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Skeletal muscle is a highly organized structure that is surrounded by well-defined fascial layers. The individual muscle is surrounded completely by a fascial layer called the epimysium. From the epimysium, extensions of the surrounding fascia (perimysium) divide the muscle belly itself into multiple fascicles. Finally, each fascicle is further subdivided into individual muscle fibers by the endomysium. The muscle fiber is the basic structural element of skeletal muscle. Skeletal muscle fibers range in size from 10 80 mm in diameter. Each muscle fiber is surrounded by a plasma membrane known as the sarcolemma. Immediately beneath the sarcolemma along the periphery of the muscle fiber, are numerous cell nuclei. There can be several hundred nuclei for each centimeter of fiber length. Satellite cells lie along the surface of the muscle and are thought to be stem cells capable of regenerating muscle tissue in the even of injury. Individual fibers are made up of smaller aubunits called myofibrils running the length of the muscle.