Making Muscles

Skeletal muscle accounts for approximately 40% of the total body mass in the human body. This type of muscle is anchored to bone by connective tissues and is involved in the movement of the body. Skeletal muscle has a tremendous ability to heal and regenerate after minor damage such as straining a muscle. When this happens, inflammation in the area activates various signals to recruit cells to the area to aid in repairing the tissue. This includes immune cells which help degrade damaged muscle cells. Next is the repair phase. In muscle tissue there is a population of adult muscle stem cells, called satellite cells, that remain inactive until they receive signals that there is an injury to the muscle. As specialized stem cells, these satellite cells have the ability to become mature muscle cells. When recruited to the damaged area, they fuse with existing heathy muscle cells and help rebuild the tissue. While the satellite cells help rebuild the muscle, cells called fibroblasts also come to the site of injury and produce collagen and connective tissue to replace the extracellular matrix (ECM). The ECM is a meshwork of proteins and molecules secreted by support cells that provides structural stability for surrounding cells as well as regulating intercellular signaling (Matthias et al., 2018). The cells working together can regenerate the muscle tissue. Though this repair mechanism works very well for small injuries, there is a different story when faced with extensive damage.

Volumetric muscle loss (VML) is defined as a large volume of muscle loss which results in permanent functional impairment. These massive injuries can result from combat-related extremity wounds or incidents such as car accidents. As seen in figure 1, extremity wounds compose the greatest percentage of injuries sustained by soldiers and have the largest projected costs of disability benefits (Masini et al., 2009). According to researchers Grogan and Hsu, VML is a significant component of combat related injuries and the cost relating to VML treatments can be in the billions of dollars (Grogan and Hsu, 2011). The prevalence of VML as well as a lack of suitable therapies make it a significant area of study.

Figure 1: Distribution of disability ratings by body region (Masini et al., 2009).

In VML, due to the volume of muscle lost, the signals that normally recruit satellite cells to help rebuild the muscle are not present. However, fibroblasts still migrate to the area and produce large amounts of collagen. This results in scar tissue formation which can prevent the muscle from contracting uniformly and cause greatly reduced functionality of the injured muscle (Grasman et al., 2015). Currently, the standard-of-care treatment is free functional muscle transfer. This requires taking muscle tissue from a healthy section of the body and transplanting it to the area of VML to attempt to restore movement. Figure 2 demonstrates a free functioning muscle transfer where muscle from the thigh was transplanted to the leg (Lin et al., 2007). Drawbacks of this method are that it requires healthy donor tissue, which may be limited on a patient suffering from severe injuries. There is also the consideration of creating two wound areas, as well as aesthetic concerns.  

Figure 2: (Left) A 46-year-old female presented with major muscle loss on the left leg. (Center) Diagram on the area of thigh to be used for free functioning muscle transfer. (Right) Leg at 8 months post-surgery (Lin et al., 2007).

Looking toward new solutions for VML, some researchers have postulated that using a scaffold, a structure designed to support another material, that mimics the muscle’s natural environment could provide environmental cues to recruit satellite cells. This would theoretically allow the large injury of the muscle to regenerate in the same manner as a small injury. One group, Sicari et al., has shown positive results with this method using a scaffold composed of a natural ECM. As seen in figure 3, after implanting mice with this ECM scaffold, the VML mice treated with ECM had a significantly higher rate of recruitment of satellite cells than the untreated mice. The treated mice also showed new formation of skeletal muscle cells while the untreated did not show any signs of new muscle formation (Sicari et al., 2014).

Figure 3: Recruitment rate of satellite cells in VML mice treated with the ECM scaffold, untreated VML mice, and an uninjured mouse (Sicari et al. 2014).

Some groups are also including cells in their scaffold to create a bioconstruct with potentially more regenerative effects. Recently published work by Quarta et al. utilizes a biological scaffold suffused with cells. The group used satellite cells, referred to here as muscle stem cells (MuSC), and muscle resident cells (MRC), which are other cell types found in proximity with the satellite cells. As seen in figure 4, they showed that by including the MRCs, the total muscle area significantly increased rather than using MuSCs alone (Quarta et al., 2017). By including a scaffold to mimic the natural environment and the different types of cells, the group has created a bioconstruct that is very physiologically similar to the native muscle. 

Figure 4: Total muscle area between mice treated with satellite cells, referred to here as muscle stem cells (MuSC), and muscle resident cells (MRC) (Quarta et al., 2017).

New research on treatments for VML are encouraging. The concept of utilizing the body’s own ability to regenerate muscle has shown promising results. However, there is still progress to be made. Some future directions for research would be to focus on scaling up the size of these constructs and work on further integrating them with the body.  

References

Grasman, J. M., Zayas, M. J., Page, R. L. and Pins, G. D. (2015). Biomimetic scaffolds for regeneration of volumetric muscle loss in skeletal muscle injuries. Acta Biomater. 25, 2–15.

Grogan, B. F. and Hsu, J. R. (2011). Volumetric Muscle Loss. J. Am. Acad. Orthop. Surg. 19, 35–37.

Lin, C. H., Lin, Y. Te, Yeh, J. T. and Chen, C. T. (2007). Free functioning muscle transfer for lower extremity posttraumatic composite structure and functional defect. Plast. Reconstr. Surg. 119, 2118–2126.

Masini, B. D., Waterman, S. M., Wenke, J. C., Owens, B. D., Hsu, J. R. and Ficke, J. R. (2009). Resource utilization and disability outcome assessment of combat causalities from operation Iraqi freedom and operation enduring freedom. J. Orthop. Trauma 23, 261–266.

Matthias, N., Hunt, S. D., Wu, J., Lo, J., Smith Callahan, L. A., Li, Y., Huard, J. and Darabi, R. (2018). Volumetric muscle loss injury repair using in situ fibrin gel cast seeded with muscle-derived stem cells (MDSCs). Stem Cell Res. 27, 65–73.

Quarta, M., Cromie, M., Chacon, R., Blonigan, J., Garcia, V., Akimenko, I., Hamer, M., Paine, P., Stok, M., Shrager, J. B., et al. (2017). Bioengineered constructs combined with exercise enhance stem cell-mediated treatment of volumetric muscle loss. Nat. Commun. 8, 1–17.

Sicari, B. M., Rubin, J. P., Dearth, C. L., Wolf, M. T., Ambrosio, F., Boninger, M., Turner, N. J., Weber, D. J., Simpson, T. W., Wyse, A., et al. (2014). An Acellular Biologic Scaffold Promotes Skeletal Muscle Formation in Mice and Humans with Volumetric Muscle Loss. Sci. Transl. Med. 6, 234ra58-234ra58.