View Full Version : Growing new hearts - why it's still not on the horizon
09-06-2011, 03:26 PM
A news article from the Aug. 26 issue of Science makes it clear that progress is being made in growing tissue-engineered blood vessels, but the process is still very poorly understood. Growing engineered hearts will present still more challenges. See http://www.sciencemag.org/content/333/6046/1088.full
09-06-2011, 03:29 PM
Am sure it is a good article, Gordon, but in order to read you either have to subscribe or pay-per-article.
09-06-2011, 05:47 PM
I can never tell, since I subscribe! I'll have to post it in 2 parts. Here's part 1:
Mending the Youngest Hearts
Researchers have begun implanting tissue-engineered blood vessels into toddlers with heart defects, and new studies of the grafts in animals show they work in unexpected ways.
View larger version:
In this page In a new window
Biodegradable scaffold. Bone marrow cells seeded on a synthetic frame attract immune cells; these signal nearby vessels to grow into and over the graft.
CREDIT: ADAPTED FROM J. D. ROH ET AL., PNAS 107, 10 (9 MARCH 2010) © NATIONAL ACADEMY OF SCIENCES
The notion that tissue engineers can provide a stock of lab-grown body parts to replace faulty tissues is still, for the most part, a dream. Ready-made hearts, livers, or kidneys that could ease the shortage of donor organs will not be available in the clinic anytime soon. But recent progress suggests the dream is not completely beyond reach. Lab-grown bladders are functioning in dozens of patients in the United States, and doctors in Europe have implanted lab-grown tracheas into several patients. In Japan, several dozen children and young adults born with severe heart defects are living with tissue-engineered cardiac blood vessels. The first received implants 10 years ago. They go to school, hold full-time jobs, play sports—in short, says Christopher Breuer, one of the implants' developers, they live active, healthy lives. This month, after an arduous approval process, surgeons are testing the blood vessels in the first U.S. patients.
The U.S. trial marks “an important signpost for the whole field,” says Joseph Vacanti, a transplant surgeon and tissue engineer at Massachusetts General Hospital in Boston. The implants, which are used to connect a major cardiac vein and the artery that carries blood to the lungs, are made of a synthetic scaffold seeded with cells from the patient's own bone marrow. In the body, the graft develops into a living blood vessel that grows with the patient.
The engineered vessels were developed by a group at Yale University led by Breuer, a pediatric surgeon, and Toshiharu Shinoka, a cardiosurgeon. Although getting approval for the trial from the U.S. Food and Drug Administration (FDA) took more than 4 years and generated more than 3000 pages of documents, the process paid off, Breuer says: Recent animal studies, arising in part from questions the FDA asked, have turned some of Breuer and Shinoka's assumptions upside down, leading to a better understanding of how the graft works and ideas for how to improve it.
The new experiments suggest that inflammation, long seen as an enemy of transplants and artificial implants alike, seems to play a key role in the transformation of the cell-filled scaffold into a healthy blood vessel. And stem cells, which have been seen as the stars of tissue engineering, play a less significant role than expected. The results are prompting tissue engineers to rethink the role of inflammation and stem cells, says Anita Driessen-Mol, a tissue engineer at Eindhoven University of Technology in the Netherlands. “It's very inspiring work,” she says.
View larger version:
In this page In a new window
Lifesaving implant. A graft bypasses the heart, redirecting low-oxygen blood from the inferior vena cava directly to the pulmonary artery.
CREDIT: B. STRAUCH/SCIENCE
Replumbing the heart
The lab-made blood vessels are meant for children whose severely malformed hearts are unable to supply their bodies with enough oxygen. At birth the children are known as “blue babies” for the skin tint that results. Unlike children with a normal heart, which has two blood-pumping chambers, or ventricles, these children have only one working ventricle. Without a repair, Breuer says, 70% of children with such defects will die before their first birthday.
In the late 1960s, the surgeon Francis Fontan and his colleagues developed a technique to make such hearts more efficient. They rearranged the organ's plumbing to concentrate pumping in the single functioning ventricle. Over the years, surgeons have improved the procedure by adding a length of blood vessel to better connect the heart's inferior vena cava, which collects blood from veins in the lower body, to the pulmonary artery, which leads to the lungs, bypassing the heart (see diagram). In a few cases, surgeons can build this diversionary vessel from the patient's own tissue. But often there isn't enough tissue available, and surgeons use tubes of synthetic materials such as Gore-Tex.
Such artificial blood vessels have significant drawbacks, Breuer says. The grafts can become calcified, trigger blood clots, and, if cells build up on the inside, can develop stenosis, a dangerous narrowing of the vessel. And because the synthetic graft doesn't grow with the child, surgeons must either delay surgery until the heart has grown larger or implant a graft that is initially too big.
For more than a decade, Breuer and Shinoka have been working to develop lab-grown vessels that act like a patient's own tissue. The team uses a scaffold made of a biodegradable polyester tube, which they incubate briefly with a patient's bone marrow mononuclear cells (BMCs)—a mix of cells including immune cells and blood-and vessel-forming stem cells. Some of the patient's cells stick, seeding the scaffold.
