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Surface and Internal Morphology of Skeletal Muscle

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Abstract

The sections in this article are:

1 Methods for Surface Morphology
1.1 Scanning Electron Microscopy
1.2 Freeze‐Replica Technique
2 Organization of Muscle Tissue
3 Fiber Surface
3.1 Endomysium
3.2 Basal Lamina
3.3 Sarcolemma
4 Fiber Interior
4.1 Exposure of Fiber Interior
4.2 Myofibrils
4.3 Sarcoplasmic Reticulum and T System
4.4 Mitochondria
5 Neuromuscular Junction
6 Summary
Figure 1. Figure 1.

Skeletal muscle fibers (M1, M2, M3) appear as cylindrical units aligned in parallel bundles. Faint cross striations are visible along individual fibers. Coarse collagenous fibers of the endomysium run in various directions over and between muscle fibers (arrows). Teased preparation of frog sartorius muscle fixed with tannic acid‐OsO4.

Figure 2. Figure 2.

Vascular corrosion cast of mouse soleus muscle. A: low‐power SEM. B: high‐power SEM. Capillary networks show a ladderlike pattern in this contracted state of muscle and are arranged in layers surrounding individual muscle fibers, which are dissolved away with all other tissue components. Note few occurrences of broken ends in the capillary casts.

From M. Kurotaki, unpublished observations
Figure 3. Figure 3.

Frog sartorius muscle fiber. Fiber surface is covered by a fibrous layer, through which cross striations are visible.

Figure 4. Figure 4.

Fibrous layer on surface of frog sartorius muscle fiber. Collagenous fibrils densely cover muscle fiber and take a predominantly longitudinal course. Cross striations can be seen through fibrous layer (arrowheads).

Figure 5. Figure 5.

Outer aspect of basal lamina of a frog sartorius muscle fiber. A: low‐power SEM. Basal lamina is exposed where fibrous layer (CF) is stripped off. Cross striations (arrows) can be seen more clearly through the lamina than through the fibrous layer. B: high‐power SEM. Outer aspect of basal lamina shows a feltlike structure, in which fine filamentous networks appear to be embedded in granular and amorphous materials.

Figure 6. Figure 6.

Appearance of end of a rat sternothyroid muscle fiber. Treatment with HCl after fixation completely removes connective tissue components from surface of muscle fiber. At the myotendon junction the conical end of a muscle fiber is characterized by formation of many longitudinal processes, clefts, and invaginations. Fingerlike processes are predominant at the peripheral portion.

From J. Desaki and Y. Uehara, unpublished observations
Figure 7. Figure 7.

Appearance of end of a frog extensor digitorum longus muscle fiber. End of this muscle fiber is characterized predominantly by invaginations and clefts.

From J. Desaki and Y. Uehara, unpublished observations
Figure 8. Figure 8.

Appearance of frog sartorius muscle sarcolemma exposed by freeze‐fracture. Freeze‐fracture of glycerol‐immersed muscle can cleave the sarcolemma in a wide expanse. Exposed surface represents P face of sarcolemma and clearly shows characteristic cross striatum of underlying myofibrils. Fibrous layer (FL) is seen where the fracture plane leaves the sarcolemma (SL). [From Sawada, Ishikawa, and Yamada .]

Figure 9. Figure 9.

Frog sartorius muscle sarcolemma exposed by freeze‐fracture. A: numerous pits are seen on the P face, distributed predominantly at the level of the I band and in interfibrillar regions. B: freeze‐fracture replica. A similar distribution of pits is observed in replica preparations. These pits represent surface openings of T tubules and caveolae. Arrows indicate level of Z band.

Figure 10. Figure 10.

Inner surface of frog sartorius muscle sarcolemma. A: low‐power SEM. True inner surface is characterized by clusters of small spherical vesicles, which represent caveolae, and by tubular and saccular structures attached on the surface. These structures tend to be distributed in a cross‐striated pattern. B: high‐power SEM of part of A. Caveolae show a uniform diameter of 60 nm (arrowheads) and often are linked to form rosettelike clusters. Tubular structures are closely associated with caveolae. Filaments appear to adhere to the surface.

Figure 11. Figure 11.

Appearance of muscle fiber interior. Interior is partly exposed (I), showing closely packed, cross‐striated myofibrils. Outer surface is covered by a filamentous layer (O). [From Sawada, Ishikawa, and Yamada .]

