The ultrastructural anatomy of the nuclear envelope in the masseter muscle indicates its role in the metabolism of the intracellular Ca+.

Specific ultrastructural anatomy of masticatory muscles is commonly referred to a general pattern assigned to striated muscles. Junctional feet consisting of calcium channels of the sarcoplasmic reticulum (i.e. the ryanodine receptors, RyRs) physically connected to the calcium channels of the t-tubules build triads within striated muscles. Functional RyRs were demonstrated in the nuclear envelopes of pancreas and of a skeletal muscle derived cell line, but not in muscle in situ. It was hypothesized that ryanodine receptors (RyRs) could also exist in the nuclear envelope in the masseter muscle, thus aiming at studying this by transmission electron microscopy. There were identified paired and consistent subsarcolemmal clusters of mitochondria, appearing as outpockets of the muscle fibers, usually flanking an endomysial microvessel. It was observed on grazing longitudinal cuts that the I-band-limited mitochondria were not strictly located in a single intermyofibrillar space but continued transversally over the I-band to the next intermyofibrillar space. It appeared that the I-band-limited transverse mitochondria participate with the column-forming mitochondria in building a rather incomplete mitochondrial reticulum of the masseter muscle. Subsarcolemmal nuclei presented nuclear envelope-associated RyRs. Moreover, t-tubules were contacting the nuclear envelope and they were seemingly filled from the perinuclear space. This could suggest that nucleoplasmic calcium could contribute to balance the cytosolic concentration via pre-built anatomical routes: (i) indirectly, via the RyRs of the nuclear envelope and (ii) directly via the communication of t-tubules and sarcoplasmic reticulum through the perinuclear space.

Early molecular response and microanatomical changes in the masseter muscle and mandibular head after botulinum toxin intervention in adult mice.

BACKGROUND: Masseter muscle paralysis induced by botulinum toxin type A (BoNTA) evokes subchondral bone loss in mandibular heads of adult rats and growing mice after 4 weeks. However, the primary cellular and molecular events leading to altered bone remodeling remain unexplored. Thus, the aim of the current work has been to assess the molecular response that precedes the early microanatomical changes in the masseter muscle and subchondral bone of the mandibular head in adult mice after BoNTA intervention. METHODS: A pre-clinical in vivo study was performed by a single intramuscular injection of 0.2 U BoNTA in the right masseter (experimental) of adult BALB/c mice. The contralateral masseter was injected with vehicle (control). Changes in mRNA levels of molecular markers of bone loss or muscle atrophy/regeneration were addressed by qPCR at day 2 or 7, respectively. mRNA levels of receptor activator of nuclear factor-κB ligand (RANKL) was assessed in mandibular heads, whilst mRNA levels of Atrogin-1/MAFbx, MuRF-1 and Myogenin were addressed in masseter muscles. In order to identify the early microanatomical changes at day 14, fiber diameters in transversal sections of masseter muscles were quantified, and histomorphometric analysis was used to determine the bone per tissue area and the trabecular thickness of subchondral bone of the mandibular heads. RESULTS: An increase of up to 4-fold in RANKL mRNA levels were detected in mandibular heads of the BoNTA-injected sides as early as 2 days after intervention. Moreover, a 4-6 fold increase in Atrogin-1/MAFbx and MuRF-1 and an up to 25 fold increase in Myogenin mRNA level were detected in masseter muscles 7 days after BoNTA injections. Masseter muscle mass, as well as individual muscle fiber diameter, were significantly reduced in BoNTA-injected side after 14 days post-intervention. At the same time, in the mandibular heads from the treated side, the subchondral bone loss was evinced by a significant reduction in bone per tissue area (-40%) and trabecular thickness (-55%). CONCLUSIONS: Our results show that masseter muscle paralysis induced by BoNTA leads to significant microanatomical changes by day 14, preceded by molecular changes as early as 2 days in bone, and 7 days in muscle. Therefore, masseter muscle atrophy and subchondral bone loss detected at 14 days are preceded by molecular responses that occur during the first week after BoNTA intervention.

