Conventional procedures to fix faulty heart valves all have drawbacks. Artificial valves are prone to blood clots and patients must take anti-clotting drugs for life. Valves from human cadavers or animals can deteriorate, requiring repeated open-heart surgeries to replace them, Hijazi said. That's especially true in children, because these valves don't grow along with the body.

Valves made from the patient's own cells are living tissue and might be able to grow with the patient, said Hayashida, a scientist at the National Cardiovascular Center Research Institute in Osaka.

The Swiss procedure has another advantage: using cells the fetus sheds in amniotic fluid avoids controversy because it doesn't involve destroying embryos to get stem cells.

"This is an ethical advantage," Hoerstrup said at the meeting.

Here's how the experiment worked:

Amniotic fluid was obtained through a needle inserted into the womb during amniocentesis, a prenatal test for birth defects that is often offered to pregnant women aged 35 and older.

Fetal stem cells were isolated from the fluid, cultured in a lab dish, then placed on a mold shaped like a small ink pen and made of biodegradable plastic. It took only four to six weeks to grow each of the 12 valves created in the experiment.

The researchers said lab tests showed they appeared to function normally.

The next step is to see if they work in sheep, a two-year experiment that Hoerstrup said is under way.

He and co-researcher Dorthe Schmidt called their method "a promising, low-risk approach enabling the prenatal fabrication of heart valves ready to use at birth."

Hoerstrup said amniotic stem cells also can be frozen for years and could potentially be used to create replacement parts for aging or diseased valves in adults, too.

The research is preliminary and experts say implanting tissue-engineered human valves in human hearts is likely years away. But it's not as far-fetched as it sounds.

Earlier this year, US scientists reported re-engineering seven diseased bladders with tissue grown from the patients' own cells.

And last year, researchers reported that two kidney dialysis patients from Argentina had received the world's first tissue-engineered blood vessels, fashioned from their own skin and vein tissue.

Dr. John E. Mayer Jr., a Children's Hospital Boston heart surgeon and tissue engineering pioneer, said scientists are optimistic that this area of research will revolutionize how people with valve disease will be cared for in the future.

About 250,000 patients worldwide have surgery to replace heart parts each year, according to Mayer.

In one of Mayer's experiments, heart valves fashioned from stem cells harvested from sheep bone marrow appeared to function normally when implanted in sheep. A similar experiment used cells harvested from sheep arteries.

Hoerstrup said amniotic fluid is potentially a richer source of stem cells than other sources.

Mayer said the big question is whether stem cells from amniotic fluid can create valves superior to those made from other cell types.

"I'm pretty sure the ball will continue to be advanced down the field," Mayer said. "We'll get there one way or the other."

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