Developmental and functional considerations of masseter muscle partitioning

Effects of age and diet consistency on the expression of myosin heavy-chain isoforms on jaw-closing and jaw-opening muscles in a rat model

BACKGROUND

Skeletal craniofacial morphology can be influenced by changes in masticatory muscle function, which may also change the functional profile of the muscles.

OBJECTIVES

To investigate the effects of age and functional demands on the expression of Myosin Heavy-Chain (MyHC) isoforms in representative jaw-closing and jaw-opening muscles, namely the masseter and digastric muscles respectively.

METHODS

Eighty-four male Wistar rats were divided into four age groups, namely an immature (n = 12; 4-week-old), early adult (n = 24; 16-week-old), adult (n = 24; 26-week-old) and mature adult (n = 24; 38-week-old) group. The three adult groups were divided into two subgroups each based on diet consistency; a control group fed a standard (hard) diet, and an experimental group fed a soft diet. Rats were sacrificed, and masseter and digastric muscles dissected. Real-time quantitative polymerase chain reaction was used to compare the mRNA transcripts of the MyHC isoforms-Myh7 (MyHC-I), Myh2 (MyHC-IIa), Myh4 (MyHC-IIb) and Myh1 (MyHC-IIx)-of deep masseter and digastric muscles.

RESULTS

In the masseter muscle, hypofunction increases Myh1 (26, 38 weeks; p < .0001) but decreases Myh4 (26 weeks; p = .046) and Myh2 (26 weeks; p < .0001) expression in adult rats. In the digastric muscle, hypofunction increases Myh1 expression in the mature adult rats (38 weeks; p < .0001), while Myh2 expression decreases in adult rats (26 weeks; p = .021) as does Myh4 (26 weeks; p = .001). Myh7 expression is increased in the digastric muscle of mature adult rats subjected to hypofunction (38 weeks; p = <.0001), while it is very weakly expressed in the masseter.

CONCLUSION

In jaw-opening and jaw-closing muscles, differences in myosin expression between hard- and soft-diet-fed rats become evident in adulthood, suggesting that long-term alteration of jaw function is associated with changes in the expression of MyHC isoforms and potential fibre remodelling. This may give insight into the role of function on masticatory muscles and the resultant craniofacial morphology.

Sex as a Biological Variable in Oral Diseases: Evidence and Future Prospects

The interest of the scientific community on sex and gender differences in health and disease has increased substantially over the past 25 to 30 y as a result of a long process of events and policies in the biomedical field. This is crucial as compelling evidence from human and animal model studies has demonstrated that sex and gender influence health, molecular and cellular processes, and response and predisposition to disease. The present scoping review aims to provide a synthesis of sex differences in oral diseases, ranging from periodontal disease to orofacial pain conditions, from risk of caries development to apical periodontitis. Overall, findings from this review further support a role for sexual dimorphism influencing disease predisposition and/or progression in oral diseases. Of note, this review also highlights the lack of consideration of additional factors such as gender and other psychosocial and external factors potentially influencing oral health and disease. New conceptual frameworks capable of capturing multiple fundamental domains and measurements should be developed in clinical and preclinical studies to inform sex-based individualized preventive and treatment strategies.

Myosin Heavy-Chain Messenger Ribonucleic Acid (mRNA) Expression and Fibre Cross-Sectional Area in Masseter, Digastric, Gastrocnemius and Soleus Muscles of Young and Adult Rats

Different demands on the muscles of mastication may influence their functional profile (size and distribution of muscle fibre types), which may change during growth and maturation, potentially influencing craniofacial growth. The aim of this study was to evaluate mRNA expression and cross-sectional area of masticatory muscle fibres compared with limb muscles in young and adult rats. Twenty-four rats were sacrificed at two different ages, namely 12 at 4 weeks (young) and 12 at 26 weeks (adult). The masseter, digastric, gastrocnemius and soleus muscles were dissected. Gene expression of myosin heavy-chain isoforms Myh7 (MyHC-I), Myh2 (MyHC-IIa), Myh4 (MyHC-IIb) and Myh1 (MyHC-IIx) in the muscles was measured using qRT-PCR RNA analysis, and immunofluorescence staining was performed to measure the cross-sectional area of different muscle fibre types. Different muscle types and ages were compared. Significant differences were found in the functional profile between masticatory and limb muscles. For the masticatory muscles, there was an increase in Myh4 expression with age, and this change was more intense for the masseter muscles, which also presented an increase in Myh1 expression, similarly to limb muscles. The fibre cross-sectional area of the masticatory muscles was generally smaller in young rats; however, this difference was less pronounced than in limb muscles.