Ultrastructural changes in human masseter muscles after botulinum neurotoxin a injection.

INTRODUCTION: Botulinum neurotoxin A (BoNTA) has long been used as a therapeutic agent and has been widely accepted as a cosmetic agent in recent years. It can inhibit function and induce structural changes in skeletal muscle. METHODS: Specimens of fresh dissected human masseter muscle were used to observe the ultrastructural changes that occurred at 6 and 12 months following BoNTA injection. RESULTS: The findings observed were muscle fiber distortion, sarcomere shortening, mitochondrial vacuolar degeneration, glycogen accumulation, and H and M band disruption in the triad of tubules. At 12 months after injection, there was still evidence of degenerative changes in muscle ultrastructure, whereas most organelles exhibited a normal structure. DISCUSSION: Profound ultrastructural and organelle disfiguring changes were observed after BoNTA injection into human masseter muscle. Most changes were transient, however, and were resolved by 12 months after injection. Muscle Nerve 57: 96-99, 2018.

Ultrastructural analysis of masseter and left medial pterigoid muscles, after unilateral mandibular molar extraction: experimental study / Análisis ultrastructural de los músculos masetero y pterigoideo medial izquierdo, después de la extracción molar mandibular unilateral: estudio experimental

Twelve adult male Wistar rats (220 g average weight) were divided in 3 experimental groups: GI -15. GII ­ 30 and GIII ­ 60 days, after mandibular molar extraction, with three experimental animals and one control per group. Qualitatively, ultrastructural changes of protein filaments from myofibrils of these muscles and ipsilateral to the extractions were observed. Ultrastructure asymmetry and disorganization of Z line and I band, in the experimental group GII, of Medial Pterigoid muscle (MPT) were observed. The temporomandibular dysfunction, stimulated by the unilateral extractions of mandibular molars in rats may lead to modifications in the Z line and I band, which showed to be sensitive to this dysfunction. Changes in the MPT muscle, probably related to its own functional characteristics and major participation in the dynamics of mastication, compared to Masseter were also observed. However, the muscular fibres seem to adapt to the new conditions along the experiment.

Doce ratas Wistar machos adultos (220 g de peso promedio) se dividieron en 3 grupos experimentales: GI - 15, GII - 30 y GIII - 60 días, después de una extracción molar mandibular. En cada grupo se dispusieron tres animales experimentales y un animal como control. Cualitativamente, se observaron cambios ultraestructurales de filamentos de proteínas de miofibrillas de estos músculos masticadores ipsilaterales a las extracciones. Se observó asimetría de la ultraestructura y desorganización de la línea Z y la banda I, en el músculo pterigoideo medial del grupo experimental GII, (MPT). La disfunción temporomandibular, estimulada por las extracciones unilaterales de los molares mandibulares en ratas, puede conducir a modificaciones en la línea Z y en la banda I, que mostraron ser sensibles a esta disfunción. Los cambios en el músculo pterigoideo medial, se debieron, probablemente, con sus propias características funcionales y una mayor participación en la dinámica de la masticación, en comparación con el músculo masetero. Sin embargo, las fibras musculares parecen adaptarse a las nuevas condiciones a lo largo del experimento.

The effects of increasing occlusal vertical dimension on the deep masseter of rat at different ages.

