The researchers used special chemicals to coax the amniotic stem cells to develop different specialized structures and functions.
Cloned lines of the cells grew readily in the laboratory, with populations doubling every 36 hours.
Like embryonic stem cells, the amniotic cells retained their genetic makeup and showed no signs of aging over multiple generations.
After being grown in culture dishes, the human cells continued to grow and take on specialized functions when implanted into living mouse tissue.
Bone cells produced bony tissue in mice, for example, and nerve cells became established in areas of mice brains that had been damaged by disease.
"It's a very encouraging and hopeful discovery," said Roger De Filippo, a stem cell researcher at the University of Southern California.
"The fact that these cells can grow in standard culture dishes to very large numbers is a huge advantage for building organs."
But Arnold Kriegstein, director of the Institute for Regeneration Medicine at the University of California, San Francisco, sounded a more cautious note.
"Some of the cell types they described were really not well developed," Kriegstein said. "We don't know yet what the true potential of these cells might be."
Potency and Potential
By harnessing the remarkable generative properties of stem cells, scientists hope to find new treatments for neurological injuries and degenerative diseases.
Controversy over embryonic stem cells led many researchers to focus on developing therapeutic techniques using adult stem cells.
(See a National Geographic magazine feature on the stem cell controversy.)
Scientists are also working to develop techniques for harvesting embryonic stem cells without destroying embryos.
(Read "Stem Cells Can Be Collected Without Destroying Embryos, Scientists Show" [August 23, 2006].)
Other studies have tried to find alternative sources of stem cells with similar properties.
(Read "Mouse Testicles Yield Promising Stem Cells" [March 24, 2006].)
If techniques for engineering a range of tissue types from amniotic stem cells can be fully developed, the effects would be far reaching.
Atala suggests that banks of amniotic fluid could one day serve the medical needs of the general population.
"Theoretically speaking, if one had a bank of 100,000 specimens, one could supply 99 percent of the U.S. population with a perfect genetic match [of engineered tissues or organs] for transplantation," Atala said.
Ronald Green is a bioethicist at Dartmouth College in Hanover, New Hampshire. He called the new development "very promising—if the science pans out."
"We are very much in need of 'ethically universal' lines [of stem cells] that anyone can use, regardless of their views on the moral status of the human embryo," Green said.
"Every step toward alternatives that don't involve the destruction of human embryos is welcome."
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