Assessments of Muscle Thickness and Tonicity of the Masseter and Sternocleidomastoid Muscles and Maximum Mouth Opening in Patients with Temporomandibular Disorder

The purpose of this study was to compare the muscle thickness and tone of the masseter and sternocleidomastoid (SCM) muscles and maximum mouth opening (MMO) in subjects with and without temporomandibular disorders (TMD), and perform a correlation comparison on the results of the TMD group. Sixty patients were allocated to the TMD group (n = 30) or the non-TMD group (n = 30). Ultrasound imaging, myotonometer, and vernier calipers were used to determine the related changes in muscle thickness and muscle tone in masseter and SCM, and MMO, respectively. The TMD group revealed a significant decrease than the non-TMD group in the muscle thickness of masseter and SCM, respectively (p < 0.001), with a significant increase in frequency (p < 0.001) and stiffness (p < 0.001) in the masseter muscle tone, with a significant increase in frequency (p < 0.001) and stiffness (p = 0.005) in the SCM muscle tone, a significant decrease in the MMO (p < 0.001). There was a moderate negative correlation between the relaxed state of masseter muscle thickness and stiffness of SCM muscle tone (r = −0.40, p = 0.002), and a moderate negative correlation between the relaxed state of SCM muscle thickness and frequency of SCM muscle tone (r = −0.42, p = 0.001). There was a moderate negative correlation between the clenching state of SCM muscle thickness and the frequency of SCM muscle tone (r = −0.47, p < 0.001). In addition, a moderate negative correlation between MMO and frequency of SCM muscle tone (r = −0.44, p < 0.001). The muscle thickness was decreased, and the muscle tone was increased in the masseter and SCM muscle, respectively. Additionally, MMO was decreased in patients with TMD compared with non-TMD.

Satellite cell capacity for functional adaptation of masseter muscle in Class II and Class III patients after orthognathic surgery-a pilot study

AIM

The aim of the prospective pilot study was to analyze the biomarkers CD34, Pax7, Myf5, and MyoD for stimulation of satellite cells (SCs), which are responsible for functional adaptation.

SUBJECTS AND METHODS

Forty-five Caucasian patients were consecutively recruited from the Maxillo-Facial-Surgery at TU Dresden. Eleven orthognathic Class III patients, 24 Class II patients, and 10 controls with Class I were involved in the study. Tissue samples from masseter muscle were taken from the patients pre-surgically (T1) and 7 months later (T2). Samples from controls were taken during the extraction of third molars in the mandible. Polymerase chain reaction (PCR) for relative quantification of gene expression was calculated with the delta delta cycle threshold (ΔΔCT) method.

RESULTS

The results show significant differences for the marker of SC stimulation between the controls, the patient groups, males, and females. The gene expression of CD34 was post-surgically upregulated for Class III (0.35-0.77, standard deviation [SD] = 0.39, P < 0.05) in comparison with controls. For Pax7, there was a significant difference shown between the retrognathic and the prognathic group because of downregulation in Class II patients (1.64-0.76, SD = 0.55, P < 0.05). In Class III patients, there was a significant upregulation for Myf5 (0.56-1.05, SD = 0.52, P < 0.05) after surgery too.

CONCLUSIONS

The significant decline of Pax7 in Class II patients indicates a deficiency of stimulated SC post-surgically. The expression of CD34 and Myf5 in Class II stayed unchanged. In contrast, there was an upregulation for all Class III patients, mainly in females, shown post-surgically. This may be one reason for weak functional adaptation and relapse in Class II patients.

Kinect-driven Patient-specific Head, Skull, and Muscle Network Modelling for Facial Palsy Patients

BACKGROUND AND OBJECTIVE

Facial palsy negatively affects both professional and personal life qualities of involved patients. Classical facial rehabilitation strategies can recover facial mimics into their normal and symmetrical movements and appearances. However, there is a lack of objective, quantitative, and in-vivo facial texture and muscle activation bio-feedbacks for personalizing rehabilitation programs and diagnosing recovering progresses. Consequently, this study proposed a novel patient-specific modelling method for generating a full patient specific head model from a visual sensor and then computing the facial texture and muscle activation in real-time for further clinical decision making.

