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Updated: May 16, 2026
Semi-automated Analysis of Mouse Skeletal Muscle Morphology and Fiber-type Composition
Published on: August 31, 2017
String mitochondria in mouse soleus muscle
Hisashi Fujioka1, Bernard Tandler, Saptarsi M Haldar
1Electron Microscopy Facility, School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106, USA. hisashi.fujioka@case.edu
Abstract:
Red myofibers in mouse soleus muscle have two spatially distinct populations of mitochondria: one where these organelles are disposed in large clusters just inside the sarcolemma and the other situated between the myofibrils. In most cases, the interfibrillar mitochondria (IFM), which are much smaller than the subsarcolemmal ones (SSM), are arranged as pairs, with each member on opposite sides of the Z-line. In some myofibers, the IFM have fused end-to-end to form greatly elongated organelles, which we call "string mitochondria." Although narrow, these can be many sarcomeres in length. The SSM do not form string mitochondria. Most of the string mitochondria exhibit many instances of "pinching," a process involved in mitochondrial division. Elements of sarcoplasmic reticulum are intimately involved with each mitochondrial membrane invagination. It appears as if the fusion:fission balance of IFM in the soleus muscle is slightly out of kilter, with end-to-end fusion predominating over fission.
Insights
Mouse soleus muscle red myofibers contain two distinct mitochondrial populations. Interfibrillar mitochondria (IFM) can fuse into elongated "string mitochondria," suggesting a fusion-fission imbalance.
Area of Science:
- Muscle physiology
- Cell biology
- Mitochondrial dynamics
Background:
- Red myofibers in mouse soleus muscle exhibit unique mitochondrial organization.
- Mitochondria exist as subsarcolemmal (SSM) clusters and interfibrillar (IFM) structures.
Purpose of the Study:
- To investigate the morphology and arrangement of mitochondria in mouse soleus red myofibers.
- To explore the phenomenon of "string mitochondria" and their relationship with mitochondrial division.
Main Methods:
- Light microscopy of mouse soleus muscle cross-sections.
- Morphological analysis of mitochondrial populations (SSM and IFM).
Main Results:
- Two distinct mitochondrial populations were identified: subsarcolemmal (SSM) and interfibrillar (IFM).
- IFM, smaller than SSM, are often paired around Z-lines.
- Some IFM fuse end-to-end forming elongated "string mitochondria," which exhibit "pinching" indicative of division.
- Sarcoplasmic reticulum interacts closely with mitochondrial membranes.
Conclusions:
- Mouse soleus red myofibers display specialized mitochondrial arrangements.
- The formation of string mitochondria suggests a fusion-predominant state in IFM dynamics.
- Mitochondrial fusion and fission balance may be altered in these muscle fibers.
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