Cells From Amniotic Fluid Used To Tissue-engineer A New Trachea

The tracheal repair technique is one of several tissue-engineeringapproaches pioneered at Children's that use the fetus's own cells,drawn from the amniotic fluid that surrounds it, to create patches tofix birth defects -- in this case, even before birth. Pediatric surgeonDario Fauza, MD, who led the study, will present the team's work onOOctober 8 at the American Academy of Pediatrics annual conference inWashington, DC.

Amniotic fluid is easily collected during pregnancy and containsunspecialized cells, known as mesenchymal stem cells, that can makemany of the tissues needed to perform repairs, Fauza says.

While tracheal defects are rare, they're life-threatening: babies bornwith incomplete, malformed or missing tracheas cannot breathe and mustimmediately go on heart-lung bypass, which can cause neurologic andother complications. Surgeons have tried various fixes, such asgrafting in pieces of the baby's rib or pelvic bone, using syntheticsubstances like Teflon, or implanting stents (in the hope that tissuewould scar around the stents and form a tube), but with limited success.

"These are all makeshift solutions, and they're fraught withcomplications -- infection, narrowing of the trachea, reoperation,"Fauza says. Working with sheep, considered a good model for humans(lambs grow quickly and are similar in size to human babies), Fauza'steam obtained a small quantity of amniotic fluid and isolatedmesenchymal stem cells. Mesenchymal stem cells descend directly fromembryonic stem cells and are abundant in the amniotic fluid. Theyspecialize in making connective tissues, including muscle, bone,cartilage, fat and tendon.

Fauza's team multiplied the amniotic mesenchymal cells inculture, then "seeded" them onto biodegradable tubes of the neededdimensions and shape. The tubes and cells were then exposed to growthfactors that caused the mesenchymal cells to differentiate intocartilage cells. When the engineered grafts were ready, they were usedto reconstruct defective tracheas in seven fetal lambs. Four to fiveweeks later, the lambs were born, and all five lambs that survived toterm were able to breathe spontaneously at birth, four of them with nosign of respiratory distress. (The other two lambs, twins, were bornprematurely and did not survive.)

While many congenital defects can be safely repaired afterbirth, Fauza's goal is to fix tracheal defects in utero. Once the babyis born, tracheal surgery requires that the baby be intubated andventilated long after the operation while the trachea heals; this canlead to many complications, including failure of the repair. Fetalsurgery would eliminate these interventions and their resultingproblems. "The fetus doesn't need the trachea, so the repair would havetime to heal in utero," Fauza explains. "And fetal healing is very good-- it's better than adult healing."

Fauza, whose research lab works closely with Children's Advanced FetalCare Center, has been investigating the idea of growing new tissues andorgans for these tiny patients for eight years. Since thetissue-engineered grafts are made from the baby's own cells, takenbefore birth, there would be no risk of the immune system rejecting thetissues, and since fetal cells are immature and not fully specialized,they can be used to generate a variety of tissues.

Currently, most tissue engineers use adult cells to create theirlab-grown tissues. While Fauza has also used cells from the ear andfrom the bone marrow to derive cartilage cells, amniotic fluid is muchmore readily available. Millions of pregnant women elect to haveamniotic fluid drawn to test for chromosome defects, the procedureknown as amniocentesis. And when a prenatal ultrasound exam revealsfetal malformations, amniocentesis is usually recommended.Complications are rare.

"In many cases, the amniotic fluid is collected anyway," says Fauza."It's a precious resource that's thrown out now, but shouldn't be."

Less than two tablespoons of amniotic fluid provide enough fetal cellsto repair a malformation in utero or after birth -- potentially, evenyears later, Fauza says. He envisions a future in which amniotic fluidis banked for everyone's use. "Fetal cells are the best cells you canhave for tissue engineering," he says. "They grow very well, andthey're very plastic -- you can coach them to do what you want."

Last year, Fauza reported using similar techniques in newbornlambs to repair congenital diaphragmatic hernia (CDH), or a hole in thediaphragm that separates the lungs from the visceral organs. If thehole is large enough, the stomach and other visceral organs can end upin the chest cavity, crowding the lungs and stunting their growth.Using mesenchymal stem cells from amniotic fluid, Fauza's teamengineered a tendon patch for the diaphragm; a year later, the lambs'diaphragms showed good healing.

The FDA is now reviewing Fauza's application to conduct a clinicaltrial in human babies with a prenatal ultrasound diagnosis of CDH; theamniotic fluid would be collected several months before birth and atissue-engineered patch made ready for use soon after delivery. Histeam is also working on stem-cell-based, tissue-engineered grafts tofix spina bifida (in which the spinal column doesn't close fully duringfetal development) and structural cardiac defects, using similarprinciples.

Founded in 1869 as a 20-bed hospital for children, Children's HospitalBoston today is the nation's leading pediatric medical center, thelargest provider of health care to Massachusetts children, and theprimary pediatric teaching hospital of Harvard Medical School. Inaddition to 347 pediatric and adolescent inpatient beds andcomprehensive outpatient programs, Children's houses the world'slargest research enterprise based at a pediatric medical center, whereits discoveries benefit both children and adults. More than 500scientists, including eight members of the National Academy ofSciences, nine members of the Institute of Medicine and 10 members ofthe Howard Hughes Medical Institute comprise Children's researchcommunity. For more information about the hospital visit: http://www.childrenshospital.org.