Age-related changes in muscle fiber types in the human thyroarytenoid muscle: An immunohistochemical and stereological study using confocal laser scanning microscopy

A decline in motor performance contributes to laryngeal dysfunction in the elderly but the pathogenetic mechanisms are unknown. Quantitative 3-dimensional, age-related changes in the muscle fiber content of the human thyroarytenoid muscle were estimated from geometric probability (stereology) by use of a technique that provided a statistically unbiased sample of all possible section orientations and locations in the entire muscle volume, There was a preferential 27% age-related loss in the length density (L-V type,L- muscle) of type 1 (slow) fibers in contrast to the selective type 2 (fast) fiber loss typical of aging limb muscles, In type 2 fibers there was no significant loss in the L-V, but there was an age-related decrease (P < 0.05) in the surface density (S-V type,S- muscle) and an increase (P < 0.05) in the atrophy factor, an index of the content of very small, atrophic fibers. There was also an age-related increase in the length fraction (L-L type,L- (all fibers)) of muscle fibers that coexpress both fast and slow myosin heavy-chain isoforms (P < 0.05). These findings demonstrate a type-specific fiber loss and atrophy that differs from that in aging limb muscles and an age-related increase in motor unit remodeling.