Analysis of muscle fiber clustering in the diaphragm muscle of sarcopenic mice

Introduction: Sarcopenia likely comprises muscle fiber denervation and re-innervation, resulting in clustering of muscle fibers of the same type (classified by myosin heavy chain isoform composition). Development of methodology to quantitatively evaluate clustering of muscle fibers according to fiber type is necessary. Methods: Fiber type specific immunofluorescence histology was used to quantify fiber clustering in murine diaphragm muscle (n=15) at ages 6 and 24 months. Results: With age, fiber type clustering is evidenced by fiber type specific changes in distances between fibers, specifically a 14% decrease to the closest fiber for type I and 24% increase for type IIx and/or IIb fibers (P<0.001). Additionally, a 34% increase to the 3 closest type IIx and/or IIb fibers was found (P<0.001). Conclusions: This novel method of analyzing fiber type clustering may be useful in examining pathophysiological conditions of motor unit loss in neuromuscular disorders, myopathies, dystrophies, injuries, or amyotrophic lateral sclerosis.