A cinefluorographic study of mandibular movement during feeding in the rat (Rattus norvegicus)

The Kinematics of Proal Chewing in Rats

Chewing kinematics are well-documented in several mammal species with fused mandibular symphyses, but relatively understudied in mammals with an unfused symphysis, despite the fact that more than half of extant Mammalia have an unfused mandibular symphysis. The Wistar brown rat (Rattus norvegicus) is widely used in human health research, including studies of mastication or neurological studies where mastication is the output behavior. These animals are known to have unfused mandibular symphyses and proal jaw (rostrocaudal) motion during occlusion, but the lack of high resolution, 3-dimensional analysis of rat chewing leaves the functional significance of symphyseal mobility unknown. We used biplanar fluoroscopy and the X-ray reconstruction of moving morphology workflow to quantify chewing kinematics in 3 brown rats, quantifying overall jaw kinematics, including motions about the temporomandibular joint and unfused mandibular symphysis. During occlusion, the teeth and the mandibular condyle translate almost exclusively anteriorly (proal) during occlusion, with little motion in any other degrees of freedom. At the symphysis, we observed minimal flexion throughout the chew cycle. Overall, there are fundamental differences in jaw kinematics between rats and other mammals and therefore rats are not an appropriate proxy for ancestral mammal jaw mechanics. Additionally, differences between humans and rat chewing kinematics must be considered when using rats as a clinical model for pathological feeding research.

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.

Three-dimensional mandibular kinematics of mastication in the marsupial Didelphis virginiana

Didelphis virginiana (the Virginia opossum) is often used as an extant model for understanding feeding behaviour in Mesozoic mammaliaforms, primarily due to their morphological similarities, including an unfused mandibular symphysis and tribosphenic molars. However, the three-dimensional jaw kinematics of opossum chewing have not yet been fully quantified. We used biplanar videofluoroscopy and the X-Ray Reconstruction of Moving Morphology workflow to quantify mandibular kinematics in four wild-caught opossums feeding on hard (almonds) and soft (cheese cubes) foods. These data were used to test hypotheses regarding the importance of roll versus yaw in chewing by early mammals, and the impact of food material properties (FMPs) on jaw kinematics. The magnitude of roll exceeds that of yaw, but both are necessary for tooth-tooth or tooth-food-tooth contact between complex occlusal surfaces. We confirmed the utility of the four vertical kinematic gape cycle phases identified in tetrapods but we further defined two more in order to capture non-vertical kinematics. Statistical tests support the separation of chew cycle phases into two functional groups: occlusal and non-occlusal phases. The separation of slow close into two (occlusal) phases gives quantitative kinematic support for the long-hypothesized multifunctionality of the tribosphenic molar. This article is part of the theme issue 'Food processing and nutritional assimilation in animals'.

Longitudinal electromyographic analysis of jaw-closing muscle activities during ingestive behaviors from pre-weaning to juvenile periods in rats

This longitudinal study investigated developmental changes in jaw-closing muscle activities during ingestive behaviors in rats. On postnatal day (P) 10, electromyography (EMG) electrodes were inserted into the masseter and temporalis muscles of rat pups. EMG activities were recorded for the following ingestive behaviors between P14 and P49: for suckling, including nipple attachment and rhythmic sucking on P14 and for pasta biting, pellet chewing, and milk licking between P21 and P49. Burst rhythms and muscle coordination (i.e., the correlation and time lag) between masseter and temporalis activities were assessed for each behavior. The burst rhythms of nipple attachment and rhythmic sucking on P14 were significantly slower than those of pasta biting, pellet chewing, and milk licking on P21. Muscle coordination differed between suckling on P14 and mastication and licking on P21. Between P21 and P49, increases were observed in burst rhythms for pasta biting and pellet chewing. The rate of increases in burst rhythms was higher for pasta biting than for pellet chewing. Muscle coordination between the two muscle activities for pasta biting did not significantly change between P21 and P49, whereas that for pellet chewing significantly changed between P21 and P24 and stabilized after P24. Burst rhythms for milk licking did not significantly change over time, while muscle coordination between the two muscle activities changed from agonist to antagonist muscle-like activity on approximately P35. The present results demonstrate that distinct patterns of rhythmic jaw-closing muscle activities emerge before weaning, they continue to change over time, and they exhibit unique developmental dynamics for each behavior after weaning.

