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.