METHODS

The modeling workflow includes (1) Kinect-to-head, (2) head-to-skull, and (3) muscle network definition & generation processes. In the Kinect-to-head process, subject-specific data acquired from a new user in neutral mimic were used for generating his/her geometrical head model with facial texture. In particular, a template head model was deformed to optimally fit with high-definition facial points acquired by the Kinect sensor. Moreover, the facial texture was also merged from his/her facial images in left, right, and center points of view. In the head-to-skull process, a generic skull model was deformed so that its shape was statistically fitted with his/her geometrical head model. In the muscle network definition & generation process, a muscle network was defined from the head and skull models for computing muscle strains during facial movements. Muscle insertion points and muscle attachment points were defined as vertex positions on the head model and the skull model respectively based on the standard facial anatomy. Three healthy subjects and two facial palsy patients were selected for validating the proposed method. In neutral positions, magnetic resonance imaging (MRI)-based head and skull models were compared with Kinect-based head and skull models. In mimic positions, infrared depth-based head models in smiling and [u]-pronouncing mimics were compared with appropriate animated Kinect-driven head models. The Hausdorff distance metric was used for these comparisons. Moreover, computed muscle lengths and strains in the tested facial mimics were validated with reported values in literature.

RESULTS

With the current hardware configuration, the patient-specific head model with skull and muscle network could be fast generated within 17.16±0.37s and animated in real-time with the framerate of 40 fps. In neutral positions, the best mean error was 1.91 mm for the head models and 3.21 mm for the skull models. On facial regions, the best mean errors were 1.53 mm and 2.82 mm for head and skull models respectively. On muscle insertion/attachment point regions, the best mean errors were 1.09 mm and 2.16 mm for head and skull models respectively. In mimic positions, these errors were 2.02 mm in smiling mimics and 2.00 mm in [u]-pronouncing mimics for the head models on facial regions. All above error values were computed on a one-time validation procedure. Facial muscles exhibited muscle shortening and muscle elongating for smiling and pronunciation of sound [u] respectively. Extracted muscle features (i.e. muscle length and strain) are in agreement with experimental and literature data.

CONCLUSIONS

This study proposed a novel modeling method for fast generating and animating patient-specific biomechanical head model with facial texture and muscle activation bio-feedbacks. The Kinect-driven muscle strains could be applied for further real-time muscle-oriented facial paralysis grading and other facial analysis applications.

Masticatory muscle function affects the pathological conditions of dentofacial deformities

The causes of dentofacial deformities include various known syndromes, genetics, environmental and neuromuscular factors, trauma, and tumors. Above all, the functional effects of muscles are important, and deformation of the mandible is often associated with a mechanical imbalance of the masticatory muscles. With the vertical position of the face, weakness of the sling of the masseter muscle and medial pterygoid muscle causes dilatation of the mandibular angle. In patients with a deep bite, excessive function of the masticatory muscles is reported. Myosin heavy chain　(MyHC) properties also affect jawbone morphology. In short-face patients, the proportion of type II fibers, which are fast muscles, is high. The proportions of muscle fiber types are genetically determined but can be altered by postnatal environmental factors. Orthognathic surgery may results in the transition of MyHC to type II (fast) fibers, but excessive stretching enhances the release of inflammatory mediators and causes a shift toward a greater proportion of slow muscle fibers. This feature can be related to postoperative relapse. Bones and muscles are in close crosstalk, and it may be possible to use biochemical approaches as well as biomechanical considerations for the treatment of jaw deformities.

Potential of Using Shear Wave Elastography in the Clinical Evaluation and Monitoring of Changes in Masseter Muscle Stiffness

The study aimed to evaluate masseter muscle stiffness in adult healthy volunteers referred to a massage treatment and also to investigate whether shear-wave elastography can be used to monitor the effect of massage on the masseter muscle. The study included 21 healthy volunteers, who were subjected to a 30-minute massage of the masseter muscle. Muscle stiffness was measured by shear-wave elastography before and directly after the massage. Pain during the massage was assessed using the visual analogue scale (VAS). The data of 20 patients (one excluded due to severe pain) with a median age of 34.5 years were analysed. The stiffness values were 11.46 ± 1.55 kPa before and 8.97 ± 0.96 kPa after the massage (p < 0.0001). The mean drop was 2.49 ± 1.09 kPa. The greatest decrease was observed in people with higher elasticity values before the massage (r = 0.79; p < 0.0001). The median intensity of pain was 7.2 (range: 6–9.5). We concluded that shear-wave elastography is a sensitive tool to monitor changes in the stiffness of the masseter muscle.

