Researchers at Children's Hospital Boston report using tissue engineering to reconstruct defective tracheas (windpipes) in fetal lambs, first using cells from the amniotic fluid to grow sections of cartilage tube, and then implanting these living grafts into the lambs while still in the womb.
The tracheal repair technique is one of several tissue-engineering approaches pioneered at Children's that use the fetus's own cells, drawn from the amniotic fluid that surrounds it, to create patches to fix birth defects -- in this case, even before birth. Pediatric surgeon Dario Fauza, MD, who led the study, will present the team's work on OOctober 8 at the American Academy of Pediatrics annual conference in Washington, DC.
Amniotic fluid is easily collected during pregnancy and contains unspecialized cells, known as mesenchymal stem cells, that can make many of the tissues needed to perform repairs, Fauza says.
While tracheal defects are rare, they're life-threatening: babies born with incomplete, malformed or missing tracheas cannot breathe and must immediately go on heart-lung bypass, which can cause neurologic and other complications. Surgeons have tried various fixes, such as grafting in pieces of the baby's rib or pelvic bone, using synthetic substances like Teflon, or implanting stents (in the hope that tissue would scar around the stents and form a tube), but with limited success.
"These are all makeshift solutions, and they're fraught with complications - 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's team obtained a small quantity of amniotic fluid and isolated mesenchymal stem cells. Mesenchymal stem cells descend directly from embryonic stem cells and are abundant in the amniotic fluid. They specialize in making connective tissues, including muscle, bone, cartilage, fat and tendon.
Fauza's team multiplied the amniotic mesenchymal cells in culture, then "seeded" them onto biodegradable tubes of the needed dimensions and shape. The tubes and cells were then exposed to growth factors that caused the mesenchymal cells to differentiate into cartilage cells. When the engineered grafts were ready, they were used to reconstruct defective tracheas in seven fetal lambs. Four to five weeks later, the lambs were born, and all five lambs that survived to term were able to breathe spontaneously at birth, four of them with no sign of respiratory distress. (The other two lambs, twins, were born prematurely and did not survive.)
While many congenital defects can be safely repaired after birth, Fauza's goal is to fix tracheal defects in utero. Once the baby is born, tracheal surgery requires that the baby be intubated and ventilated long after the operation while the trachea heals; this can lead to many complications, including failure of the repair. Fetal surgery would eliminate these interventions and their resulting problems. "The fetus doesn't need the trachea, so the repair would have time 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 Fetal Care Center, has been investigating the idea of growing new tissues and organs for these tiny patients for eight years. Since the tissue-engineered grafts are made from the baby's own cells, taken before birth, there would be no risk of the immune system rejecting the tissues, 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 their lab-grown tissues. While Fauza has also used cells from the ear and from the bone marrow to derive cartilage cells, amniotic fluid is much more readily available. Millions of pregnant women elect to have amniotic fluid drawn to test for chromosome defects, the procedure known as amniocentesis. And when a prenatal ultrasound exam reveals fetal 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 cells to repair a malformation in utero or after birth - potentially, even years later, Fauza says. He envisions a future in which amniotic fluid is banked for everyone's use. "Fetal cells are the best cells you can have for tissue engineering," he says. "They grow very well, and they're very plastic - you can coach them to do what you want."
Last year, Fauza reported using similar techniques in newborn lambs to repair congenital diaphragmatic hernia (CDH), or a hole in the diaphragm that separates the lungs from the visceral organs. If the hole is large enough, the stomach and other visceral organs can end up in the chest cavity, crowding the lungs and stunting their growth. Using mesenchymal stem cells from amniotic fluid, Fauza's team engineered 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 clinical trial in human babies with a prenatal ultrasound diagnosis of CDH; the amniotic fluid would be collected several months before birth and a tissue-engineered patch made ready for use soon after delivery. His team is also working on stem-cell-based, tissue-engineered grafts to fix spina bifida (in which the spinal column doesn't close fully during fetal development) and structural cardiac defects, using similar principles.