The present thesis evaluated qualitative and quantitative age related changes in muscle fibretype and myosin heavy chain compositions in the human masseter, lateral pterygoid and digastricmuscles; three jaw muscles with different developmental origin, nerve supply and functionalconditions. Muscle specimens from young adult and elderly subjects were studied usingmorphological, enzyme-histochemical, immuno-cytochemical and biochemical techniques. Forcomparison, the biceps brachii muscle of the same subjects was analysed.
The masseter and the lateral pterygoid muscles of elderly were composed of type I, IM/IIC,IIA, IIAB and IIB fibres, with significant intramuscular differences in proportion and diameter.The old digastric and biceps brachii muscles comprised of type I, ITA, IIAB and IIB and few typeIM/IIC fibres.
All muscles in both age groups contained slow, fast A and fast X MyHCs, and the masseteralso contained fetal MyHC isoform. Single masseter fibres contained fetal and occasionallyembryonic and a-cardiac MyHC isoforms. Single fibres of the lateral pterygoid muscle containedo-cardiac and occasionally fetal MyHCs.
The masseter and the lateral pterygoid muscles contained a large proportion of fibres withmixtures of up to four MyHCs. Single digastric and biceps brachii fibres contained one or twoMyHCs. The largest proportion of fibres with mixtures of MyHCs, more than half of the fibrepopulation, was found in the lateral pterygoid muscle of both age groups.
The most prominent differences in fibre composition between muscles of young adults andelderly were found in the masseter, followed by the lateral pterygoid and the digastric muscles.Such extensive age-related changes in both fibre type and MyHC content as those detected in thehuman masseter have not been reported for any human skeletal muscle. The masseter of elderlyshowed 1) lower proportions of type I and slow MyHC, ii) larger proportions of type IM and typeII fibres and fast and fetal MyHCs, iii) larger proportion of fibres with mixtures of various MyHCisoforms and iiii) fibre atrophy. These alterations were pronounced in the anterior part of itssuperficial portion and less marked in its deep portion. The old masseter not only expressed fetalMyHC but also showed a significantly increased proportion of this myosin isoform, indicating thatthis protein is a natural component of the human masseter phenotype.
The lateral pterygoid showed an age related increase in the amount of type IIA fibres and fibreswith solely fast A MyHC, and fibre atrophy. The old digastric muscle showed changes in linewith the biceps brachii muscle, a decrease in the proportion of type IIB fibres.
Age-related alterations in the masseter and the lateral pterygoid muscles were opposite to thosereported for limb and trunk muscles, whereas changes in the anterior and posterior bellies of thedigastric muscle resembled those of limb and trunk muscles.
In conclusion, the present results showed that the three human jaw muscles not only haveunique fibre type and MyHC compositions but also undergo muscle and region specific changes infibre composition during aging, probably reflecting muscular differences in genetic programs andepigenetic influences. The similarities in fibre composition and phenotypic changes followingaging between the masseter and the lateral pterygoid muscles, and between the two bellies of thedigastric muscle suggest that although nerve supply and developmental origin are essential formuscle fibre composition, genetic control in relation to tasks and functional demands on muscleare of superior importance.