Anatomical Considerations When Treating Compensatory Hypertrophy of the Upper Part of the Masseter after Long-Term Botulinum Neurotoxin Type A Injections

The masseter is the most targeted muscle when treating hypertrophy to produce a smooth face shape. Compensatory hypertrophy is a well known clinical sequela that occurs in botulinum neurotoxin (BoNT) treatments and is limited to the lower part of the masseter. Based on the masseteric hypertrophy procedure, which targets a confined area, we predicted the possibility of compensatory hypertrophy occurring in the upper part of the masseter. If the patient complains about an unexpected result, additional injections must be performed, but the involved anatomical structures have not been revealed yet. The aim of this study was to identify the morphological patterns of the masseter. Deep tendons were observed in most specimens of the upper part of the masseter and mostly appeared in a continuous pattern (69.7%). The superficial and deep tendons could be classified into a simply connected form and forms surrounding part of the muscle. In 45.5% of cases there were tendon capsules that completely enclosed the muscle, which can interfere with how the injected toxin spreads. Interdigitation patterns in which the tendons could be identified independently between the muscles were present in 9.1% of cases. The present findings provide anatomical knowledge for use when injecting BoNT into the masseter.

Great potential of ultrasound elastography for the assessment of the masseter muscle in patients with temporomandibular disorders. A systematic review

Objective:

To summarize the available evidence on the use of elastography in the assessment of the masseter muscle in healthy individuals and patients with masseter muscle disorders.

Methods:

Systematic literature review has been performed according to the Preferred Reporting Items for Systematic Reviews and Meta-analyses guidelines.

Results:

16 of 142 studies identified were analyzed. Elastography was used in seven studies. Heterogeneity was observed in terms of study protocols, devices, patients, units of measure, and results. Elasticity values showed a correlation between the left and right masseter muscle side in healthy people, but not in patients with temporomandibular disorders (TMDs). Elasticity values increased in TMD and were correlated with the severity of TMD symptoms. Phantom studies proved the high reliability of elastography.

Conclusion:

Elastography is a promising tool for the assessment of the masseter muscle elasticity, but the evidence is insufficient. Studies on larger groups are needed to determine the accuracy of elastography to characterize masticatory muscle disorders.

Influence of age and gender on sex steroid receptors in rat masticatory muscles

The temporomandibular muscle dysfunction is characterized by myofascial pain and is more prevalent in women of reproductive age. Sex steroid hormones are hypothetically involved in the dysfunction, but few are the studies of steroid receptors in masticatory and mastication-related muscles. Our aim was to determine estrogen and testosterone receptor expression in rat masticatory and mastication-related muscles within the context of age and gender. Twelve rats were equally divided into four groups: (a) 10-month-old females; (b) 10-month-old males; (c) 24-month-old females; and (d) 24-month-old males. Euthanasia of the females was performed in the proestrous phase (vaginal smears) and the masticatory and accessory muscles were removed for immunohistochemical analysis. Statistical analysis was performed with ANOVA and the Tukey test. Estrogen receptor expression was similarly low in all muscles and groups. Testosterone receptor expression in the Masseter muscle of the 24-month-old male rats was higher than that in the other groups and significantly superior to its expression in the Posterior Digastric muscle. In short, testosterone receptor expression was highest in old male rats. If we generalize to humans, this fact could indicate age- and sex-related hormonal influence on temporomandibular muscle dysfunction. Further studies, however, are necessary to strengthen this hypothesis.

Ultrasonography of the internal architecture of the superficial part of the masseter muscle in vivo

It is unclear whether the deep inferior tendon (DIT) is equally present in vivo, and little anatomical information is available regarding the existence and morphology of the DIT in healthy young subjects. The aim of this study was to characterize the DIT of the masseter muscle in healthy young subjects using ultrasonography and to compare the morphology of this tendon with previously reported data for healthy young subjects in order to provide the most-effective injection methods for botulinum neurotoxin treatments of masseteric hypertrophy. This study investigated two fresh cadavers and 30 healthy subjects. Ultrasonography scanning in both longitudinal and transverse directions was applied to the masseter muscle. The DIT within the superficial part of the masseter was observed in both the fresh cadavers and the living subjects. The posterior region of the masseter muscle was compartmentalized (entirely covered) by the DIT in 26.7% of the specimens. The superficial part of the masseter muscle was divided by the DIT transversely and longitudinally into the superficial and deep muscle bellies in 35% and 38.3% of the specimens, respectively. The present findings suggest applying a DIT-based injection technique under guidance by ultrasonography prior to treating masseteric hypertrophy. Clin. Anat. 32:446-452, 2019. © 2019 Wiley Periodicals, Inc.

