The masticatory system under varying functional load. Part 1: Structural adaptation of rabbit jaw muscles to reduced masticatory load.

Skeletal muscle fibres can change their myosin heavy-chain (MyHC) isoform and cross-sectional area, which determine their contraction velocity and maximum force generation, respectively, to adapt to varying functional loads. In general, reduced muscle activity induces transition towards faster fibres and a decrease in fibre cross-sectional area. In order to investigate the effect of a reduction in masticatory load on three functionally different jaw muscles, the MyHC composition and the corresponding cross-sectional area of fibres were determined in the superficial masseter, superficial temporalis, and digastric muscles of male juvenile New Zealand White rabbits that had been raised on a soft diet (n=8) from 8 to 20 weeks of age and in those of normal diet controls (n=8). Differences between groups were tested for statistical significance using a Mann-Whitney rank sum test. The proportion and cross-sectional area of fibres co-expressing MyHC-I and MyHC-cardiac alpha were significantly smaller in the masseter muscles of the animals that had been fed soft food than in those of the controls. In contrast, the proportions and cross-sectional areas of the various fibre types in the temporalis and digastric muscles did not differ significantly between the groups. The results suggest that reducing the masticatory load during development affects the contraction velocity and maximum force generation of the jaw-closing muscles that are primarily responsible for force generation during chewing. These muscles adapt structurally to the reduced functional load with changes in the MyHC composition and cross-sectional area mainly within their slow fibre compartment.

Relationship between function of masticatory muscle in mouse and properties of muscle fibers.

Mammals exhibit marked morphological differences in the muscles surrounding the jaw bone due to differences in eating habits. Furthermore, the myofiber properties of the muscles differ with function. Since the muscles in the oral region have various functions such as eating, swallowing, and speech, it is believed that the functional role of each muscle differs. Therefore, to clarify the functional role of each masticatory muscle, the myofiber properties of the adult mouse masticatory muscles were investigated at the transcriptional level. Expression of MyHC-2b with a fast contraction rate and strong force was frequently noted in the temporal and masseter muscles. This suggests that the temporal and masseter muscles are closely involved in rapid antero-posterior masticatory movement, which is characteristic in mice. Furthermore, expression of MyHC-1 with a low contraction rate and weak continuous force was frequently detected in the lateral pterygoid muscle. This suggests that, in contrast to other masticatory muscles, mouse lateral pterygoid muscle is not involved in fast masticatory movement, but is involved in functions requiring continuous force such as retention of jaw position. This study revealed that muscles with different roles function comprehensively during complicated masticatory movement.

Craniofacial morphology and growth in the ferret: effects from alteration of masticatory function.

INTRODUCTION: Our hypothesis is that any effects on craniofacial growth and morphology induced by altering the masticatory function will vary according to the species involved. The general aim of this thesis was to test the possible effects of altering the masticatory function on craniofacial growth and morphology, as well as features of masticatory muscles in ferrets. To better elucidate these mechanisms involved, the anatomy of the skull, craniofacial growth pattern and development of occlusion in the ferret were investigated to serve as baseline data. MATERIALS AND METHODS: The description of the anatomy and the radiographic features of the skull was based on observations of 100 skulls of adult ferrets of both sexes. The craniofacial growth pattern was studied in 16 young ferrets of both sexes using serial cephalograms taken on 5 occasions according to age: 25 days, 35 days, 55 days, 80 days and 300 days. To follow the development of occlusion, these animals underwent dental examinations every other day. Forty male ferrets were divided into two groups from the age of 5 weeks. One group was fed hard pellets (hard-diet group), and the other the same diet but softened with water (soft-diet group). After six months, specimens of masticatory muscles were dissected and analysed histochemically, and cephalograms were analysed cephalometrically. RESULTS: The ferret skull is relatively elongated with a short facial region, being longer and broader in males than in females. The viscero- and neurocranium follow an orderly pattern of expansive growth. The growth of the mandible is mainly characterized by an anteroposterior body elongation, and enlargement of the coronoid process. Craniofacial growth in ferrets ceases earlier in female than in male animals. The deciduous teeth erupted between the 19th and 31st postnatal day, and exfoliated approximately between days 51 and 76. The time of eruption of the permanent teeth ranged from 42 to 77 days. The female ferrets were generally ahead of the males regarding the eruption age of permanent teeth. The alteration of the masticatory function by feeding the animals a soft diet caused a hard palate plane more distant from the cranial base plane, smaller inter-frontal and inter-parietal widths, and a slenderer zygomatic arch, as well as a generally shorter and narrower coronoid process. Variations of most variables are greater in animals fed on soft diet. The mean cross-sectional area of type I and type II fibres in the temporalis and the masseter, and type II fibres in the digastricus, as well as the number of capillaries per fibre were significantly smaller in the soft-diet animals. CONCLUSION: Altered masticatory function has induced changes in masticatory muscles and certain effects on craniofacial growth in ferrets. These changes are not the same as those reported in other species (e.g. rats). It appears that reduced masticatory function leads to smaller fibres in the elevators and transverse skull dimensions. The different effects are presumably related to the differences in craniofacial anatomy and growth pattern, as well as the masticatory system, in the various species studied previously.

