Skeletal muscle tissue engineering

The skeletal muscle is the most abundant type of muscle found in the human body. It is responsible for voluntary movement, support, and protection. Cross-section of striated muscles shows that they are composed of multiple muscle fibers bundled in parallel to form fascicles. Each muscle fiber is composed of thousands of myofibrils containing sarcomeres, which are the basic functional unit of the muscle. The sarcomeres are in turn composed of filament of actin and myosin. Upon activation via an action potential and calcium-influx, cross-linked actin and myosin filaments slide past one another generating the calcium-dependent contractile ability of muscle. Skeletal muscles are supplied by a network of branched capillaries surrounding the fascicles which supply oxygen, nutrients, and remove waste products. From a functional stand point, two types of muscle fibers exist depending on energy expenditure. Slow twitch fibers depend on oxidative metabolic pathways which are required for longer periods of energy expenditure and fast fibers depend on glycolytic metabolic pathways for more powerful bursts but with shorter endurance. The sarcolemma is a specialized cell membrane that surrounds each muscle fiber in which ion regulation occurs. The activation and contraction of the skeletal muscle is under the synchronous control of motor neurons dispersed across the muscle fibers. The motor neuron synapses onto the sarcolemma of the muscle fiber to form a neuromuscular junction (NMJ). Depolarization occurs after the release of acetylcholine across the NMJ, which triggers calcium release and thus the contraction of the muscle. Each layer of skeletal muscle is surrounded by a connective tissue sheath that serves a critical role in support and maintenance as part of the extracellular matrix (ECM) of skeletal muscle. The epimysium surrounds the muscle. The perimysium surrounds the fascicles while the endomysium surrounds the fiber. Collagen is the most abundant structural component of the sheath. The epimysium is primarily composed of extracellular fluids, while the perimysium is composed of collagen type III, and perimysium composed of collagen type II. The ECM is composed of glycoproteins, proteoglycans, and elastin. These components of the structure of skeletal muscle are important to mimic in engineering a functional and effective skeletal muscle.