title: Developing muscle spindles in the mouse masseter muscle studied by electron microscopy.
abstract: The morphogenesis of the muscle spindles of the masseter muscle was investigated by electron microscopy. The mice used in this study were those from the 16-day-old fetuses to the four-day-old young, and the three-month-old adult ICR mice. At the time of observation the formation of the masseter muscle spindle had already started in the 16-day-old fetus. In the 17-day-old fetus, the sensory nerve terminated not only in the myotube but also in the myoblast in which no myofilaments were yet formed. From the observations on the 16-day-old and 17-day-old fetuses, it is suggested that the myotubes of the intrafusal muscle fibers are formed by the fusion of the myocytes already containing the myofilaments. The gamma motor end-plate appeared two days later than the sensory nerve terminal formation in the muscle spindle. The initial inner capsule of the spindle was observed in the four-day-old young. The cell without the myofilaments appeared amidst the cluster of the intrafusal muscle fibers in the three-day-old young. It is considered that this cell may be the origin of the inner capsule cell. The periaxial space formation and the myelination of the masseteric nerve fiber occurred in the four-day-old young. The formation of the postsynaptic folds was observed in the extrafusal muscle fiber at the same stage. Thus, the muscle spindles, nerve fibers and the extrafusal muscle fibers showed a remarkable development at the same time in this early postnatal stage.
mesh_terms: D000818:Animals; D005333:Fetus / Q000648:ultrastructure; D005865:Gestational Age; D008406:Masseter Muscle / Q000196:embryology / Q000254:growth & development / Q000294:innervation*; D008410:Masticatory Muscles / Q000294:innervation*; D051379:Mice; D008813:Mice, Inbred ICR; D008854:Microscopy, Electron; D024510:Muscle Development; D009470:Muscle Spindles / Q000196:embryology / Q000254:growth & development* / Q000648:ultrastructure