OBJECTIVE: To investigate the influence of increasing the occlusal vertical dimension (iOVD) on the fibre-type distribution and ultrastructure of deep masseter of rat at different ages. DESIGN: A total of forty-eight male Wistar rats were divided into two groups according to age: 'teenage' group (n=24, 1.5 months) and 'young adult' group (n=24, 8 months). Both the teenage and the young adult rats were then randomly divided into the control group (n=12) and the experimental group (n=12). The occlusal vertical dimensions of the rats in the experimental groups were increased by placing composite resin on all maxillary molars. The fibre-type distribution and ultrastructure of the deep masseter were subsequently observed on day 7 and day 14 after iOVD. RESULTS: In the teenage experimental group, the proportion of type IIa fibres increased, while the proportion of type IIb and type IIx fibres decreased by day 7 after iOVD (P<0.05). However, no significant fibre phenotype transformation was observed in the young adult experimental group until day 14 after iOVD. In addition, the proportion of type IIa in the teenage experimental group was higher than that of the young adult experimental group on day 7 and 14 (P<0.05). Under the transmission electron microscope, muscle fibre reconstruction and the compensatory increase in the number and volume of mitochondria appeared earlier in the teenage experimental group. The cellular traumatic reaction was less than that in the young adult experimental group. CONCLUSION: The teenage rat alters masseter muscle structure to a slower phenotype earlier and to a greater degree than that of the young adult rat when increasing the occlusal vertical dimension.

Botulinum neurotoxin effects on masseter muscle fibre in WNIN obese rats-Scanning electron microscope analysis.

WNIN/Ob obese mutant rats are unique in comparison to similar rodent models of obesity established in the West. The present study is aimed to evaluate the masticatory function and histological changes in masseter muscle fibres treated with botulinum toxin type A (BoNT/A) in WNIN/Ob rats. Twelve WNIN/Ob obese rats and 12 lean rats at 35 days of age were taken and divided into four groups (6 rats in each group): Group-I (WNIN/Ob) and Group-II (lean) rats were injected with BoNT/A (1 unit) into right side of masseter muscle. For control left masseter of both phenotypes was injected with saline. Group-III (WNIN/Ob) and Group-IV (lean) rats were without any treatment. Growth and food intake was monitored daily for 45 days. Rats were euthanized and gross necropsy was carried out to check any abnormalities. Masseter muscles were dissected and mean muscle mass was recorded. Small portion of muscle was stored in 10% formalin for hematoxylin-eosin (H&E) staining and remaining tissue stored in gluteraldehyde for scanning electron microscopy (SEM). There is a significant decrease in the body weights and food intake of BoNT/A treated obese rats. The H&E staining of the masseter muscle in both groups showed normal morphology and orientation. The SEM analysis showed that, fibre size in BoNT/A treated masseter muscle of obese rats increased more than the saline treated side and in control rats. The increase in the muscle fibre size and transition of muscle fibre subtypes may be due to the reduced masticatory function of the masseter muscle. SCANNING 38:396-402, 2016. © 2015 Wiley Periodicals, Inc.

Characterization of Longissimus thoracis, Semitendinosus and Masseter muscles and relationships with technological quality in pigs. 1. Microscopic analysis of muscles.

Three porcine muscles (Longissimus thoracis, Semitendinosus, Masseter), known to have large differences in biochemical and histological traits, were fully characterized and the link between muscle structure and quality evaluated. The oxidative Masseter had more pigment, higher content of metmyoglobin, haem iron, protein and collagen, and was redder with higher fibre numbers, fibre circularity, pH and water holding capacity than the glycolytic Longissimus. Fibre type distribution showed predominance of type IIB in Longissimus and Semitendinosus white, type I in Semitendinosus red and IIA in Masseter. Type I fibres were larger than type IIB and IIA in Semitendinosus and Masseter, respectively, but not in the Longissimus, indicating that fibre size is muscle dependent. Muscle redness was positively correlated with type I fibre traits, haem iron and metmyoglobin, and negatively associated with type II fibre characteristics, non-haem iron and oxymyoglobin. Expressible juice had positive correlation with fibre size and negative with fibre number and connective tissue.

Zymographic and ultrastructural evaluations after low-level laser irradiation on masseter muscle of HRS/J strain mice.

