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Cytoarchitecture of the fetal murine soleus muscle
1Department of Neurobiology, Anatomy and Cell Science, University of Pittsburgh School of Medicine, Pennsylvania 15261.
Abstract:
The organogenesis of the soleus muscle of the 129 ReJ mouse (a mixed muscle, which in the adult contains approximately equal numbers of slow-twitch oxidative and fast-twitch oxidative-glycolytic myofibers) was studied in spaced, serial transverse, and longitudinal sections of muscles of 14-, 16-, and 18-day in utero and 1- and 5-day postnatal mice. A discrete soleus muscle was distinguished by 14 days in utero. It consisted of groups of closely apposed primary myotubes displaying junctional complexes and a pleomorphic population of mononucleated cells. Between 14 and 16 days in utero there was little de novo myotube formation. At 16 days in utero, basal lamina surrounded groups of primary myotubes; and primitive motor endplates were found on these myotubes. At 18 days in utero, the basal-lamina-enclosed groups of primary myotubes were no longer present. At this stage, basal lamina surrounded clusters (consisting of one primary myotube and one or more secondary myotubes) or independent myotubes (single myotubes surrounded by their own basal lamina). Cluster formation and cluster dispersal occurred concurrently, beginning at 18 days in utero and extending until birth. At birth, there was still a substantial population of immature, secondary myotubes that interdigitated with larger, more mature primary myofibers. At this stage, intermuscular axons had begun to myelinate, and postsynaptic specialization of the motor endplates had begun. Cluster dispersal and myonuclear migration was completed during the first 5 days postnatally with the muscle taking on adult characteristics. Beginning at 16 days in utero and extending into the neonatal period, there was evidence of myotube death in the soleus muscle.
Insights
The development of the soleus muscle in mice involves myotube formation, clustering, and dispersal, with significant changes occurring before birth and continuing postnatally. This study details the organogenesis of this mixed muscle type.
Area of Science:
- Developmental Biology
- Muscle Biology
- Histology
Background:
- The soleus muscle in 129 ReJ mice is a mixed muscle type with both slow-twitch oxidative and fast-twitch oxidative-glycolytic myofibers.
- Understanding muscle organogenesis is crucial for regenerative medicine and understanding neuromuscular disorders.
Purpose of the Study:
- To investigate the temporal and spatial aspects of soleus muscle organogenesis in mice.
- To characterize the cellular and structural changes during soleus muscle development from embryonic to early postnatal stages.
Main Methods:
- Histological analysis of serial transverse and longitudinal sections.
- Examination of soleus muscles from mice at various developmental stages: 14, 16, and 18 days in utero, and 1 and 5 days postnatal.
Main Results:
- A distinct soleus muscle was identifiable by 14 days in utero, composed of primary myotubes and mononucleated cells.
- Between 16 and 18 days in utero, basal lamina formation, primitive motor endplate development, and a shift from grouped primary myotubes to clustered and independent myotubes occurred.
- Postnatally, cluster dispersal, myonuclear migration, and myelination of axons were completed, with evidence of myotube death observed throughout development.
Conclusions:
- Soleus muscle organogenesis involves dynamic processes of myotube organization, including clustering and dispersal, alongside cellular maturation and axonal myelination.
- The developmental timeline reveals critical stages of soleus muscle formation, from initial myotube organization to the establishment of adult muscle characteristics.
- Myotube death is a component of soleus muscle development, suggesting a role in muscle remodeling during the transition from embryonic to postnatal life.