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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.
Insights
This study details the development of mouse masseter muscle spindles from fetal to adult stages. Key findings reveal the timing of sensory nerve endings, muscle fiber fusion, and capsule formation during early development.
Area of Science:
- Developmental Biology
- Neuroscience
- Histology
Background:
- Muscle spindles are sensory mechanoreceptors crucial for proprioception.
- The development of the masseter muscle spindle is not well-documented.
- Understanding morphogenesis aids in comprehending neuromuscular function.
Purpose of the Study:
- To investigate the electron microscopic morphogenesis of the masseter muscle spindle in mice.
- To determine the developmental timeline of key components including nerve terminals, muscle fibers, and capsule formation.
Main Methods:
- Electron microscopy was used to examine masseter muscle spindles.
- Tissues were collected from mice at various developmental stages: 16-day fetuses to 4-day-old young, and 3-month-old adults.
- Morphological changes were analyzed at the cellular and subcellular levels.
Main Results:
- Masseter muscle spindle formation begins by the 16th fetal day.
- Sensory nerve terminals associate with myoblasts and myotubes.
- Intrafusal muscle fibers likely form via myocyte fusion; gamma motor end-plates appear later.
- Inner capsule formation, periaxial space development, and nerve myelination occur in early postnatal life.
- Extrafusal muscle fibers also show significant development concurrently.
Conclusions:
- The masseter muscle spindle undergoes complex morphogenesis from fetal to early postnatal stages.
- Coordinated development of muscle spindles, nerve fibers, and extrafusal muscle fibers occurs during a critical early postnatal period.
- The study provides a detailed timeline for the formation of these neuromuscular structures.