Low-level laser therapy (LLLT) has been widely used in the treatment of the stomatognathic system dysfunction; however, its biological effect remains poorly understood. This study evaluated the effect of LLLT (GaAlAs, 780 nm, 20 J/cm(2), 40 mW) on masseter muscle of HRS/J mice after different numbers of laser irradiations (three, six, and ten) for 20 s in alternate days. Three experimental groups were defined according to the number of laser irradiations and three control groups (n=5) were used. On the third day after the last irradiation, all animals were killed and the masseter muscle was removed and processed for the following analysis: (a) transmission electron microscopy, (b) zymography, (c) immunohistochemistry for vascular endothelial growth factor (VEGF) and VEGFR-2. The results showed: (a) with six laser applications, a dilation of T tubules, and sarcoplasmic reticulum cistern, increased pinocytosed vesicles in the endothelium; with ten laser applications, few pinocytic vesicles in the endothelium and condensed mitochondria. (b) Under the conditions of this study, the synthesis of other matrix metalloproteinases was not observed, only the MMP-2 and -9. (c) After ten laser irradiations, immunostaining was observed only for VEGFR-2. We conclude that after six laser applications, ultrastructural changes may facilitate the Ca(+2) transfer to cytosol and increase the fluid transport from one surface to another. The ultrastructural changes and no immunostaining for VEGF with ten applications may decrease the metabolic activity as well as damage the angiogenic process, suggesting that an effective number of laser applications may be less than ten, associating to this therapy a better cost-benefit.

Influence of laser therapy and muscle relaxant on the masseter muscle under occlusal wear: an ultrastructural study / Influencia de la terapia laser y relajante muscular en el músculo masetero bajo oclusión: un estudio ultraestructural

The aim of this study was to analyze the influence of low-intensity laser therapy and muscle relaxant in the characteristic ultra structural masseter muscle occlusal wear. Animals and Methods: 40 male Wistar rats were randomly divided into four groups: the control group (GI), occlusal wear (G-II), laser occlusal wear (G-III), and the muscle relaxant occlusal wear (G-IV). Under general anesthesia given intraperitoneally, animals in groups II, III and IV had unilateral amputation of upper and lower molar cusps to simulate an occlusal wear situation. The masseter muscle G-III received laser therapy (830nm, 4J/cm2, 40mW, f ~ 2mm) and the procedure was subsequently repeated every other day for 14/30 days. G-IV animals were treated with daily injection of dantrolene ® (2.5 mg / kg in 0.5 ml of H2O). From 24 hours after the elimination peak. The animals were euthanized with an overdose of anesthesia on days 14 and 30 after the removal of the cusps and the ipsilateral masseter muscle was excised and divided in two, one half was routinely processed for light microscopy and other for electron microscopy. There was no statistical difference between each experimental group and the control and between periods in each experimental group. However, the muscle fibers in the G-II showed the most pronounced changes. There is no causal relationship between muscles fibers injuries and occlusion and, despite signs of muscular tissue injury were more evident in the occlusal wear group. Results indicates a moderate action of laser therapy and muscle relaxants in skeletal muscle...

El objetivo del estudio fue analizar la influencia de la terapia láser de baja intensidad y del relajante muscular sobre las características ultraestructurales del músculo masetero en el desgaste oclusal. 40 ratas macho Wistar, se dividieron al azar en cuatro grupos: grupo de control (GI), desgaste oclusal (G-II), laserterapia desgaste oclusal (G-III), y relajante muscular desgaste oclusal (G-IV). Bajo anestesia general por vía intraperitoneal, los animales de los grupos II, III y IV sufrieron amputación unilateral de las cúspides de los molares superiores e inferiores para simular una situación de desgaste oclusal. El músculo masetero del G-III recibió la terapia con laser (830nm, 4J/cm2, 40mW, f ~ 2mm) después del procedimiento el cual se repitió durante 14/30 días. Los animales del G-IV fueron tratados con una inyección diaria de Dantroleno® (2,5 mg/Kg en 0,5 ml de H2O). Los animales fueron sacrificados con una sobredosis de anestesia general en los días 14 y 30. Después de la remoción de las cúspides el músculo masetero ipsilateral se extirpó y se dividió en dos, una mitad fue procesada para microscopía de luz y la otra para microscopía electrónica. No hubo diferencias estadísticamente significativas entre cada grupo experimental y el control, así como, entre los períodos en cada grupo experimental. Sin embargo, las fibras musculares en el G-II mostraron los cambios más pronunciados. En conclusión no existe relación causal entre las lesiones de las fibras musculares y la oclusión, a pesar que los signos de lesión de los tejidos musculares fue más evidente en el grupo con desgaste oclusal. Los resultados indican una acción moderada ejercida por la terapia láser y de los relajantes musculares sobre el músculo esquelético...