Insulin signaling pathway in the masseter muscle of dexamethasone-treated rats

Background and aims

The treatment with glucocorticoids may induce molecular changes in the level and/or degree of phosphorylation of proteins located downstream of the insulin receptor/insulin-like growth factor receptor (IR/IGF1R) in many tissues. However, few studies have investigated the intracellular insulin pathway in the masseter muscle. Therefore, this study aimed to analyze the IR/IGF1R signaling pathway in the masseter muscle of rats treated with dexamethasone.

Materials and methods

Male Wistar rats were divided into two groups: control group (intraperitoneally injected with 0.9% NaCl solution) and dexamethasone group [intraperitoneally injected with 1 mg/kg (bw) dexamethasone solution] for 10 consecutive days. Sections of the masseter muscle were removed at time zero and after the infusion of regular insulin into the portal vein.

Results

Dexamethasone administration induces body weight loss without changing masseter muscle weight and reduces the expression of total IR and PI3K proteins; total levels of IRS1, Akt, and ERK1 remain unchanged between groups. The degree of phosphorylation/activity of IRS1 after insulin stimulus increased only in the control group; degree of phosphorylation of Akt increased in both groups, but this increase was attenuated in the dexamethasone group.

Discussion and conclusion

The degree of phosphorylation/activity in the masseter muscle is different from that in other muscle territories.

Spectral components in electromyograms from four regions of the human masseter, in natural dentate and edentulous subjects with removable prostheses and implants

OBJECTIVE

To compare the frequency or spectral components between different regions of the superficial masseter in young natural dentate and total edentulous older adults rehabilitated with removable prostheses and fixed-implant support. A secondary objective was to compare these components between the three groups.

DESIGN

21 young natural dentate and 28 edentulous (14 with removable prostheses and 14 with fixed-implant support) were assessed. High-density surface electromyography (sEMG) was recorded in four portions of the superficial masseter during submaximal isometric bites. Spectral components were obtained through a spectral analysis of the sEMG signals. An analysis of mixed models was used to compare the spectral components.

RESULTS

In all groups, the spectral components of the anterior portion were lower than in the posterior region (p < 0.05). Both edentulous groups showed lower spectral components and median frequency slope than the natural dentate group (p < 0.05). The removable prostheses group showed the greatest differences with natural dentate group.

CONCLUSIONS

There were significant differences in the spectral components recorded in the different regions of the superficial masseter. The lower spectral components and fatigability of older adults rehabilitated with prostheses could be a cause of a greater loss of type II fibers, especially in the removable prostheses group.

Comparison of the expression of neurotransmitter and muscular genesis markers in the postnatal male mouse masseter and trigeminal ganglion during development

Calcitonin gene-related peptide (CGRP) is released by motor neurons and affects skeletal muscle fiber and transient receptor potential cation channel subfamily V member 1 (TRPV1), an important marker of pain modulation. However, the expression of CGRP and TRPV1 in the trigeminal ganglion (TG) during changes and in feeding patterns has not been described. We used real-time reverse transcription polymerase chain reaction and in situ hybridization to investigate the mRNA expression levels of CGRP and TRPV1 in the TG. The expression of myosin heavy-chain (MyHC) isoforms was also investigated in the masseter muscle (MM) during the transition from sucking to mastication, an important functional trigger for muscle. The mRNA and protein levels of CGRP increased in the MM and TG from postnatal day 10 (P10) to P20 in male mice. The protein levels of TRPV1 were almost constant in the TG from P10 to P20, in contrast to increases in the MM. The mRNA abundance of TRPV1 in the TG and MM was increased from P10 to P20. The localization of an antisense probe was used to count CGRP cell numbers and found to differentiate the ophthalmic, maxillary, and mandibular nerve divisions of the TG. In particular, the number of CGRP⁺ cells per 10,000 μm² in the maxillary and mandibular divisions of the TG gradually changed from P10 to P20. The expression of CGRP and TRPV1 in the TG and MM and the patterns of expression of different MyHC isoforms were affected by changes in feeding during male mouse development.