Masticatory biomechanics of red and grey squirrels (Sciurus vulgaris and Sciurus carolinensis) modelled with multibody dynamics analysis

The process of feeding in mammals is achieved by moving the mandible relative to the cranium to bring the teeth into and out of occlusion. This process is especially complex in rodents which have a highly specialized configuration of jaw adductor muscles. Here, we used the computational technique of multi-body dynamics analysis (MDA) to model feeding in the red (Sciurus vulgaris) and grey squirrel (Sciurus carolinensis) and determine the relative contribution of each jaw-closing muscle in the generation of bite forces. The MDA model simulated incisor biting at different gapes. A series of 'virtual ablation experiments' were performed at each gape, whereby the activation of each bilateral pair of muscles was set to zero. The maximum bite force was found to increase at wider gapes. As predicted, the superficial and anterior deep masseter were the largest contributors to bite force, but the temporalis had only a small contribution. Further analysis indicated that the temporalis may play a more important role in jaw stabilization than in the generation of bite force. This study demonstrated the ability of MDA to elucidate details of red and grey squirrel feeding biomechanics providing a complement to data gathered via in vivo experimentation.

Avoiding the lockdown: Morphological facilitation of transversal chewing movements in mammals

The evolution of mammals is characterized, amongst other developments, by an increasing relevance of effective food processing in form of an increasingly durable dentition, complex occlusal surfaces, and transverse chewing movements. Some factors have received increasing attention for the facilitation of the latter, such as the configuration of the jaw joint, the chewing muscle arrangement and lever arms, or the reduction of interlocking cusps on the cheek teeth occlusal surface. By contrast, the constraining effect of the anterior dentition (incisors and canines) on transverse chewing motions, though known, has received less comprehensive attention. Here, we give examples of this constraint in extant mammals and outline a variety of morphological solutions to this constraint, including a reduction of the anterior dentition, special arrangements of canines and incisors, the nesting of the mandibular cheek teeth within the maxillary ones, and the use of different jaw positions for different dental functions (cropping vs. grinding). We suggest that hypselodont anterior canines or incisors in some taxa might represent a compensatory mechanism for self-induced wear during a grinding chewing motion. We propose that the diversity in anterior dentition among mammalian herbivores, and the evolutionary trend towards a reduction of the anterior dentition in many taxa, indicates that the constraining effect of the anterior dentition, which is rigidly linked to the cheek teeth by the osseous jaws, represents a relevant selective pressure in mammalian evolution.

Anterior Mandibular Displacement in Growing Rats—A Systematic Review

Simple Summary

Mandibular deficiency is a very common orthodontic problem. Many different types of appliances have been utilized to correct such malocclusions. Most are appliances that alter the function of the mandible resulting in a more forward positioning of the lower jaw. Many researchers state that such an alteration results in a different rate of mandibular growth, due to condyle endochondral ossification, thus correcting the orthodontic anomaly. Their effect though remains controversial. The aim of the present study was to investigate the effect of such functional appliances in the mandible of growing rats by reviewing the existing literature up to March of 2022. Most of them stated that true condylar growth is observed, although there are many limitations due to the nature of such experiments.

Abstract

Skeletal Class II malocclusion is the most common skeletal anomaly in orthodontics. Growth in the body of the deficient mandible is induced by periosteal apposition and endochondral ossification in the condyle. Functional appliances have been used in the correction of Class II malocclusions by inducing mandibular growth. Despite their utilization though, their effect still remains controversial. The aim of the present study is to review the existing literature regarding the effects of mandibular protrusion in mandibular growth of growing rats. A protocol was followed according to the guidelines of the Cohrane Handbook for Systematic Reviews. Databases were searched using a specific algorithm. From the ten studies finally analyzed, we conclude that the use of a functional appliance in growing rats induces cell proliferation and bone formation in their condyles, resulting in mandibular growth.

Experimental approaches to assess the effect of composition of abrasives in the cause of dental microwear

Dental microwear is used to investigate feeding ecology. Animals ingest geological material in addition to food. The full effect of geological abrasives on tooth wear is unknown. To evaluate mineralogical abrasives as tooth wear agents, rats were fed food manufactured with quartz silt, diatomaceous earth, and calcium carbonate. Rats were assigned to treatments and fed for 15 days. Molars were scanned with a Sensofar Plu Neox confocal microscope and evaluated using ISO-25178-2 parameters and traditional microwear variables using light microscopy. Using a pellet-diet as the control, all treatments had influence on microwear and discriminant function analyses indicated that unique surface textures had been produced. ISO variables with high discriminatory values were correlated to scratch and pit frequencies, but more ISO parameters identified changes associated with numbers of scratches than changes associated with pits. The microwear changes associated with the abrasive inclusions were co-dependent on the type of diet that the abrasives had been added to. The abrasives had less effect with pellets but produced more modified and more differentiated microwear when added to the transgenic dough. Although abrasives produce distinctive surface textures, some knowledge of the properties of food with the abrasives is needed to identify the abrasive agent.