Ultrastructural and proteomic alteration of superficial masseter muscle after lower jaw sagittal advancement in rat.

OBJECTIVE: The purpose of this study was to investigate the ultrastructural and proteomic alteration of the superficial masseter muscle (SM) after lower jaw sagittal advancement in a rat model. METHODS: Six 35-day-old male Sprague-Dawley rats were utilized in a transmission electron microscopy study, and six more in a proteomic study. The rats were randomly allocated to two experimental and two control groups (n=3). The experimental groups were fitted with fixed devices that protruded the mandible, whereas the control groups were not fitted with the devices. The rats were sacrificed 3 days after mandibular advancement. Transmission electron microscopy analysis, Western blot analysis, comparative proteomic analysis, and matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF) mass spectrometry (MS) were applied to profile qualitative and quantitative differences in the morphology and proteome of rat SM. RESULTS: Under the transmission electron microscope, morphological changes of the SM were associated with deregulation of the myofibrillar network accompanied by fibre rupture, interlaced myofilaments, and changes in the mitochondria and sarcoplasmic reticulum cisternae. The shape of mitochondria changed. Proteomic analysis identified fourteen differentially expressed proteins involved in metabolism, contraction or the cytoskeleton, and transport. Amongst these, the expression of 13 proteins increased and 1 decreased. According to Western blot analysis, myosin heavy chain II was down-regulated. CONCLUSION: Three days after functional mandibular advancement, the ultrastructure of rat SM had changed. This change paralleled changes in metabolic, contractile and cytoskeletal, and transport proteins, which affected SM physiology. The energy metabolism of SM increased, and the muscle velocity and force of contraction decreased.

Human masseter muscle fiber type properties, skeletal malocclusions, and muscle growth factor expression.

PURPOSE: We identified masseter muscle fiber type property differences in subjects with dentofacial deformities. PATIENTS AND METHODS: Samples of masseter muscle were collected from 139 young adults during mandibular osteotomy procedures to assess mean fiber areas and percent tissue occupancies for the 4 fiber types that comprise the muscle. Subjects were classified into 1 of 6 malocclusion groups based on the presence of a skeletal Class II or III sagittal dimension malocclusion and either a skeletal open, deep, or normal bite vertical dimension malocclusion. In a subpopulation, relative quantities of the muscle growth factors IGF-I and GDF-8 gene expression were quantified by real-time polymerase chain reaction. RESULTS: Fiber properties were not different in the sagittal malocclusion groups, but were very different in the vertical malocclusion groups (P ≤ .0004). There were significant mean fiber area differences for type II (P ≤ .0004) and type neonatal-atrial (P = .001) fiber types and for fiber percent occupancy differences for both type I-II hybrid fibers and type II fibers (P ≤ .0004). Growth factor expression differed by gender for IGF-I (P = .02) and GDF-8 (P < .01). The ratio of IGF-I:GDF-8 expression associates with type I and II mean fiber areas. CONCLUSION: Fiber type properties are very closely associated with variations in vertical growth of the face, with statistical significance for overall comparisons at P ≤ .0004. An increase in masseter muscle type II fiber mean fiber areas and percent tissue occupancies is inversely related to increases in vertical facial dimension.

Effects of low-level laser irradiation in ultrastructural morphology, and immunoexpression of VEGF and VEGFR-2 of rat masseter muscle.