Neonatal imitation in context: Sensorimotor development in the perinatal period

More than 35 years ago, Meltzoff and Moore (1977) published their famous article, “Imitation of facial and manual gestures by human neonates.” Their central conclusion, that neonates can imitate, was and continues to be controversial. Here, we focus on an often-neglected aspect of this debate, namely, neonatal spontaneous behaviors themselves. We present a case study of a paradigmatic orofacial “gesture,” namely tongue protrusion and retraction (TP/R). Against the background of new research on mammalian aerodigestive development, we ask: How does the human aerodigestive system develop, and what role does TP/R play in the neonate's emerging system of aerodigestion? We show that mammalian aerodigestion develops in two phases: (1) from the onset of isolated orofacial movementsin uteroto the postnatal mastery of suckling at 4 months after birth; and (2) thereafter, from preparation to the mastery of mastication and deglutition of solid foods. Like other orofacial stereotypies, TP/R emerges in the first phase and vanishes prior to the second. Based upon recent advances in activity-driven early neural development, we suggest a sequence of three developmental events in which TP/R might participate: the acquisition of tongue control, the integration of the central pattern generator (CPG) for TP/R with other aerodigestive CPGs, and the formation of connections within the cortical maps of S1 and M1. If correct, orofacial stereotypies are crucial to the maturation of aerodigestion in the neonatal period but also unlikely to co-occur with imitative behavior.

Expression of CGRP in embryonic mouse masseter muscle

Neuropeptide calcitonin gene-related peptide (CGRP) is a mediator of inflammation and head pain that influences the functional vascular blood supply. The CGRP also regulate myoblast and acetylcholine receptors on neuromuscular junctions in development. However, little is known about its appearance and location during mouse masseter muscle (MM) development. We detected the mRNA abundance of CGRP, vascular genesis markers (Vascular endothelial growth factor A (VEGF-A), PECAM (CD31), lymphatic vessel endothelial hyaluronan receptor-1 (LYVE-1)) and embryonic and adult myosin heavy chain (MyHCs) (embryonic, IIa, IIb, and IIx) using real-time RT-PCR during development from the embryonic stage to after birth (E12.5, E14.5, E17.5, E18.5, P0, P1 and P5). We also endeavored to analyze the expression and localization of CGRP in situ hybridization in the developing mouse MM during development from the embryonic stage to after birth (E12.5, E14.5, E17.5, and P1). The antisense probe for CGRP was detected by in situ hybridization at E12.5, E14.5 E17.5 and then no longer detected after birth. The CGRP, CD31, embryonic MyHC abundance levels are highest at E17.5 (p<0.001) and they show a pattern similar to that of the other markers from E12.5 to P5. PCA analysis indicates a specific relation between CGRP and embryonic MyHC, CD31, and LYVE-1 in MM development. Cluster analyses identified the following distinct clusters for mRNA abundance in the MM: cluster 1, P5; cluster 2, E12.5, E14.5, E17.5, E18.5, P0, and P1. The positive correlation between CGRP and embryonic MyHC (Pearson's r>0.65; p<0.01) was analyzed. These data suggested that CGRP may have an influence on embryonic MyHC during mouse MM development. CGRP also affects the angiogenesis markers at embryonic stages.

Tenomodulin regulated the compartments of embryonic and early postnatal mouse masseter muscle

The masseter muscle (MM) is a complex tendinous laminar structure during development; however, the stage of the laminar structure formation is unknown. Tenomodulin (TeM) is a useful marker of tendons and has an anti-angiogenic cysteine-rich C-terminal domain. Therefore, we analyzed mRNA of TeM and angiogenesis markers (CD31 and vascular endothelial growth factor (VEGF)) and performed in situ hybridization for the TeM genes in MM from on embryonic day 12.5 (E12.5) to postnatal day 5 (P5). The TeM expression is at first detectable in the middle region of the mesenchymal connective tissue in the MM at E 12.5. The expression domains of the TeM during development typically include the middle region of the MM, particularly surrounding the vascular regions. The level of TeM mRNA in the MM increased from E12.5 to E17.5 and decreased after birth. In contrast, the levels of CD31 and VEGF mRNAs were almost constant from E12.5 to E18.5 and then low from birth onward. Therefore, the development of the laminar tendinous structure in the middle region between superficial and deeper regions of the MM first occurs during the process of tendon formation at embryonic day 12.5. In our study of MM development, the laminar structure regulating TeM also prevents vascular invasion during the formation of compartment of the MM. The tendon may relate to the components of muscle mass of MM.

