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Cytological differentiation of human fetal skeletal muscle
Insights
Human fetal muscle development involves myotube and satellite cell fusion around 16 weeks gestation. Organelle development and filament synthesis are key to myofiber formation, with similarities across species.
Area of Science:
- Developmental Biology
- Cell Biology
- Human Anatomy
Background:
- Muscle development, or myogenesis, is a complex process involving cellular differentiation and structural organization.
- Understanding the ultrastructural changes during human fetal myogenesis provides insights into normal muscle formation and potential developmental abnormalities.
Purpose of the Study:
- To investigate the ultrastructural differentiation of various human muscles during fetal development.
- To characterize the sequence of organelle development, filament synthesis, and structural organization in developing human muscle fibers.
Main Methods:
- Transmission electron microscopy was used to examine muscle tissue from human fetuses aged 13 weeks to neonatal.
- Detailed ultrastructural analysis focused on myoblasts, myotubes, myofibers, and associated organelles.
Main Results:
- Myotube and satellite cell fusion, enclosed by basal lamina, observed around 16 weeks gestation.
- Organelle development, filament synthesis (thin and thick filaments, intermediate filaments), and Z-line formation were detailed.
- Sarcoplasmic reticulum (SR) development, transverse tubule formation, and their relationship with Z-lines were elucidated, suggesting SR does not initiate Z-line formation.
- Cytological differentiation showed similarities across different muscles, with variations in Z-line width appearing in the last trimester.
Conclusions:
- The sequence and stages of human myogenesis are comparable to those observed in other species.
- Ultrastructural evidence supports specific roles for vesicles in SR formation and highlights the association between transverse tubules and lateral sacs.
Abstract:
The ultrastructural differentiation of several different muscles was investigated in human fetuses ranging in age from 13 weeks to neonatal. At approximately 16 weeks of gestation cell cluster containing both myotubes and satellite cells lie enclosed by a newly formed basal lamina and show evidence of fusion. The development of organelles is evident in myoblasts, proceeds as the cells transform into myofibers, and continues in the neonate. Filament synthesis occurs primarily in the cell periphery where thin filaments appear to align themselves in relations to parallel arrays of ribosome-studded thick filaments: Z line formation follows the appearance of thin filaments. Intermediate filaments, approximately 10-12 nm thick, were also consistently observed in perinuclear regions and distal to filament assembly. Although sarcoplasmic reticulum (SR) development is closely related to fibril formation, connections between Z lines and SR are not consistent, thus supporting the conclusion that SR does not evoke the formation of the Z line. Bristlecoated vesicles appear to be the precursors of elements of the SR, possibly the lateral sacs. Development of the transverse tubules, as invaginations of the sarcolemma, is closely associated with the formation of lateral sacs since the latter occur along the sarcolemma as soon as transverse tubules appear. Cytological differentiation is similar, though not identical, in several different muscles. During the last trimester muscle fibers show some evidence of diversity mainly of variation in Z line width. In gerneral the results suggest that the sequence and stages of human myogenesis are similar to those of other species.