Figure 12. Figure 12.

Appearance of myofibril. Where myofibrils are longitudinally split, characteristic sarcomere pattern (A, I, Z, M) and filament organization clearly are seen.

Figure 13. Figure 13.

Myofibrils of frog sartorius muscle. A: SEM. B: freeze‐etch replica. Replica prepared by rapid freezing of an unfixed, fresh tissue and by rotary shadowing after freeze‐fracture etching. Note banding pattern of myofibrils (A, I, Z, M) in different preparations.

Figure 14. Figure 14.

Frog sartorius muscle sarcoplasmic reticulum and T tubule. A: SEM. B: freeze‐fracture replica. Regional differentiation of sarcoplasmic reticulum (SR) is clearly discernible in both preparations [see Peachey ]. T, T tubule.

Figure 15. Figure 15.

Frog sartorius muscle triad. Note granular projections (arrows) on terminal cisternae of sarcoplasmic reticulum (SR) facing the T tubule. Behind these structures are thin myofilaments in the I band. [From Sawada, Ishikawa, and Yamada .]

Figure 16. Figure 16.

Innervating nerve and neuromuscular junction of Chinese hamster sternothyroid muscle. Nerve (N) forms side branches (B) that terminate on muscle fibers (M) to form neuromuscular junctions (asterisks). Cap, capillary. [From Desaki and Uehara .]

Figure 17. Figure 17.

Thin‐section electron micrographs of neuromuscular junction of mouse diaphragm. A: branch (N) of a motor axon approaches a muscle fiber (M) to form neuromuscular junction (asterisks). Myelin sheath is lost just before terminal arborization (arrows). Cap, capillary. Compare with Fig. B: en face view of subneural apparatus showing characteristic pattern of junctional folds (JF). Compare with Fig. A.

Figure 18. Figure 18.

Neuromuscular junction. En face views of subneural apparatuses from adult (A) and 10‐day‐old (B) rats. Note extent of elaboration of synaptic troughs (ST) with junctional folds in adult and developing muscles. M, muscle fiber.

From J. Desaki and Y. Uehara, unpublished observations
Figure 19. Figure 19.

Neuromuscular junction. A: frog sartorius muscle. Subneural apparatus reflects en plaque type of nerve ending. Synaptic troughs (ST) are elongated along the muscle fiber (M). B. finch latissimus dorsi anterior muscle. En grappe type of nerve ending.

From J. Desaki and Y. Uehara, unpublished observations


Figure 1.

Skeletal muscle fibers (M1, M2, M3) appear as cylindrical units aligned in parallel bundles. Faint cross striations are visible along individual fibers. Coarse collagenous fibers of the endomysium run in various directions over and between muscle fibers (arrows). Teased preparation of frog sartorius muscle fixed with tannic acid‐OsO4.



Figure 2.

Vascular corrosion cast of mouse soleus muscle. A: low‐power SEM. B: high‐power SEM. Capillary networks show a ladderlike pattern in this contracted state of muscle and are arranged in layers surrounding individual muscle fibers, which are dissolved away with all other tissue components. Note few occurrences of broken ends in the capillary casts.

From M. Kurotaki, unpublished observations


Figure 3.

Frog sartorius muscle fiber. Fiber surface is covered by a fibrous layer, through which cross striations are visible.



Figure 4.

Fibrous layer on surface of frog sartorius muscle fiber. Collagenous fibrils densely cover muscle fiber and take a predominantly longitudinal course. Cross striations can be seen through fibrous layer (arrowheads).



Figure 5.

Outer aspect of basal lamina of a frog sartorius muscle fiber. A: low‐power SEM. Basal lamina is exposed where fibrous layer (CF) is stripped off. Cross striations (arrows) can be seen more clearly through the lamina than through the fibrous layer. B: high‐power SEM. Outer aspect of basal lamina shows a feltlike structure, in which fine filamentous networks appear to be embedded in granular and amorphous materials.



Figure 6.

Appearance of end of a rat sternothyroid muscle fiber. Treatment with HCl after fixation completely removes connective tissue components from surface of muscle fiber. At the myotendon junction the conical end of a muscle fiber is characterized by formation of many longitudinal processes, clefts, and invaginations. Fingerlike processes are predominant at the peripheral portion.

From J. Desaki and Y. Uehara, unpublished observations


Figure 7.

