Intercostal muscle from fetal and newborn rats was examined with the electron microscope. At 16 days' gestation, the developing muscle was composed of primary generations of myotubes, many of which were clustered together in groups. Within these groups, the membranes of neighboring myotubes were interconnected by specialized junctions, including tight junctions. Morphologically undifferentiated cells surrounded the muscle groups, frequently extended pseudopodia along the interspace between adjacent myotubes, and appeared to separate neighboring myotubes from one another. At 18 and 20 days' gestation, the muscle was also composed of groups of cells but the structure of the groups differed from that of the groups observed at 16 days. Single, well differentiated myotubes containing much central glycogen and peripheral myofibrils dominated each group. These large cells were interpreted as primary myotubes. Small, less differentiated muscle cells and undifferentiated cells clustered around their walls. Each cluster was ensheated by a basal lamina. The small cells were interpreted as primordia of new generations of muscle cells which differentiated by appositional growth along the walls of the large primary myotubes. All generations of rat intercostal muscle cells matured to myofibers between 20 days' gestation and birth. Coincidentally, large and small myofibers diverged from each other, leading to disintegration of the groups of muscle cells. Undifferentiated cells frequently occurred in the interspaces between neighboring muscle cells at the time of separation. Myofibers arising at different stages of muscle histogenesis intermingled in a checkerboard fashion as a result of this asynchronous mode of development. The possibility of fusion between neighboring muscle cells in this developing system is discussed.

The fine structure of the developing neuromuscular junction of rat intercostal muscle has been studied from 16 days in utero to 10 days postpartum . At 16 days, neuromuscular relations consist of close membrane apposition between clusters of axons and groups of myotubes . Focal electron-opaque membrane specializations more intimately connect axon and myotube membranes to each other . What relation these focal contacts bear to future motor endplates is undetermined . The presence of a group of axons lying within a depression in a myotube wall and local thickening of myotube membranes with some overlying basal lamina indicates primitive motor endplate differentiation . At 18 days, large myotubes surrounded by new generations of small muscle cells occur in groups . Clusters of terminal axon sprouts mutually innervate large myotubes and adjacent small muscle cells within the groups . Nerve is separated from muscle plasma membranes by synaptic gaps partially filled by basal lamina . The plasma membranes of large myotubes, where innervated, simulate postsynaptic membranes . At birth, intercostal muscle is composed of separate myofibers . Soleplate nuclei arise coincident with the peripheral migration of myofiber nuclei . A possible source of soleplate nuclei from lateral fusion of small cells' neighboring areas of innervation is suspected but not proven . Adjacent large and small myofibers are mutually innervated by terminal axon networks contained within single Schwann cells . Primary and secondary synaptic clefts are rudimentary. By 10 days, some differentiating motor endplates simulate endplates of mature muscle . Processes of Schwann cells cover primary synaptic clefts . Axon sprouts lie within the primary clefts and are separated from each other . Specific neural control over individual myofibers may occur after neural processes are segregated in this manner .

The effect of increasing age on the completeness of anterior tibial muscle regeneration from autotransplants of minced muscle has been studied in Swiss Webster and C57/B6J mice aged 18 to 120 days. A progressively declining capability to regenerate new myofibers was associated with a decreasing phagocytic clearance of implanted myofiber debris. Concurrently, there was decreased presumptive myoblast proliferation and new myofiber formation. The importance of age-related host factors, including nonspecific macrophage activity, in muscle mince regeneration was demonstrated by: (1) the successful regeneration of muscle in heterotransplanted muscle minces from older mice implanted in younger animals and (2) the failure of muscle regeneration when the reverse experiment was performed in syngeneic animals. Heterologous striated muscle from the diaphragm regenerated in the bed of the excised anterior tibial muscle, whereas heterologous cardiac muscle failed to regenerate as expected because of the absence of satellite cells. The failure of phagocytic clearance of implanted myofiber mince and concurrent retardation of regeneration suggests a major age-related nonimmune role of phagocytic macrophages in the early stages of regeneration of anterior tibial muscle from isotopic minced muscle implants.