McMaster University in Hamilton and Andras Nagy at Toronto's Mount Sinai Hospital, tweaked the stem cell internally by adding a master gene, known as SOX17, which has the power to turn on a number of other genes.

Dr. Séguin compared it to a domino effect, with SOX17 as the block that tips first "and sets off the cascade" for that cell to become an endoderm cell that gives rise to the respiratory and digestive tracts only.

The cells are so committed that, despite researchers' efforts, they could not make them become neurons, gut or muscle cells after SOX17 is added, she said.

Researchers are now using chemical cues to guide the endoderm cells to become the specific tissues they hope to study, such as those of the lung and liver.

Dr. Rossant explained their method may be successful because it mimics the step-by-step biology of what happens in an embryo, with genes turning on in particular sequences. Dr. Séguin acknowledged, however, that researchers would have to find a way around genetically modifying stem cells before they can be used in patients.

Still, the work marks the first major discovery to come from the two and only human embryonic stem cell lines generated in Canada. The lines, or batches that can reproduce limitlessly in a lab, were derived under strict guidelines in 2005 from human embryos donated for research through fertility clinics.

Their origins remain controversial - and indeed, they would be banned from use by government-funded researchers in the U.S. - because embryos were destroyed in the process of harvesting the stem cells.

However, Dr. Séguin confirmed that efforts are under way to test their findings with the ethically acceptable "reprogrammed" stem cells recently announced. Scientists in Japan and the U.S. reported last fall they had turned ordinary human cells into embryonic-like stem cells with the addition of four genes.

While their potential and safety are still being investigated, the reprogrammed cells are considered morally palatable and more valuable for medical treatments than embryonic stem cells. This is because a patient could donate her own cells to be reprogrammed for use in a stem cell therapy that would be a perfect tissue match for that patient.

Dr. Séguin explained they have already added the SOX17 gene to a reprogrammed cell and are awaiting results. Similarly, in April, Canadian researchers who found a way to grow large quantities of cardiac tissue from embryonic stem cells are also trying to work with the reprogrammed cells.