Task-dependent control of the jaw during food splitting in humans

Although splitting of food items between the incisors often requires high bite forces, rarely do the teeth harmfully collide when the jaw quickly closes after split. Previous studies indicate that the force-velocity relationship of the jaw closing muscles principally explains the prompt dissipation of jaw closing force. Here, we asked whether people could regulate the dissipation of jaw closing force during food splitting. We hypothesized that such regulation might be implemented via differential recruitment of masseter muscle portions situated along the anteroposterior axis because these portions will experience a different shortening velocity during jaw closure. Study participants performed two different tasks when holding a peanut-half stacked on a chocolate piece between their incisors. In one task, they were asked to split the peanut-half only (single-split trials) and, in the other, to split both the peanut and the chocolate in one action (double-split trials). In double-split trials following the peanut split, the intensity of the tooth impact on the chocolate piece was on average 2.5 times greater than in single-split trials, indicating a substantially greater loss of jaw closing force in the single-split trials. We conclude that control of jaw closing force dissipation following food splitting depends on task demands. Consistent with our hypothesis, converging neurophysiological and morphometric data indicated that this control involved a differential activation of the jaw closing masseter muscle along the anteroposterior axis. These latter findings suggest that the regulation of jaw closing force after sudden unloading of the jaw exploits masseter muscle compartmentalization.

Evidence for the functional compartmentalization of the temporalis muscle: a 3-dimensional study of innervation

PURPOSE

The temporalis muscle is commonly used for functional transfer. It is architecturally complex, but few studies have examined its intramuscular innervation and none has used 3-dimensional modeling techniques. Understanding neuromuscular compartmentalization may allow the design of local muscle transfers to minimize donor-site morbidity. The purpose of the present study was to document the intramuscular innervation patterns throughout the volume of the temporalis muscle and define functional units within the muscle.

MATERIALS AND METHODS

In 10 formalin-embalmed cadaveric specimens, the foramen ovale was exposed and the branches of the mandibular nerve were identified. Each branch was digitized in short segments extramuscularly and intramuscularly. Three-dimensional models were reconstructed from the digitized data using Maya software, and the innervation patterns were documented.

RESULTS

The temporalis muscle was found to have superior and inferior parts that were further grouped by innervation into regions, with each receiving its innervation from 1 primary nerve. The nerves originated directly from the mandibular nerve, except in 3 specimens, where the posterior deep temporal nerve arose from the masseteric nerve.

CONCLUSION

These results provide a detailed mapping of innervation patterns and suggest there are at least 5 functional compartments. Each of these has the capacity for selective activation, 3 of which have clinical value. These findings may allow for decreased donor-site morbidity and more functionally sophisticated designs in clinical practice.

Lake Malawi cichlid evolution along a benthic/limnetic axis

Divergence along a benthic to limnetic habitat axis is ubiquitous in aquatic systems. However, this type of habitat divergence has largely been examined in low diversity, high latitude lake systems. In this study, we examined the importance of benthic and limnetic divergence within the incredibly species-rich radiation of Lake Malawi cichlid fishes. Using novel phylogenetic reconstructions, we provided a series of hypotheses regarding the evolutionary relationships among 24 benthic and limnetic species that suggests divergence along this axis has occurred multiple times within Lake Malawi cichlids. Because pectoral fin morphology is often associated with divergence along this habitat axis in other fish groups, we investigated divergence in pectoral fin muscles in these benthic and limnetic cichlid species. We showed that the eight pectoral fin muscles and fin area generally tended to evolve in a tightly correlated manner in the Lake Malawi cichlids. Additionally, we found that larger pectoral fin muscles are strongly associated with the independent evolution of the benthic feeding habit across this group of fish. Evolutionary specialization along a benthic/limnetic axis has occurred multiple times within this tropical lake radiation and has produced repeated convergent matching between exploitation of water column habitats and locomotory morphology.

A ³¹P-magnet resonance spectroscopy study on the metabolism of human masseter in individuals with different vertical facial pattern

OBJECTIVE

The objective of this study was to investigate differences in masseter metabolism by (31)P-Chemical Shift Imaging (CSI) in adult individuals with different vertical facial patterns. The clinical study should be supported by functional findings at the mRNA level after orthognathic surgery.

STUDY DESIGN

Twenty-two male volunteers (mean age 24.6) were divided into a deep-bite (NL/ML 11.8 ± 2.3°) and open-bite group (NL/ML 34.1 ± 2.6°). Vertical jaw relationship, gonial angle, and masseter volume were defined and compared with the phosphate values obtained from the (31)P spectra. Student t test and regression analysis were used.

RESULTS

Phosphocreatine related strongly to muscle volume (P < .001), gonial angle (P < .001), and ML/NL angle (P < .01). Pi was found to be related to gonial angle (P < .05). Muscle volume was found to be inversely related to ML/NL (P < .01) and to the gonial angle (P < .01).