Myosin heavy-chain isoform composition of human single jaw-muscle fibers.

Diversity in muscle contractile properties is based on the variability of contractile properties of single muscle fibers which in turn is related to the presence of different myosin heavy-chain (MyHC) isoforms. Human jaw muscles are featured by many hybrid fibers expressing more than one MyHC isoform. The purpose of this study was to determine the proportion of each isoform within these fibers for evaluation of the fiber's capacity of producing a large diversity in contractile properties. Electrophoretic separation of MyHC isoforms was performed on 218 single fibers of the temporalis and digastric muscles. Of these fibers, 100 were classified as hybrid fibers. Most hybrid fibers co-expressed MyHC-IIA and -IIX (n = 62); a smaller number co-expressed MyHC-I and -IIA (n = 14), MyHC-I and -IIX (n = 12), and MyHC-I, -IIA, and -IIX (n = 12). The proportions of the individual MyHC isoforms in the hybrid fibers varied highly, suggesting a large range of contractile properties among these fibers.

Effects of repeated + Gz forces on masticatory muscles.

OBJECTIVE: To study the effects of repeated + Gz forces on masticatory muscles. METHODS: 48 male Wistar rats were randomly divided into 4 groups. Group A was normally fed. Group B was only fixed with rat-kept devices for 5 minutes. Group C was borne + 1 Gz for 5 minutes. Group D was repeatedly exposed + 10 Gz (each for 30 s, onset rate about 0.5 G/s, 5 times/d with + 1 Gz 1 minute intervals, 4 d/wk, 3 weeks in total). The histological changes of the masseter, temporal and lateral pterygoid muscles were observed. RESULTS: No abnormal changes were observed in Group A, B and C. But pathological changes could be found in group D. The wrench and deformation of muscular fibers, the dissolution of partial myofibril, the swelling of mitochondria, the reduce of hepatin from the masseter and lateral pterygoid muscles could be found. CONCLUSIONS: Repeated + Gz stresses could induce the damage of masticatory muscles in different degrees.

[Histological and ultrastructural characteristics of jaw-closing muscles. A review]. / Caratteristiche istologiche ed ultrastrutturali dei muscoli elevatori mandibolari. Revisione della letteratura.

The aim of this work is giving, through a wide literature review, a detailed analysis of the histological and ultrastructural characteristics that distinguish masseter and temporal muscles from the other skeletal muscles. Furthermore we'll explain the functional meanings of these differences. We developed the following points: fibre type composition and relative frequency of the various fibre types, fibre size, myosin composition, capillarization and age-related changes. With standard staining method for the myofibrillar ATPase, besides the two main fibre types, I and II, in the masticatory muscles a moderate share of IM fibres with intermediate stainability, which usually don't appear in adult skeletal muscles, are shown. The relative frequency of the various fibre types is also peculiar, with a prevalence of type I fibres in almost every portion of the masseter and temporal muscles, which therefore are functionally slow muscles. Another unusual characteristic is also the mean diameter of type I fibres, that are commonly larger than type II fibres. This finding suggests that masticatory muscles are adapted to carry out specially prolonged and fatiguing tasks. The findings about contractile protein patterns and the changes in myosin heavy chain composition during ageing are also relevant. The deep differences between jaw-closing and limb and trunk muscles are reviewed on the basis of their special functional activities.

The blood supply of the human temporalis muscle: a vascular corrosion cast study.

Knowledge as to the blood supply of the human temporalis muscle is limited to its extramuscular path and relations, little information existing about the intramuscular vascular architecture. To investigate the 3-dimensional vascular network in the human temporalis muscle, in 5 fresh cadavers an infusion of methylmethacrylate resin was made via the carotid vessels with subsequent removal of the organic tissues by a corrosion process. The vascular corrosion casts of the temporalis muscle were studied by stereomicroscopy and scanning electron microscopy. In 6 well perfused muscle specimens, the temporalis muscle was found to be consistently supplied by 3 arteries: the anterior and posterior deep temporal arteries, and the middle temporal artery. Each primary artery branched into the secondary arterioles and then terminal arterioles. The venous network accompanied the arteries, and double veins pairing a single artery was a common finding. Arteriovenous anastomosis was absent, whereas arterioarterial and venovenous anastomoses were common. The capillaries formed a dense interlacing network with an orientation along the muscle fibres. Understanding of the intramuscular angioarchitecture of the temporalis provides the vascular basis for surgical flap manipulation and splitting design.

Venous hemangioma of the temporalis muscle.

