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Surrounding each muscle fiber (cell) is a delicate external lamina composed primarily of reticular fibers. This external lamina is produced by the muscle fiber and, when observed through the light microscope, cannot be distinguished from the cell membrane of the muscle fiber, the sarcolemma.
Looking at one myofibril, we see that it is divided into segments called sarcomeres. These are the contractile units of a muscle. A dark stripe called a Z disc marks the ends of one sarcomere and the beginning of the next. Sarcomeres are composed of thick filaments and thin filaments.The thin filaments are attached at one end to a Z disc and extend toward the center of the sarcomere. The thick filaments, by contrast, lie at the center of the sarcomere and overlap the thin filaments.t is through this connective tissue that the force generated by the individual cells is transmitted to the bone. A group of muscle cells are bundled together by collagen to form a fascicle. Since muscle cells are elongated and cylindrical, each muscle cell is usually called a muscle fiber.