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

Abstract

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, 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, 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, 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. (Otolaryngol Head Neck Surg 1999;121:441-51.)