A 14-year-old girl presented with a diffuse venous hemangioma of the right temporalis muscle. The muscle had become swollen, thinning the underlying zygomatic and temporal bones. Magnetic resonance (MR) imaging demonstrated a diffuse isointense area containing high intensity foci on the T1-weighted images, and a serpiginous high intensity pattern on the T2-weighted images. A biopsy specimen revealed irregularly dilated veins in the fibrous stroma of the muscle. Diffuse abnormal signals on MR images may be a pathognomonic feature of intramuscular venous hemangiomas.

Ultrastructural finding of vascular degeneration in fibrodysplasia ossificans progressiva (FOP).

The masseter and temporalis muscles were investigated ultrastructurally in 2 patients having fibrodysplasia ossificans progressiva (FOP). Results showed atrophy and necrosis of muscle fibres. Both cases contained cartilage which was calcified in the second case and was associated with degenerating chondrocytes. The blood vessels showed various changes, with haematoma formation and large fibrin deposits. Some of the fibrin deposits seen in the second case were surrounded with cartilage. This suggests that the cartilage probably forms as a reaction to an old haemorrhage. It is possible that the effect of the mutant gene found in FOP is on blood vessels rather than the normal regulatory mechanisms of the inducible osteogenic precursor cells. These degenerating blood vessels often result in haemorrhage and fibrin deposits; the fibrosis, cartilage or bone formation are probably secondary changes. Amianthoid fibres found in the tendon of the temporalis muscles of the second case, together with the other changes of fibrosis and cartilage formation, account for the severe restricted mouth opening suffered by these patients.

[Anatomical observations on the attachment of human temporalis and masseter muscles].

It is very important for orthodontics to investigate the bone remodeling pattern in the area of the attachment of the masticatory muscles induced by the changing muscular function. In the present study, the insertion pattern of Sharpey's fibers and the bone structure are comparatively studied on the temporalis and the masseter muscles macroscopically, light microscopically and electron microscopically, respectively. The following results are obtained: 1) The periosteum on the squamous part of the temporal bone is thinner, while the one on the lower ridge of the zygomatic arch is relatively thicker. 2) The fine and loose Sharpey's fibers are found mostly on the squamous part of the temporal bone, while the ones on the lower ridge of the zygomatic arch are relatively coarse and dense. 3) Comparing with the squamous part of the temporal bone, the fibrous bone is thinner and also few reversal lines and resting lines are observed at the lower ridge of the zygomatic arch. These findings seem to prove the fact that the bone remodeling proceeds frequently within the limited narrow area. These differences are directly due to the differences of the anatomical site and the muscular function.

Degenerative changes in masseter and temporalis muscles in limited mouth opening and TMJ ankylosis.

The masseter and temporalis muscles were studied ultrastructurally in eight cases suffering from restricted mouth opening and TMJ ankylosis. The changes varied from slight degeneration of muscle fibers, such as accumulation of lipid or lipofuscin deposits, Z-band streaming and derangement of myofilaments, in one case to moderate or severe changes of muscle atrophy and necrosis in the remaining seven cases. Nemaline bodies were also observed in one of these seven cases. In addition to these muscle changes neural degeneration especially of myelinated nerves were found in three of the cases studied. The present findings showed that in spite of the diversity of the underlying factors affecting the joint, there was many similarities in the changes observed within the muscle. It is postulated that these degenerative muscle changes probably play an important role in restricted mouth opening and TMJ ankylosis, and that some of these muscle pathology may be the result of neural involvement.

Incidence of centrally positioned nuclei in mouse masticatory muscle fibers.

Cross-sections of normal digastric, temporalis and masseter muscles from 7- and 30-week-old mice were studied for centrally positioned nuclei. Such nuclei were inhomogeneously distributed throughout each muscle and varied markedly between specimens. The incidence of centrally positioned nuclei in the digastric muscle (mean +/- SD: 0.029 +/- 0.015, n = 25) was significantly higher (p less than 0.001) than that in the temporalis (mean +/- SD: 0.011 +/- 0.010, n = 25) and masseter muscles (mean +/- SD: 0.005 +/- 0.007, n = 9), but did not differ between the two latter muscles (p = 0.41). Furthermore, the frequency in a given muscle was apparently age-independent. A connection between fiber type and centrally positioned nuclei is suggested.

Amianthoid fibres in muscle tissue associated with temporomandibular joint ankylosis.

The masseter and temporalis muscles were studied ultrastructurally in 3 patients having extra and intra-articular ankylosis of the TMJ. Amianthoid fibres (thick collagen fibres) varying from 280-430 nm in diameter were found in the connective tissue of both muscles. These fibres appeared to form by fusion of collagen fibrils of normal thickness rather than representing an active dynamic process. The presence of amianthoid fibres in these muscles may account for the restricted mouth opening associated with TMJ ankylosis and could be of some value in future management of this condition.