Bat Dentitions: A Model System for Studies at The Interface of Development, Biomechanics, and Evolution

The evolution of complex dentitions was a major innovation in mammals that facilitated the expansion into new dietary niches that imposed selection for tight form-function relationships. Teeth allow mammals to ingest and process food items by applying forces produced by a third-class lever system composed by the jaw adductors, the cranium, and the mandible. Physical laws determine changes in jaw adductor (biting) forces at different bite point locations along the mandible (outlever), thus individual teeth are expected to experience different mechanical regimes during feeding. If the mammal dentition exhibits functional adaptations to mandible feeding biomechanics, then teeth are expected to have evolved to develop mechanically-advantageous sizes, shapes, and positions. Here, we present bats as a model system to test this hypothesis and, more generally, for integrative studies of mammal dental diversity. We combine a field-collected dataset of bite forces along the tooth row with data on dental and mandible morphology across 30 bat species. We (1) describe, for the first time, bite force trends along the tooth row of bats, (2) use phylogenetic comparative methods to investigate relationships among bite force patterns, tooth and mandible morphology, and (3) hypothesize how these biting mechanics patterns may relate to the developmental processes controlling tooth formation. We find that bite force variation along the tooth row is consistent with predictions from lever mechanics models, with most species having the greatest bite force at the first lower molar. The cross-sectional shape of the mandible body is strongly associated with the position of maximum bite force along the tooth row, likely reflecting mandibular adaptations to varying stress patterns among species. Further, dental dietary adaptations seem to be related to bite force variation along molariform teeth, with insectivorous species exhibiting greater bite force more anteriorly, narrower teeth and mandibles, and frugivores/omnivores showing greater bite force more posteriorly, wider teeth and mandibles. As these craniodental traits are linked through development, dietary specialization appears to have shaped intrinsic mechanisms controlling traits relevant to feeding performance.

Commonalities and evolutionary divergences of mandible shape ontogenies in rodents

In mammals, significant changes take place during postnatal growth, linked to changes in diet (from sucking to gnawing). During this period, mandible development is highly interconnected with muscle growth and the epigenetic interactions between muscle and bone control the spatialization of bone formation and remodelling in response to biomechanical strain. This mechanism contributes to postnatal developmental plasticity and may have influenced the course of evolutionary divergences between species and clades. We sought to model postnatal changes at a macroevolutionary scale by analysing ontogenetic trajectories of mandible shape across 16 species belonging mainly to two suborders of Rodents, Myomorpha and Hystricomorpha, which differ in muscle attachments, tooth growth and life-history traits. Myomorpha species present a much stronger magnitude of changes over a shorter growth period. Among Hystricomorpha, part of the observed adult shape is set up prenatally, and most postnatal trajectories are genus-specific, which agrees with nonlinear developmental trajectories over longer gestational periods. Beside divergence at large scale, we find some collinearities between evolutionary and developmental trajectories. A common developmental trend was also observed, leading to enlargement of the masseter fossa during postnatal growth. The tooth growth, especially hypselodonty, seems to be a major driver of divergences of postnatal trajectories. These muscle- and tooth-related effects on postnatal trajectories suggest opportunities for developmental plasticity in the evolution of the mandible shape, opportunities that may have differed across Rodent clades.

Insectivory leads to functional convergence in a group of Neotropical rodents

The mandible of vertebrates serves as insertion area for masticatory muscles that originate on the skull, and its functional properties are subject to selective forces related to trophic ecology. The efficiency of masticatory muscles can be measured as mechanical advantage on the mandible, which, in turn, has the property of correlating with bite force and shape. In the present work, we quantify the mechanical advantage of the mandible of akodontine rodents, which present a diverse radiation of insectivorous specialists, to assess their relationship to the estimated bite force and diet. We also tested the degree of morphofunctional convergence in response to insectivory on the group. We found the mechanical advantages to be convergent on insectivorous species, and associated with the estimated bite force, with higher mechanical advantages in species with a stronger bite and short, robust mandibles and lower mechanical advantages in insectivorous species with weaker bites and more elongated, dorso-ventrally compressed mandibles. Insectivorous species of Akodontini are functional specialists for the consumption of live prey and may exploit the resources that shrews, moles and hedgehogs consume elsewhere.

