Engineered skeletal muscle — the muscle under a person’s voluntary control — could help treat muscle disease and injury. However, previous in vitro attempts to engineer skeletal muscle with all the properties of actual muscle have failed.
Now scientists have developed muscle tissue that mimics the highly organized structure of real skeletal muscle. In addition, this novel muscle includes a reserve of functional stem cells that helps the tissue heal itself, a first for engineered muscle. When transplanted into mice, this engineered tissue rapidly integrated with the host blood vessel network over two weeks and developed structural, functional and regenerative properties comparable to real muscle. The findings are detailed in the Proceedings of the National Academy of Sciences.
The engineered muscle bundles were grown from rat myogenic cells — the cells that fuse together to form muscle fibers — within silicon rubber molds. Just like natural muscle, these bundles contained densely packed, highly aligned, cross-striated muscle fibers that each contained multiple cell nuclei.
Over time, the contractile force-generating capacity of this engineered muscle surpassed that of newborn or neonatal rat muscle and was 10 to 100 times greater than previous attempts at engineering skeletal muscle.
“For me, the most surprising and exciting finding was that our muscle was able to maintain a functional population of muscle stem cells responsible for tissue regeneration and growth, known as satellite cells,” says researcher Nenad Bursac, a tissue engineer at Duke University in Durham, NC. “Within our engineered muscle, these satellite cells responded to toxin-induced injury by proliferating and repairing damaged muscle fibers as well as contributing to the growth of new muscle fibers over time. Another very exciting thing was to observe day after day in live animals how our implanted engineered muscle becomes invaded by host blood vessels and is stronger.”
The scientists noted that developing engineered muscle can also enable them to perform novel studies of muscle growth and repair in a dish. It could also help them model different diseases in vitro, study them, and perform screening for new drug and gene therapies, Bursac says.
The researchers suggest that differentiating muscle cells before implanting them enhanced how well the engineered muscle did in live animals. For instance, fully differentiating them before implantation led to muscle with improved structure, function, and vascularization — that is, infiltration with blood vessels.
Bursac cautions their engineered muscle in its current form “is not applicable for clinical use, due to its small size and the cell source used for its production — neonatal rats. However, we believe that our work still shows significant advance in the field of skeletal muscle tissue engineering and identifies cell types and techniques that will accelerate the development of clinically relevant muscle.”
The researchers are now addressing the issues of cell source and size of the engineered muscle. “We have already started to make functional muscle from human muscle stem cells,” Bursac says. “While this engineered human muscle shows many features of normal muscle, we still need to improve its strength — that is, force that it can produce — to achieve what we achieved with neonatal rat cells.”
To create larger muscle tissues, the researchers will need to develop ways to incorporate a bed of capillary blood vessels within the muscle to help feed a significantly larger number of cells. “Many groups are working on this topic, as it pertains to all engineered tissues and not only the muscle,” Bursac says. “Another important part of the future development is the question if the implanted muscle will be able to integrate into the host neuronal system, such that it can be voluntarily moved as most of our muscles in the body are. For this we will need to develop methods for efficient innervation of engineered muscle after implantation.”