In their last trip to Washington embryonic stem cells found themselves in quite a political and ethical quagmire.

So much so, in fact, that the broughaha resulted in a search for ethically less contentious sources of stem cells. There is a steady trickle of reports on possible sources, but embryonic stem cells have remained unique to date in their versatility.

Many stem cells "can drive to one specific cell type," Anthony Atala, director of the Institute for Regenerative Medicine at Wake Forest University School of Medicine in Winston-Salem, N.C., told reporters at a press conference.

But "there is only one stem cell type today which can be identified as truly pluripotent, and that is the human embryonic stem cell."

Or at least, there was only one such stem cell type until last Sunday. That's when Atala, and his colleagues at Wake Forest University and Harvard Medical School, published a paper in the Jan. 7, 2007, online edition of Nature Biotechnology on a stem cell that combines the advantages of embryonic and adult stem cells, while being available from a plentiful and noncontroversial source: amniotic fluid.

"It's been known scientifically for decades that the amniotic fluid, and the placenta, have a large number of cells from the developing embryo and fetus," Atala said. So "we were interested to see whether there was a true stem cell population within this fluid - a pluripotent stem cell that would give rise to multiple cell types."

The scientists did indeed manage to find such a cell. Atala called the stem cells "a different stem cell class - neither embryonic nor adult." Indeed, they may be first-class. From the teams' results, it appears that the cells combine some of the advantages of embryonic with those of adult stem cells.

Like embryonic stem cells, the amniotic stem cells divide vigorously. They can double their population every 36 hours, and keep dividing for at least 250 times, which is far beyond the limits of about 50 divisions that most post-embryonic stem cells are able to undergo.

The amniotic stem cells also were able to differentiate into cell types of all three major embryonic cell lines. The researchers confirmed through cloning and retroviral marker studies that single cells could give rise to nerve, muscle and liver cells, which are derived from the three embryonic germ layers ectoderm, mesoderm and endoderm, respectively.

Unlike human embryonic stem cells, though, the amniotic fluid cells do not form tumors when implanted into tissue. "We actually thought they would" because they divide quite vigorously in cell culture, Atala said. "But it turns out they don't."

The amniotic stem cells also are easier to grow than embryonic stem cells. They do not require feeder layers, but will grow directly on plastic.

That could be an advantage if the cells ever make it to clinical applications. In 2005, scientists reported that currently approved embryonic stem cell lines were contaminated with possible immunogens via the mouse feeder cell layers used to grow them. (See BioWorld Today, January 31, 2005.)

As a final advantage, the cells are almost absurdly easy to come by.

There are 4.5 million live births yearly in the United States, and the cells can be harvested from amniotic fluid throughout pregnancy, "all the way until after birth," Atala said.

Despite the amniotic stem cells' obvious potential, Atala also expressed his strong support for embryonic stem cell research. Asked whether the amniotic fluid stem cells might replace embryonic stem cells, he answered that "I don't feel that way at all at this point."

The reason, he elaborated, is that for all the hopes attached to it, stem cell research still is in its early stages.

"We don't know what the extent of therapy will be with these cells," he said. "Of course, we don't know that with embryonic stem cells either - time will tell with all of these cell types. You know, there is strength with all of these cells, so we just need to keep working with all of them."