The present study evaluates by ultrastructural and immunohistochemical methods, the possible changes on muscular tissue affected by LLLI during a treatment, for example, in cases of temporomandibular joint disorders. Sixty male Wistar rats divided into 6 groups (n=10) received ten laser irradiations, with different energy densities (groups I-0; II-0.5; III-1.0; IV-2.5; V-5.0; and VI-20 J/cm(2)). Muscles were removed and processed for transmission electron microscopic and immunohistochemical (VEGF and VEGFR-2) analyses. Captured photomicrographs of immunohistochemistry and transmission electron microscopy were evaluated. It was observed in the irradiated muscles, mitochondria of different shapes and sizes, with increased plasticity evidenced by organelles in fusion, division and the presence of elongated structures with characteristics of mitochondria, proximity with the dilated sarcoplasmatic reticulum, suggesting organelles with large amounts of energy, and the presence of cytoplasmic protrusions in the capillaries with high dosages. All studied groups showed immunostainings for both markers (VEGF and VEGFR-2), but in general those who received higher doses also showed the markings more pronounced, suggesting dose-dependent biomodulation. It was concluded that the LLLI was able to modify the ultrastructural characteristics and immunohistochemical pattern of VEGF and VEGFR-2 in the masseter muscle of rats.

Ring fibers visualized by electron microscopy in a Japanese patient with malignant hyperthermia.

This report describes an episode of malignant hyperthermia (MH) in a 53-year-old Japanese man during general anesthesia that was triggered by isoflurane and succinylcholine. His past history and family history were unremarkable. From our analysis of ten exons, he had no recognizable mutation in the ryanodine receptor gene, but compatible with his MH reaction, he showed positive sensitivity to the Ca-induced Ca-release test. Histochemical and electron microscopic studies of muscle biopsy tissue demonstrated unusual "ring fibers," which have never before been reported to be associated with MH. The presence of ring fibers in this patient might indicate muscle regeneration, suggesting a recovery process from the MH episode.

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.

[A preliminary study on the effects of the exogenous creatine phosphate on rat masseter muscle after unilateral chew].

OBJECTIVE: To study the effect of energy therapy on Ca2+ concentration and Ca2+ -ATP enzyme activity in rat master muscle after unilateral chew, and to discuss the protective action of the exogenous creatine phosphate on rat masseter muscle after unilateral chew. METHODS: The 20 rats were randomly divided into 4 groups, A: Creatine phosphate normal control group; B: Creatine phosphate experimental group; C: Saline normal control group; D: Saline experimental group. The Ca2+ concentration were determined by atomic absorption spectrophotometry, the activity of the Ca2+ -ATP enzyme were determined by super-micro volume Ca2+ -ATP enzyme kit. RESULTS: (1) The Ca2+ concentration of the extraction side of group D which received the saline injection had significant difference compared with the non-extraction side (P = 0.007), the group C (P = 0.009) and the extraction side of group B (P = 0.01); (2) Ca2+ -ATP enzyme activity of group D were higher than its non-extraction side (P = 0.001), group C (P = 0.003) and the extraction side of group B (P = 0.001); (3) The ultrastructural changes of the rat masseter muscle under transmission electron microscope were as follows: The extraction side of group D have more severe pathological manifestations than non-extraction side. Both the extraction side and the non-extraction side of group B had a similar manifestation to the normal control group. CONCLUSION: Exogenous energy material, creatine phosphate, may have certain degree of protective effect on rat masseter muscles after unilateral chew. And it may become a possible way to improve the injury of the masseter muscle.

Two dogs with juvenile-onset skin diseases with involvement of extremities.

Two dogs of juvenile-onset skin diseases with involvement of extremities were examined by histopathological, immunohistochemical and ultrastructural analyses. Clinically, both cases showed alopecia and crusts on the face and extremities. Case 1 showed histopathology of dermo-epidermal separation. Ultrastructural analysis revealed that the clefts were recognized between hemidesmosomes and lamina densa. In case 2, histopathology showed follicular atrophy, vacuolar degeneration at lower epidermis and masseter muscle degeneration, without remarkable ultrastructural abnormalities in basement membrane zone of the skin. Thus, the findings in case 1 were compatible to those in junctional epidermolysis bullosa, while those in case 2 were compatible to dermatomyositis-like disease. Combination of histopathological and ultrastructural analyses was useful to distinguish the diseases in two dogs.

