Multinuclear myocytes are multinucleated muscle cells formed by fusion of multiple embryonic mononuclear myocytes. During the early stage of embryonic development, these cells are called primary myofibers. Later, secondary myofibers arise upon fusion of fetal myoblasts with primary myofibers. The myofibers then elongate when additional myocytes fuse to their ends. The resulting myofibers are large and can include hundreds of nuclei, where each nucleus reflects the fusion of a single myocyte. The nuclei work in synchrony to produce multiple copies of the genes essential to proper muscle contraction. The myofiber cell membrane is called the sacrolemma and the cytoplasm is called the sacroplasm. Transverse tubules, or projections of the sacrolemma, extend into the sacroplasm and conduct electrical impulses, triggering muscle contraction. The transverse tubules divide the myofiber into sub-compartments, called myofibrils, which are comprised of multiple myofilaments composed of actin and myosin. Secondary myofibers (~E14.5) acquire differential expression of myosin heavy chain isoforms, which later support fast- versus slow-contracting muscles.