Appearance of end of a frog extensor digitorum longus muscle fiber. End of this muscle fiber is characterized predominantly by invaginations and clefts.

From J. Desaki and Y. Uehara, unpublished observations


Figure 8.

Appearance of frog sartorius muscle sarcolemma exposed by freeze‐fracture. Freeze‐fracture of glycerol‐immersed muscle can cleave the sarcolemma in a wide expanse. Exposed surface represents P face of sarcolemma and clearly shows characteristic cross striatum of underlying myofibrils. Fibrous layer (FL) is seen where the fracture plane leaves the sarcolemma (SL). [From Sawada, Ishikawa, and Yamada .]



Figure 9.

Frog sartorius muscle sarcolemma exposed by freeze‐fracture. A: numerous pits are seen on the P face, distributed predominantly at the level of the I band and in interfibrillar regions. B: freeze‐fracture replica. A similar distribution of pits is observed in replica preparations. These pits represent surface openings of T tubules and caveolae. Arrows indicate level of Z band.



Figure 10.

Inner surface of frog sartorius muscle sarcolemma. A: low‐power SEM. True inner surface is characterized by clusters of small spherical vesicles, which represent caveolae, and by tubular and saccular structures attached on the surface. These structures tend to be distributed in a cross‐striated pattern. B: high‐power SEM of part of A. Caveolae show a uniform diameter of 60 nm (arrowheads) and often are linked to form rosettelike clusters. Tubular structures are closely associated with caveolae. Filaments appear to adhere to the surface.



Figure 11.

Appearance of muscle fiber interior. Interior is partly exposed (I), showing closely packed, cross‐striated myofibrils. Outer surface is covered by a filamentous layer (O). [From Sawada, Ishikawa, and Yamada .]



Figure 12.

Appearance of myofibril. Where myofibrils are longitudinally split, characteristic sarcomere pattern (A, I, Z, M) and filament organization clearly are seen.



Figure 13.

Myofibrils of frog sartorius muscle. A: SEM. B: freeze‐etch replica. Replica prepared by rapid freezing of an unfixed, fresh tissue and by rotary shadowing after freeze‐fracture etching. Note banding pattern of myofibrils (A, I, Z, M) in different preparations.



Figure 14.

Frog sartorius muscle sarcoplasmic reticulum and T tubule. A: SEM. B: freeze‐fracture replica. Regional differentiation of sarcoplasmic reticulum (SR) is clearly discernible in both preparations [see Peachey ]. T, T tubule.



Figure 15.

Frog sartorius muscle triad. Note granular projections (arrows) on terminal cisternae of sarcoplasmic reticulum (SR) facing the T tubule. Behind these structures are thin myofilaments in the I band. [From Sawada, Ishikawa, and Yamada .]



Figure 16.

Innervating nerve and neuromuscular junction of Chinese hamster sternothyroid muscle. Nerve (N) forms side branches (B) that terminate on muscle fibers (M) to form neuromuscular junctions (asterisks). Cap, capillary. [From Desaki and Uehara .]



Figure 17.

Thin‐section electron micrographs of neuromuscular junction of mouse diaphragm. A: branch (N) of a motor axon approaches a muscle fiber (M) to form neuromuscular junction (asterisks). Myelin sheath is lost just before terminal arborization (arrows). Cap, capillary. Compare with Fig. B: en face view of subneural apparatus showing characteristic pattern of junctional folds (JF). Compare with Fig. A.



Figure 18.

Neuromuscular junction. En face views of subneural apparatuses from adult (A) and 10‐day‐old (B) rats. Note extent of elaboration of synaptic troughs (ST) with junctional folds in adult and developing muscles. M, muscle fiber.

From J. Desaki and Y. Uehara, unpublished observations


Figure 19.

Neuromuscular junction. A: frog sartorius muscle. Subneural apparatus reflects en plaque type of nerve ending. Synaptic troughs (ST) are elongated along the muscle fiber (M). B. finch latissimus dorsi anterior muscle. En grappe type of nerve ending.

From J. Desaki and Y. Uehara, unpublished observations
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How to Cite

Harunori Ishikawa, Hajime Sawada, Eichi Yamada. Surface and Internal Morphology of Skeletal Muscle. Compr Physiol 2011, Supplement 27: Handbook of Physiology, Skeletal Muscle: 1-21. First published in print 1983. doi: 10.1002/cphy.cp100101