ABSTRACT
Skeletal muscle cells are of mesodermal origin. Mature cells in adult skeletal muscle are myotubes or muscle fibres. They are differentiated multinucleated cells formed by cytoplasmic fusion of immature mononucleated myoblasts. In muscle growth, these mononucleated precursor cells also fuse to the myotubes, adding to both the ends and the side of the cell. Muscle fibres vary in size and length between the sexes and between differing muscle groups. Fibres in muscles that have a precision requirement, such as the small muscles of the hand, tend to be smaller than in power muscles, such as the quadriceps. The fibre size is likely to be related intimately to the innervation of the muscle. Muscle fibres are bounded by a plasma membrane, the sarcolemma, and have a cytoplasm termed the sarcoplasm.