A salamander that chews using complex, three-dimensional mandible movements

Most non-mammal tetrapods have a hinge-like jaw operation restricted to vertical opening and closing movements. Many mammal jaw joints, by contrast, operate in more complex, three-dimensional (3D) ways, involving not only vertical but also propalinal (rostro-caudal) and transverse (lateral) movements. Data on intraoral food processing in lissamphibians and sauropsids has prompted a generally accepted view that these groups mostly swallow food unreduced, and that in those cases where lissamphibians and sauropsids chew, they mostly use simple vertical jaw movements for food processing. The exception to this generally accepted view is the occurrence of some propalinal chewing in sauropsids. We combined 3D kinematics and morphological analyses from biplanar high-speed video fluoroscopy and micro-computed tomography to determine how the paedomorphic salamander Siren intermedia treats captured food. We discovered not only that S. intermedia uses intraoral food processing but also that the elaborated morphology of its jaw joint facilitates mandibular motions in all three planes, resulting in complex 3D chewing. Thus, our data challenge the commonly held view that complex 3D chewing movements are exclusive to mammals, by suggesting that such mechanisms might have evolved early in the tetrapod evolution.

A rat model for inducing temporomandibular anterior disc displacement experimentally

The aim of this study was to establish an experimental rat model of temporomandibular joint (TMJ) anterior disc displacement (ADD). A pilot study was conducted to determine the most appropriate surgical protocol. In the main experiment, 40 rats were used. Twenty-four rats were subjected to ADD in the right TMJ, and subsequently thereafter six, nine, and nine rats were sacrificed at 1, 4, and 8 weeks, respectively, for gross evaluation. Twelve rats that underwent a sham operation were equally divided and sacrificed at each of the above time points. Four non-treated control rats were sacrificed at the beginning of the study. TMJ blocks were harvested for radiological and histological assessment. Gross examination showed that 14 rats in the ADD group (58.3%) had anterior displacement of the TMJ disc. In the ADD joints, posterior condylar cartilage thickness decreased during the follow-up period; however, there was no significant difference between the sham-treated and ADD joints, or among the follow-up time points (P > 0.05). The anterior condylar cartilage exhibited obvious qualitative alterations. Radiologic signs of osteoarthrosis appeared after ADD surgery, but this became attenuated with time. The model investigated in this study successfully induced ADD in rats, and should be useful for assessment of progressive changes in the TMJ following ADD.

Superficial, suprahyoid, and infrahyoid neck musculature in naked mole-rats (Heterocephalus glaber): Relative size and potential contributions to independent movement of the lower incisors

Naked mole-rats (Heterocephalus glaber) are fossorial, eusocial rodents that exhibit the unusual capability of moving their lower incisors independently in lateral and rostroventral directions. The evolution of this trait would presumably also involve concurrent alterations in neck musculature to support and control movements of the lower incisors. In order to assess morphological adaptations that might facilitate these movements, we performed detailed dissections of the neck musculature of adult naked mole-rats. In addition to characterizing attachment sites of superficial, suprahyoid, and infrahyoid musculature, we also quantified muscle mass and mandibular features thought to be associated with gape (condyle height, condyle length, and jaw length). Based on muscle attachment sites, the platysma myoides may contribute to lateral movement of the lower incisor and hemi-mandible in naked mole-rats. The large digastric muscle is likely to be a main contributor to rostroventral movement of each lower incisor. The geniohyoid and mylohyoid muscles also likely contribute to rostroventral movements of the lower incisors, and the mylohyoid may also produce lateral spreading of the hemi-mandibles. The transverse mandibular (intermandibularis) muscle likely serves to reposition the lower incisors back to a midline orientation following a movement.