Effects of increased occlusal vertical dimension on daily activity and myosin heavy chain composition in rat jaw muscle.

Mammalian skeletal muscles change their contractile-protein phenotype in response to mechanical loading and/or chronic electrical stimulation, implying that the phenotypic changes in masticatory muscles might result from new masticatory-loading conditions. To analyze the effects of increased occlusal vertical dimension (OVD) on daily activities and fibre-type compositions in jaw muscles, we measured the total duration of daily activity (duty time) and the myosin heavy chain (MyHC) compositions in the masseter and digastric muscles of freely moving control and bite-opened rats. In the control state, the duty time of the digastric muscle was higher than that of the masseter muscle at activity levels exceeding 5 and 20% of the day's peak activity. The opposite was true at activity levels exceeding 50 and 80% of the day's peak activity. The MyHCs consisted of a mixture of fast and slow types in the digastric muscle. The masseter consisted of mostly fast-type MyHC. The increment of OVD increased not only the duty time at activity levels exceeding 5, 20, 50 and 80% of the day' peak activity in both muscles but also the proportion of MyHC IIa in the masseter muscle and MyHC I in the digastric muscle at the expense of that of MyHC IIb. These results suggest that the increment of OVD changes masseter and digastric muscles towards slower phenotypes by an increase in their daily activities.

Masseter muscle inserted in the temporomandibular joint discocapsular complex, a case report

The presence of muscle insertions in the temporomandibular joint disc have a great importance in the dynamic joint. This article presents a case of bilateral insertion of deep fascicle of the masseter muscle in the temporomandibular joint capsule and disc in a spain corpse, describes the microscopic and macroscopic appearance of variation and a brief review of the functional implications.

La presencia de inserciones musculares en el disco de la articulación temporomandibular tiene gran importancia en la dinámica de la articulación. En este artículo se presenta un caso de la inserción bilateral del fascículo profundo del músculo masétero en la cápsula y disco de la articulación temporomandibular en un cadáver español, se describe el aspecto macroscópico y microscópico de la variación y se realiza una breve revisión de las implicancias funcionales.

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.

Three-dimensional observation of connective tissue of bovine masseter muscle under concentrate- and roughage-fed conditions by using immunohistochemical/confocal laser-scanning microscopic methods.

We investigated changes in connective tissue components of masseter (MA) muscle in Japanese black heifers (n= 6) in concentrate- and roughage-fed groups (groups C and R, respectively). Body weight, at slaughter, of experimental heifers in group C (272.3 +/- 22.3 kg) was higher (P < 0.05) than that of group R (213.8 +/- 27.5 kg). However, muscle weight and myofiber diameter (superficial and deep layers) of MA muscle did not differ between groups C and R. In contrast, total mastication duration of group R was longer (P < 0.05) than that of group C. MA muscle of groups C and R was composed only of type I myofiber. Using immunohistochemical/confocal laser-scanning microscopy, type I collagen was observed mainly in perimysium, and type V and VI collagen were observed in perimysium and endomysium of both groups. Type IV collagen and laminin were observed only in the endomysium in both groups. However, type III collagen and fibronectin were strongly apparent in the perimysium and endomysium in group R. Connective tissue components in the perimysium of groups C and R were observed to form plate-shaped layers. On the other hand, honeycomb-shaped connective tissue components were seen in the endomysium-surrounded muscle fibers. In particular, fibronectin was strongly observed in the perimysium and endomysium in group R. These results indicate that there are different developmental changes among connective tissue components in MA muscle in response to mastication. The immunohistochemical/confocal laser-scanning microscopic method is useful to investigate the structural relationship among connective tissue components in skeletal muscle.