Source:Annals of Anatomy - Anatomischer Anzeiger
Author(s): M. Meznaric, I. Eržen, P. Karen, E. Cvetko
The myosin heavy chain (MyHC) composition of ageing limb muscles is transformed into a slower phenotype and expresses fast-twitch fiber type atrophy, presumably due to age-related motor unit remodeling and a change in the patterns of physical activity. It is not known if ageing affects the sternocleidomastoid muscle (SCM) in a similar way. The goal of the study was to analyze the MyHC composition and the size of muscle fibres in the ageing SCM by immunohistochemical methods and quantitative analysis and stereology using our own software for morphometry. We hypothesize that with ageing the MyHC composition of SCM transforms similarly as in ageing limb muscles, but the size of the muscle fibres is less effected as in limb muscles. The study was performed on the autopsy samples of the SCM in 12 older males. The results were compared with those published in our previous study on 15 young adult males.An ageing SCM transforms into a slower MyHC profile: the percentage of slow-twitch fibers is enhanced (numerical proportion 44.6 vs. 31.5%, P<0.05; area proportion 57.2 vs. 38.4%, P<0.05). The share of hybrid 2a/2x fibers is diminished (numerical proportion 14.1 vs. 26.8%, P<0.05), the area proportion of all fast-twitch fibers expressing MyHC-2a and 2x is smaller (50.6 vs. 63.5%, P<0.05), and the area proportion of fibers expressing the fastest myosin isoform MyHC-2x is smaller too (19.0 vs. 34.5%, P<0.05).The slower phenotype with the preferential reduction of the fibers expressing the fastest MyHC-2x provide circumstantial evidence for: (i) more fast-twitch than slow-twitch motor units being lost; and (ii) reinnervation by the surviving motor units. There appears to be no significant influence on muscle fiber size, which is congruent with relatively unchanged SCM activity during life.
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