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The ultrastructure of normal myogenesis in the limb of the mouse
Abstract:
The ultrastructure of myogenesis in the mouse hind limb has been studied from day 12 to birth. Mononucleated cells with myofilaments are small and infrequent during myogenesis and never line myotubes. Only mononucleated cells without myofilaments cover the myotubes are involved in mass fusion. These mononucleated cells are pleomorphic and undifferentiated during early myogenesis. They have a heterochromatic nucleus and many accumulate mitochondria and lengthen during days 16-18. After day 18 they are normally elongate with a heterochromatic nucleus and packed with free ribosomes. The multinucleated cell development may be divided into four stages. 1. Immature myotube (days 14-15). Their fibrils are small, out of register and some poorly defined. Many myofilaments are not in rigid hexagonal alignment. The sarcoplasmic reticulum is simple and randomly oriented and triads are absent. Large clumps of glycogen occur between euchromatic nuclei which are in chain formation. 2. Mature myotube (days 16-18). Small groups of fibrils show alignment. Glycogen granules and sarcoplasmic reticulum become numerous between fibrils. Triads are sparse and mitochondria cluster in subsarcolemmal regions and between nuclei. 3. Young myofibers are present by day 19. More fibrils are aligned and compact. The filaments are in a rigid hexagonal array, the glycogen is dispersed and nuclei are peripherally located and moderately heterochromatic. Triads are frequent but often obliquely oriented and mitochondria are elongate and numerous between fibrils. 4. Mature myofibers occur postnatally (2 weeks).
Insights
This study details mouse hind limb myogenesis, revealing that undifferentiated mononucleated cells fuse to form myotubes. Muscle fiber development progresses through distinct ultrastructural stages from embryonic day 12 to postnatal maturity.
Area of Science:
- Developmental Biology
- Cell Biology
- Muscle Biology
Background:
- Myogenesis, the process of muscle formation, involves complex cellular differentiation and structural organization.
- Understanding the ultrastructural changes during myogenesis is crucial for comprehending muscle development and potential disorders.
Purpose of the Study:
- To investigate the ultrastructural characteristics of myogenesis in the mouse hind limb from embryonic day 12 to birth.
- To describe the morphological changes in mononucleated and multinucleated muscle cells during development.
Main Methods:
- Transmission electron microscopy was used to examine the ultrastructure of developing muscle cells.
- Samples were collected from mouse hind limbs at various developmental stages (days 12-18) and postnatally.
Main Results:
- Mononucleated cells without myofilaments are responsible for fusion; those with myofilaments do not line myotubes.
- Multinucleated cell development progresses through four distinct stages: immature myotube, mature myotube, young myofiber, and mature myofiber.
- Key ultrastructural features, including fibril alignment, sarcoplasmic reticulum development, triad formation, glycogen distribution, and nuclear morphology, change progressively during myogenesis.
Conclusions:
- The study provides a detailed ultrastructural timeline of mouse hind limb myogenesis.
- The findings elucidate the cellular dynamics and morphological transformations underlying skeletal muscle development.