Capillaries within human skeletal muscle fibers.

Area of Science:

• Muscle pathology
• Myopathology
• Capillary biology

Background:

• Internalized capillaries, defined as capillaries within muscle fibers, are a rare myopathological finding.
• Their occurrence and associated conditions require systematic investigation.

Purpose of the Study:

• To systematically investigate the prevalence, characteristics, and potential mechanisms of internalized capillaries in muscle biopsy specimens.
• To explore the association of internalized capillaries with specific muscle types, demographic factors, and neuromuscular diseases.

Main Methods:

• Systematic analysis of 923 muscle biopsy specimens.
• Histopathological examination to identify and characterize internalized capillaries.
• Correlation of findings with clinical data, including muscle type, patient demographics, and diagnosed conditions.

Main Results:

• Internalized capillaries were found in 24 specimens, predominantly in the gastrocnemius muscle.
• They were more common in males and associated with juvenile spinal muscular atrophy and Becker's muscular dystrophy.
• Internalized capillaries were typically located within type I myofibers, often along the fiber axis, and appeared to originate at sites of fiber splitting, without penetrating the sarcolemma.

Conclusions:

• The presence of internalized capillaries in type I myofibers, coupled with increased capillary density, suggests a role for hypoxia in their formation.
• Morphogenetic mechanisms may involve active capillary proliferation into myofibers or myofiber fusion, with both processes potentially contributing to internalization.
• Further research is needed to fully elucidate the precise mechanisms of internalized capillary formation.