Glycogen-membrane complexes in denervated human skeletal muscle

Area of Science:

• Muscle biology
• Cellular ultrastructure
• Human skeletal muscle physiology

Background:

• Intracellular complexes of glycogen with smooth, cisternal, cytoplasmic membranes, termed glycogen-membrane complexes (GMCs), are described.
• These complexes were observed in denervated human skeletal muscle.
• The association of glycogen particles with these structures varied with fixation conditions, potentially explaining prior unrecognized observations.

Purpose of the Study:

• To characterize the ultrastructure of glycogen-membrane complexes (GMCs) in denervated human skeletal muscle.
• To investigate the relationship between GMCs and other cellular components, particularly the sarcoplasmic reticulum and transverse tubules.
• To compare the morphology of GMCs with known muscle cell structures using advanced electron microscopy techniques.

Main Methods:

• Electron microscopy was employed to examine denervated human skeletal muscle.
• Tannic acid enhancement was used for thin-section electron microscopy.
• Freeze-fracture electron microscopy was utilized to analyze membrane structures.
• Specimens were prepared under various fixation conditions to assess glycogen-membrane complex visibility.

Main Results:

• Glycogen particles were consistently found intimately associated with GMCs.
• Freeze-fracture replicas revealed similarities between GMCs and sarcoplasmic reticulum terminal cisternae regarding particle size and distribution.
• Tannic acid-enhanced sections showed GMCs with intense cisternal content staining and irregular cytoplasmic leaflet staining, mirroring sarcoplasmic reticulum terminal cisternae.
• Triad-like junctions were observed between GMCs and transverse tubule system elements.

Conclusions:

• Glycogen-membrane complexes (GMCs) in denervated human skeletal muscle exhibit ultrastructural similarities to sarcoplasmic reticulum terminal cisternae.
• The observed triad-like junctions suggest a functional relationship between GMCs, sarcoplasmic reticulum, and transverse tubules.
• These findings provide new insights into the organization and potential function of glycogen within muscle cells.