Muscle (labels) - histology illustration

Skeletal muscles move by contracting. Each muscle is composed of tightly packed bundles of muscle fibers, each containing proteins, which are responsible for muscle movement. Differentiation of muscle cells begins early in embryonic development. Myoblasts (A), the precursors of skeletal muscle cells, fuse together (B), to form multinucleated cells which eventually develop into myofibrils containing contractile fibers (C).

Courtesy of NASA.