Muscle cells and muscle fibers have many nuclei because these cells arise from a fusion of myoblasts. Before being fused the myoblasts each have their own nucleus. After being fused together, the multiple myoblasts become a muscle fiber that has multiple nuclei.
Muscles come in three different types in animals: cardiac, smooth and skeletal. Each muscle type is comprised of muscle cells, but only skeletal muscle is constructed of muscle fibers that contain multiple nuclei. These fibers are bound together by connective tissues that are innervated by blood vessels and nervous tissue. The muscle fibers in skeletal muscle also contain a large concentration of mitochondria to produce energy for the muscles to move.