In August, a toddler born with only one functioning heart ventricle went under anesthesia at Yale so surgeons could repair the defect by attaching a new blood vessel to her heart.
Inserting artificial blood vessels, which are normally made of the same synthetic materials used for bypass surgeries, has become a relatively routine operation to help children with single-ventricle heart defects improve their circulation and live longer and healthier lives. But this child’s operation, performed by Toshiharu Shinoka, M.D., Ph.D., associate professor of surgery and pediatrics and director of pediatric cardiovascular surgery at Yale-New Haven Hospital (YNHH), and Gary S. Kopf, M.D., professor of surgery, was very different. It was the first on U.S. soil to use a tissue-engineered blood vessel, or graft. Made of a biodegradable framework seeded with living cells, the graft is expected not merely to integrate into the child’s heart, but to actually grow along with it.
“They’re living vascular grafts. They respond to all the body’s signals, so they’re able to grow, repair, and remodel just like a regular blood vessel,” says head researcher Christopher K. Breuer, M.D., associate professor of pediatric surgery and pediatrics at the School of Medicine, of the small tube-shaped devices.
In the 1990s, Shinoka worked alongside Breuer at Children’s Hospital Boston, where they bathed biodegradable scaffolds shaped like heart valves with blood vessel cells; the cells took, and the valves worked well in experiments with sheep. However, this method was impractical for clinical use, because forming usable tissue could take months.
Several years later, after having moved back to his native Japan, Shinoka devised a new technique using bone marrow cells, which yielded vessels in just hours, and over the ensuing years he implanted them into 25 Japanese children with single-ventricle defects. After six or more years of follow-up, all the children who received these grafts in Japan are doing well.
In 2007 Shinoka came Yale, which he calls “an ideal place for me to work,” to continue refining the technique with Breuer. After four years of testing their tissue-engineered blood vessels in the lab, the two surgeons set out to obtain the FDA approval that led to the September operation at YNHH.
“By seeding these cells onto the scaffold,” Breuer says, “we can actually induce the body to regenerate a blood vessel or grow a blood vessel, just like a salamander can regrow its tail or a starfish can regrow one of its arms.”
Unlike synthetic grafts, which must be either deliberately oversized to accommodate a child’s growth or replaced in risky repeat operations as a child develops, the tissue-engineered vessels seem to grow right along with the child. As the scaffolds dissolve and are replaced by collagen, the grafts develop the same cell layers seen in natural veins. Breuer and Shinoka presumed that stem cells in the bone marrow they used must be the source of those cell layers, but “we began to study how these grafts actually formed,” says Breuer, “and we surprised ourselves.”
In the lab they noticed that almost all bone-marrow cells had disappeared from the graft after a week. Yet during those crucial first few days, they learned, the stem cells were using molecules normally seen in inflammation to signal cells to leave nearby blood vessels and start building a new vessel around the scaffold (see illustration below). What the surgeons were witnessing was, in essence, vein regeneration.
Breuer, who directs the medical school’s tissue engineering program, first became interested in tissue engineering trying to create an intestinal graft that could be transplanted into babies who had lost portions of their bowel to the lethal disease necrotizing enterocolitis. Though surgeons can save some of these babies by removing portions of dead bowel, many do not survive, and he hoped to be able to engineer replacement tissue that could save their lives. But a year’s worth of bowel experiments led nowhere, Breuer says, so he turned his attention instead to blood vessels and heart valves, which are structurally simpler tissues. Within a year he was able to prove it was feasible to tissue-engineer a blood vessel.
When he came to Yale in 2003 after fulfilling a military service obligation in Afghanistan, where he says he found a “very fertile environment” in which to resume his research.
The child treated in August is doing well, and Shinoka and Breuer now aim to implant the grafts in six more children, carefully documenting the grafts’ growth using MRIs, and monitoring the children for complications. As they decipher how the signaling molecules that are sent out by bone-marrow cells help to form the grafts, they hope one day to be able to skip the bone-marrow step entirely, creating an “off-the-shelf” graft.
While they cautiously designed the current graft to be used in a low-pressure area of the heart, they hope to engineer arteries and valves that can withstand higher blood pressures and turbulence. Such grafts could improve on existing technology for dialysis catheters or cardiac bypass operations. But they could be lifesaving in more difficult-to-treat forms of congenital heart disease by preventing the need for dangerous replacement surgeries in growing children. “That,” says Breuer, “is where our work could potentially have the biggest impact.”