SKELETAL MUSCLE

Histogenesis: Skeletal muscle arises from mesenchyme of mesodermal origin. The mesenchymal cells retract their long cytoplasmic processes and assume a shortened spindle shape to become myoblasts; these fuse to form multinucleated myotubes, Myotubes elongate by incorporating additional myoblasts while myoflilaments accumulate in their cytoplasm. Eventually, the accumulated myofilaments organize into myofibrils and displace the nuclei and other cytoplasmic components peripherally.

Skeletal Muscle Cells: Mature skeletal muscle fibers are elongated, unbranched, cylindrical, multinucleated cells. The flattened, peripheral nuclei lie just under the sarcolemma (muscle cell plasma membrane); most of the organelles and sarcoplasm (muscle cell cytoplasm) are near the poles of the nuclei. The sarcoplasm contains many mitochondria, glycogen granules, and an oxygen-binding protein called myoglobin, and it accumulates lipofuscin pigment with age. Mature-skeletal muscle fibers cannot divide.