09-06-2011, 05:48 PM
And here's part 2:
Preclinical experiments in lambs showed that the grafts soon formed a normal-looking blood vessel, with endothelial cells lining the inside and smooth muscle cells surrounding them. As expected, the scaffold degraded within a few months; new collagen fibrils, the connective tissue that helps blood vessels hold their shape, replaced it. And just like a normal blood vessel, the new tissue could grow. Based on those positive results, the clinical trial in Japan was approved and went ahead. Results have been very promising. The only complications were a few cases of stenosis, Shinoka and his colleagues have reported.
View larger version:
In this page In a new window
Engineer surgeons. Shinoka (left) and Breuer launched a U.S. trial this month.
CREDIT: ©ROBERT A. LISAK, 2011
Still, researchers weren't sure exactly how the new blood vessel formed. Were the seeded cells growing and differentiating? Or were new cells migrating into the graft? To better understand what happens after implantation—and to prove to the FDA “that each step of the way we were doing what we thought we were,” Breuer says, he and his colleagues went back to the lab. Using new, more precise fabrication techniques, they developed a mouse-sized version of their blood vessel scaffold. They seeded it with human BMCs and implanted the vessel in mice. To their surprise, though the vessel remained intact, the human cells disappeared within a week.
To test whether the human cells disappeared because they were attacked by the mouse immune system, the researchers seeded a vessel scaffold with mouse cells genetically matched to the recipient but tagged with green fluorescent protein (GFP). In a paper in The FASEB Journal this month, they confirm their earlier results: A week after implantation, almost all of the GFP-tagged cells had disappeared.
The observation “was a huge eye opener for the field,” Driessen-Mol says. “We thought that the cells you put in there would still be around for weeks.” Instead, it seems, the blood vessel that forms somehow comes from cells in the host's body. Although the seeded cells don't stick around for long, they do provide an advantage to the implant, the researchers reported last year in the Proceedings of the National Academy of Sciences. They secrete a protein that attracts monocytes, immune cells that modulate inflammation and can help prompt the formation of new blood vessels. Breuer's team showed that the seeded cells “are essential to initiate the proper kind of inflammation response. Somehow they attract the right kind of initial cells,” Driessen-Mol says.
To pinpoint the origin of the cells that build the new blood vessel, the Yale researchers created special chimeric mice. They gave female mice a lethal dose of radiation to destroy the rodents' immune and blood-forming stem cells, then rescued the mice with bone marrow stem cells from males that carried the GFP gene. The donated cells repopulated the animals' bone marrow, and soon the female mice had GFP-tagged male cells in their blood.
The researchers implanted their cell-seeded polyester tube into the chimeric mice and tracked what happened. In the first few weeks after implantation, the researchers found male, GFP-expressing immune cells in the grafts. But by 6 months, the endothelial cells and smooth muscle cells that formed the new stable blood vessel were all female. They also found no sign of stem cell markers in the vessel, suggesting that the cells growing into the graft were differentiated cells from the female host—not her stem cells at all. Further experiments with tagged cells showed that the new cells come from the adjacent blood vessels.
Taken together, Breuer says, the evidence suggests that the graft prompts the adjacent vessels to expand into and over the implanted scaffold in a process similar to normal blood vessel growth. Robert Nerem, a tissue engineer at the Georgia Institute of Technology in Atlanta, says these experiments represent “exactly the kind of study that needs to be done so we understand what's happening mechanistically in these therapeutic approaches.” Still, he would like the results confirmed in larger animals.
The results from these animal studies overturn the belief, held by many tissue engineers, that rare stem cells in the seeded BMCs would differentiate and grow on the implanted scaffold to form new tissue. The results are consistent with other evidence that BMCs can prompt tissue repair without contributing to the new tissue directly; something similar seems to happen when the cells are injected into diseased hearts. Breuer says that a more precise understanding may help researchers design safer and more effective lab-grown tissues. He and Shinoka are working to develop grafts that wouldn't need seeded cells but instead would contain a combination of signaling molecules to attract the needed response from monocytes.
That could make the implants easier to produce and much less expensive, Vacanti says, increasing the chance that they would be widely adopted. He says the new understanding might also simplify efforts to construct the tissue engineer's ultimate challenge: whole organs, with multiple cell types and a full set of blood vessels. For researchers trying to construct entire hearts, for example, “you could imagine that you wouldn't need to preseed with vascular cells, just with muscle cells,” he says.
In the meantime, the new trial will track the six U.S. patients after they receive their implants, following up with regular magnetic resonance imaging scans to watch for signs of stenosis. The team proposed starting small, Breuer says, to signal to the FDA that “we're willing to get started very slowly and carefully.” That's the right approach, Vacanti says. “It's terrific that they are going ahead,” he says. “Their work is thoughtful, rigorous, and very carefully done.” Setting a positive precedent is crucial for the field, he says. “They'll do it properly.”
09-06-2011, 05:50 PM
And a comment: Science is both a prestigious journal and a news magazine about science. This article, obviously, is in the latter category. It's not peer-reviewed science, it's journalism. But since it's in Science, it WILL come under very heavy criticism from scientists if there are serious problems with it.
09-06-2011, 06:31 PM
Interesting. Thanks for posting.
09-06-2011, 07:32 PM
Powered by vBulletin® Version 4.2.0 Copyright © 2013 vBulletin Solutions, Inc. All rights reserved.