CONCLUSIONS

A difference in masseter muscle metabolism between long- and short-faced subjects was confirmed at rest position.

Genetic stretching factors in masseter muscle after orthognathic surgery

Up to 30% of patients relapse after orthognathic operations, and one reason might be incomplete neuromuscular adaptation of the masticatory muscles. Displacement of the mandible in sagittal or vertical directions, or both, leads to stretching or compression of these muscles. The aim of this study was to analyse stretching factors in 35 patients with retrognathism or prognathism of the mandible (Classes II and III). Tissue samples were taken from both sides of the masseter muscle (anterior and posterior) both before and 6 months after operation. Developmental myosin heavy chains MYH3 and MYH8, the fast and slow MYH 1, 2, and 7, and cyclo-oxygenase (COX) 2, forkhead transcription factor (FOX)O3a, calcineurin, and nuclear factor of activated T cells (NFAT)1c (stretching and regeneration-specific), were analysed by real time polymerase chain reaction (PCR). Correlations of Class II and III with sagittal and vertical cephalometric measurements ANB and ML-NL-angle were examined, and the results showed significant differences in amounts of MYH8 (p<0.05), MYH1 (p<0.05), and FOXO3a (p<0.05) between the 2 groups. Regeneration factor COX2 is more dominant in Class II. Surgically, bite opening (ML/NL angle) correlated with stretching indicators FOXO3a, calcineurin, and NFAT1c only in Class II patients. This means that stretching of the masseter muscle caused by lengthening of the mandible and raising of the bite in Class II patients was more likely to lead to relapse (similar to that in patients with open bite) than in Class III patients. In conclusion, deep bite should be reduced more by incisor intrusion than by skeletal opening. The focus in these patients should be directed towards physiotherapeutic strengthening of the muscles of mastication, and more consideration should be given to change in the vertical dimension.

Human masseter muscle fibers from the elderly express less neonatal Myosin than those of young adults

In contrast to limb muscles where neonatal myosin (MyHC-neo) is present only shortly after birth, adult masseter muscles contain a substantial portion of MyHC-neo, which is coexpressed with mature MyHC isoforms. Changes in the numerical and area proportion of muscle fibers containing MyHC-neo in masseter muscle with aging could be expected, based on previously reported findings that (i) developmental MyHC-containing muscle fibers exhibit lower shortening velocities compared to fibers with exclusively fast MyHC isoforms and (ii) transformation toward faster phenotype occurs in elderly compared to young masseter muscle. In this study, we detected MyHC isoforms in the anterior superficial part of the human masseter muscle in a sufficiently large sample of young, middle-aged, and elderly subjects to reveal age-related changes in the coexpression of MyHC-neo with adult MyHC isoforms. MyHC isoforms were visualized with immunoperoxidase method and the results were presented by (i) the area proportion of fibers containing particular MyHC isoforms and (ii) the numerical proportion of fiber types defined by MyHC-1, -2a, -2x, and -neonatal isoform expression from a successive transverse sections. We found a lower numerical and area proportion of fibers expressing MyHC-neo as well as a lower area proportion of fibers containing MyHC-1 in elderly than in young subjects. We conclude that the diminished expression of MyHC-neo with age could point to a lower regeneration capacity of masseter muscle in the elderly.

Developmental myosin heavy chain mRNA in masseter after orthognathic surgery: a preliminary study

INTRODUCTION

A microarray study showed an increase of developmental myosin heavy chain (MyHC) mRNA in the masseter muscle after surgery. The aim of the study was to determine the expression of the embryonic MYH3 and perinatal MYH8 for use as potential marker for muscle adaptation after orthognathic surgery.

MATERIAL AND METHODS

24 adult patients with a prognathic (11) or retrognathic (13) mandible were involved in the study. 192 biopsies were taken from two parts of the muscles presurgically and 6 months following. The expression of MYH3 and MYH8 were quantified using real-time polymerase chain reaction (RT-PCR). Absolute quantification was done by CT-values.

RESULTS

MYH3 was up-regulated in prognathia (8.5, P<0.001) and in retrognathia (2.8, P<0.043). MYH8 was up-regulated in retrognathia only (4.0, P<0.063) and down-regulated in prognathia (-1.6). MYH3 values correlated in retrognathic patients (P<0.005) before and after surgery.

CONCLUSION

MYH3 and MYH8 could play a role in functional adaptation after orthognathic surgery and orofacial orthopaedics.