Advers changes in fibre type and myosin heavy chain compositions of human jaw muscle vs. limb muscle during ageing
M . M O N E M I ,1F . K A D I ,2 , 3J.-X. L I U ,2 , 3L.-E. T H O R N E L L2 , 3and P.-O. E R I K S S O N1 , 3
- Departments of Odontology, Clinical Oral Physiology, Umea University, Sweden
- Department of Integrative Medical Biology, Anatomy, Umea University, Sweden
- Department of Musculoskeletal Research, National Institute for Working Life, Umea, Sweden
This review shows that human jaw muscles not only have unique fibre type and myosin heavy chain (MyHC) compositions but also undergo muscle and region-specific changes in fibre composition during ageing. Alterations in the masseter and the lateral pterygoid muscles in the elderly are opposite to those reported for limb and trunk muscles, whereas changes in the anterior and posterior bellies of the digastric muscle resemble those of limb and trunk muscles. We conclude that age-related alterations in fibre type composition and MyHC expression are muscle and region specific, probably reflecting muscular differences in genetic programs and epigenetic influences.
Successful performance of everyday oral activities suchas biting, chewing, swallowing and oro-facial commu-nication requires bilateral involvement of an ensembleof jaw and facial muscles. The muscle fibre compositionof these muscles has been investigated in young adultsand the elderly (cf. Ringqvist 1974, Eriksson et al. 1981,1982, Eriksson 1982, Eriksson & Thornell 1983,Thornell et al. 1984, ButlerBrowne et al. 1988, Sta˚l et al.1994, Monemi et al. 1996, 1998, 1999a, b, c). This review compares the fibre type and MyHC compositions of themasseter, the lateral pterygoid and the digastric muscles (Fig. 1) in young adults and elderly, and for comparisonthe biceps brachii of the same subjects, with specialemphasis on muscular changes during ageing.
Figure 1 Schematic illustration of (a) the masseter (b) the lateral pterygoid and (c) the digastric muscle. Regions from which muscle specimens have been obtained are labelled; the anterior (1) and posterior (2) regions of the superficial portion and the deep (3) portion of the masseter, the superior (4) and inferior (5) portions of the lateral pterygoid and the anterior (6) and posterior (7) bellies of the digastric muscle.
These jaw muscles are structurally and functionally different (Møller 1966, Schumacher 1985, McDevitt 1989).The masseter is divided into a superficial and a deep portion and has a multipennate arrangement of relatively short muscle fibres. The superficial portion is active in force production whereas the deep portion with vertically oriented muscle fibres, and a high density of large and complex muscle spindles (Eriksson & Thornell 1987, Eriksson et al. 1994a, b), is probably more involved in the postural controlof the mandible (Eriksson et al.1984, Hannam & McMillan 1994). The superior and inferior portions of the lateral pterygoid muscle have a near-parallel relationship between their relatively long fibres. They are active in depression, elevation and lateral movements of the mandible, and concerned with stabilizing the temporomandibular joint during jaw function. The digastric muscle is formed by an anterior and a posterior belly linked by an intermediate tendon and is active in depression and horizontal positioning of the mandible. The masseter, the lateral pterygoid and the anterior portion of the digastric originate from the first branchial arch and are supplied by the trigeminal nerve, whereas the posterior belly of the digastric comes from the second branchial arch and is supplied by the facial nerve (Noden 1983).
Fibre type composition
Enzyme-histochemical investigations in young adults have shown that human jaw muscles have unique fibre type compositions different from those of limb and trunk muscles (Fig 2).
Figure 2 Seria muscle cross sections from young adults (a,c,e,g) and elderly (b,d,f,h) subjects, stained for mATPase at pH 10.3. The anterior region of the superficial portion of the masseter (a,b), the superior portion of the lateral pterygoid (c,d), the anterior belly of the digastric (e,f) and the biceps brachaii (g,h). Type I, lightly, and type II darkly stained. The type IM fibers in the masseter and the lateral pterygoid muscles are intermediately stained . Magnification x160.
The masseter contains a predominance of type I fibres, fibres with an interme-diate ATPase staining at alkaline pH, type IM, and small-sized type IIB fibres. In this muscle, regional differences in fibre type proportion and size are typical. The lateral pterygoid is mainly composed of type I and type IM fibres and in addition small-sized type IIB fibres. The digastric contains type I, type IIA and type IIB fibres of about equal proportions and size. Compared with young masseter, the masseter of elderly contained a significantly lower proportion of type I fibres. Type IIA and type IIAB fibres, which are absent or rare in young adults, were generally present in the old masseter. The masseter of the elderly con- tained a significantly larger proportion of type II (IIA + IIAB + IIB) fibres in its superficial portion, whereas no statistically significant difference between young and elderly muscles was found in its deep portion. The lateral pterygoid of the elderly contained a significant proportion of type IIA fibres, which are rare or absent in young adults. The fibre diameter (all fibre types included) was significantly smaller in the superficial portion of the old masseter and in the inferior region of the old lateral pterygoid, as compared with young adults. For separate fibre types, a significantly smaller diameter in the elderly was found only for the type IM and the type IIB fibres in the inferior portion of the lateral pterygoid. The digastrics and the biceps brachii muscles contained significantly lower proportions of the type IIB fibres in the elderly than in young adults. For the two muscles, the mean fibre diameter of each fibre type was smaller in the elderly, although the differences were not statistically significant.
Myosin heavy chain composition
Immunocytochemical analyses have shown that the young adult masseter and lateral pterygoid muscles contain, in addition to slow and fast MyHC isoforms, fetal, embryonic and alpha-cardiac-like MyHC iso- forms (Figs 4 and 5). These isoforms are not normally expressed in adult limb muscles. Based on the MyHC composition in individual fibres, nine MyHC-based fibre types have been distinguished in the young masseter and seven and five in the young lateral pterygoid and digastric muscles, respectively. In young adults, fibres expressing two to four MyHC isoforms (hybrid fibres) were found in 23% in the masseter and in 64% in the lateral pterygoid muscle. Hybrid fibres in the digastric and biceps muscles contained only two MyHCs. The old masseter comprised a larger proportion of hybrid fibres than the young masseter (39 vs. 23%), and contained 16 MyHC-based fibre types. A characteristic feature of the old lateral pterygoid was the remarkable increase of fibres with solely fast A MyHC (13% in elderly and scarce in young adults). No significant difference in the MyHC composition of muscle fibres between elderly and young adults was found in the digastric and the biceps brachii.
We conclude that the muscle fibre type composition and changes of these during aging are muscle and region specifc, probably reflecting muscular differences in genetic programs and epigenetic influences.
