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Variation of mitochondrial volume fraction along multiply innervated fibers in rabbit extraocular muscle
Abstract:
Mitochondrial volume fraction was compared among three regions along the length of six multiply innervated fibers (MIFs) in the orbital surface layer of rabbit superior rectus. These MIFs are of about 5 micrometer diameter toward the middle of their length, and of about 15 micrometer diameter toward their proximal and distal ends. The region of highest volume fraction (26%) was located toward the proximal end of their segment of minimal diameter, in apparent association with endplate-like nerve junctions. The region of lowest volume fraction (8%) was located at their distal segment of maximal diameter. The region toward the distal end of their segment of minimal diameter displayed an intermediate volume fraction (15%). These mitochondrial volume fractions were further analyzed in terms of the relative contributions of the I-band, the A-band, and the subsarcolemmal mitochondrial clusters. Comparable changes in mitochondrial content occur in both the I-band and A-band; in the fibers' distal segment of maximal diameter, however, the mitochondrial volume fraction in the A-band (5%) is lower than in the I-band (11%). These modifications of mitochondrial content along the fibers' length occur irrespective of the contributions of the subsarcolemmal mitochondrial clusters.
Insights
Mitochondrial distribution varies in rabbit superior rectus muscle fibers. Highest mitochondrial volume fraction is near nerve junctions, while lowest is in larger diameter distal segments.
Area of Science:
- Muscle physiology
- Cell biology
- Mitochondrial research
Background:
- Multiply innervated fibers (MIFs) in the superior rectus muscle exhibit regional variations in mitochondrial content.
- Understanding mitochondrial distribution is crucial for comprehending muscle function and energy metabolism.
Purpose of the Study:
- To investigate the longitudinal distribution of mitochondrial volume fraction in rabbit superior rectus MIFs.
- To analyze how mitochondrial content varies across different fiber segments and sarcomere regions.
Main Methods:
- Quantitative analysis of mitochondrial volume fraction in three distinct regions along MIFs.
- Comparison of mitochondrial distribution within the I-band, A-band, and subsarcolemmal regions.
- Microscopic examination of MIFs from the orbital surface layer of rabbit superior rectus.
Main Results:
- Mitochondrial volume fraction varied significantly along the length of MIFs, ranging from 8% to 26%.
- The highest mitochondrial volume fraction (26%) was observed near proximal endplate-like nerve junctions.
- The lowest mitochondrial volume fraction (8%) was found in the distal, larger diameter segment, with lower A-band content compared to I-band content.
Conclusions:
- Mitochondrial distribution in superior rectus MIFs is non-uniform, correlating with fiber diameter and proximity to nerve junctions.
- Regional differences in mitochondrial content, particularly within sarcomeric bands, suggest specialized energy demands along the fiber length.
- Subsarcolemmal mitochondrial clusters do not solely account for the observed longitudinal variations in mitochondrial content.