Researchers have repaired large muscle wounds in mice by growing and implanting “microthreads” coated with human muscle cells. The microthreads—made out of the same material that triggers the formation of blood clots—seem to help the cells grow in the proper orientation, which is vital for rebuilding working muscle tissue.
“We hypothesize that cells migrate along these scaffolds, which act like a conduit,” says George Pins, associate professor of bioengineering at Worcester Polytechnic Institute. Pins developed the microthread technology. The implanted cells quickly integrate into the existing muscle and reduce formation of scar tissue. “The cells grow into the space where muscle used to be, but they grow in a guided way.”
Currently, there’s not much doctors can do when someone suffers massive injury to a muscle, such as in a car crash or an explosion. Thick bands of scar tissue can form in the wound, leaving the muscle severely and permanently impaired.
Scientists are developing numerous approaches to creating replacement muscle, including growing patches of cells in a dish, injecting stem cells into damaged muscle, and implanting cell-seeded scaffolds designed to mimic native tissue. While all of these efforts show promise for certain applications, one of the major challenges has been growing enough cells in the correct structure to heal large muscle wounds.
“Muscle alignment is very important,” says Kevin “Kit” Parker, a bioengineer at Harvard University who wasn’t involved in the research. “You want the sarcomeres [the basic functional unit of muscle] to be aligned, that’s how you get muscle contractions.”
Pins and his collaborators, including Ray Page, an assistant professor at WPI’s Bioengineering Institute, aim to solve this problem by growing cells along microthreads. These hair-thin strands are made of fibrin, a protein polymer that the body uses to initiate wound healing, and a common ingredient in tissue engineering. To make the microthreads, the researchers simultaneously extrude fibrinogen, the building block of fibrin, and thrombin, an enzyme that catalyzes the soluble fibrinogen proteins into a polymer, from two small tubes. (Microthreads are also being studied for other applications, such as growing patches of heart muscle to repair damage after heart attacks.)
The threads were seeded with human muscle cells derived from tissue discarded during surgery. Prior to seeding, Page’s team grew the cells under conditions that pushed them to de-differentiate—or to become more juvenile, less specialized cells—which in turn made them better able to regenerate.
To test the technology in mice, researchers cut out about 30 percent of the animals’ tibialis anterior muscle, which lies at the front of the lower leg. They then implanted cell-seeded microthreads into the wound. (The diameter of the thread, about 50 to 100 microns, is five to 10 times the size of the cells.)
Researchers believe that the fibrin scaffold sends signaling cues that mimic native wound healing, binding to growth factors and other molecules found in blood clots. It also attracts an enzyme that breaks down the fibrin, releasing fibrinogen proteins that signal the surrounding cells to migrate in and grow new tissue, says Pins.
The cells appeared to integrate into the host tissue in just a couple of days. After a week, the microthreads began to degrade, and researchers saw that muscle fibers had grown into the area left behind. At 10 weeks, the wound bed was full of human cells, which looked like mature muscle fibers. Page presented the research at a bioengineering symposium at WPI earlier this month.
The researchers are now trying to determine whether the new tissue behaves like normal muscle. Early evidence suggests that the implants also spurred the growth of native muscle cells, though Page says they still need to confirm this.
In addition, mice implanted with microthreads had much less scar tissue than animals left to heal on their own. The microthreads “dramatically reduced the amount of collagen [the major component of scar tissue] deposited in the wound area,” says Page. “Instead of collagen, we see a lot of [well-organized] muscle tissue.”
Page says that while other scientists have been able to repair muscle to a certain extent, the WPI technology healed a much larger area of injury than previous research. This may be because the microthreads help solve one of the major challenges in growing larger swaths of new tissue—drawing in an adequate supply of blood, vital for cell survival. “One of the reasons we wanted to investigate microthreads was, we felt having space between threads would give room for vasculature to form and for muscle cells to grow,” says Page.
Harvard’s Parker, who is growing heart muscle using even smaller fibers, agrees, adding that few people in tissue engineering are taking this approach. “If I put a solid chunk of meat in there, the center will become hypoxic [or oxygen-starved],” says Parker. “If I leave space between the cells, it is easier to recruit local blood vessels.”
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