Functional Morphology and Behavioral Correlates to Postcranial Musculature

In this the second issue of a two-volume set of the Anatomical Record on the relationship between muscle functional morphology and behavior, the focus is on the postcranial musculature. Traditionally, when talking of the postcranium we think of the skeletal parts that primarily provide the lever system necessary for body movements. However, without the force produced by muscle, the postcranial skeleton could not perform these or most other tasks. In this special issue, our colleagues present ten papers that focus on postcranial muscle morphology and function from different perspectives. They include papers on forelimb and hindlimb muscle functional morphology of vertebrates, including lizards, bats, primates, a carnivoran and a rodent, and involved in different substrate use (arboreal, terrestrial, and flying) and locomotion behavior (quadrupedal, leaper, and suspensory) along with a historical overview to help bookend the contextualization of the issues. The picture that these papers provide is one of great liveliness in the field of muscle functional morphology where both young students and affirmed professors continue to contribute with both traditional approaches and new techniques to further our knowledge of muscle morphology and its relationship with animal behavior. Anat Rec, 301:419-423, 2018. © 2018 Wiley Periodicals, Inc.

A Bio-Realistic Finite Element Model to Evaluate the Effect of Masticatory Loadings on Mouse Mandible-Related Tissues

Mice are arguably the dominant model organisms for studies investigating the effect of genetic traits on the pathways to mammalian skull and teeth development, thus being integral in exploring craniofacial and dental evolution. The aim of this study is to analyse the functional significance of masticatory loads on the mouse mandible and identify critical stress accumulations that could trigger phenotypic and/or growth alterations in mandible-related structures. To achieve this, a 3D model of mouse skulls was reconstructed based on Micro Computed Tomography measurements. Upon segmenting the main hard tissue components of the mandible such as incisors, molars and alveolar bone, boundary conditions were assigned on the basis of the masticatory muscle architecture. The model was subjected to four loading scenarios simulating different feeding ecologies according to the hard or soft type of food and chewing or gnawing biting movement. Chewing and gnawing resulted in varying loading patterns, with biting type exerting a dominant effect on the stress variations experienced by the mandible and loading intensity correlating linearly to the stress increase. The simulation provided refined insight on the mechanobiology of the mouse mandible, indicating that food consistency could influence micro evolutionary divergence patterns in mandible shape of rodents.

The influence of altered functional loading and posterior bite-blocks on the periodontal ligament space and alveolar bone thickness in rats

OBJECTIVES

Posterior bite-blocks are resin-based structures elevating the occlusion and creating intrusive force on the posterior teeth. Bite-blocks were applied to the molars of growing rats and a hard and soft diet was used to create altered functional masticatory forces. The aim of the present investigation was to study the effect of this appliance on the periodontal ligament space and alveolar bone thickness when combined with altered masticatory forces.

MATERIAL AND METHODS

Fifty-two four-week-old rats were divided into two groups, hard and soft diet. Two weeks later, half of them received a bite-block appliance, creating four groups: control hard (CH), control soft (CS), bite-block hard (BH) and bite-block soft (BS). All were sacrificed at age of 10 weeks. Their heads were scanned by micro-CT and periodontal ligament space (PDL) width, cross-sectional alveolar socket surface and alveolar bone thickness were measured. Analysis of variance (ANOVA) was used to compare the groups.

RESULTS

The PDL was 9.2% thinner in the CS group (p < 0.001) and 20.7% in the bite-block groups (p  < 0.001) compared to the CH group. Within each of the four groups, the mesiodistal PDL space was larger than the palatobuccal. The alveolar bone was thinner by 5.8% (p = 0.018) in the CS group, 10.7% in the BH group (p < 0.001) and 16.7% in the BS group (p < 0.001) compared to the CH group.

CONCLUSIONS

Young rats wearing posterior bite-blocks have narrower PDL space and thinner alveolar bone compared to controls. When fed a soft diet, the alveolar bone is even thinner but the PDL showed no difference.

Age-related changes in mastication are not improved by tongue exercise in a rat model

OBJECTIVES/HYPOTHESIS

Aging results in progressive changes in deglutitive functions, which may be due in part to alterations in muscle morphology and physiology. Mastication is a critical component of bolus formation and swallowing, but aging effects on masticatory function have not been well studied.

STUDY DESIGN

The purpose of this study was to 1) quantify the effects of aging on mastication, and 2) determine the effects of tongue exercise on mastication in young adult and old rats. We hypothesized that there would be significant differences in mastication characteristics (number of bites, interval between bites, time to eat) as a function of age, and that tongue exercise would resolve preexercise differences between age groups.

METHODS

We expanded the established model of progressive, 8-week tongue exercise to include a mastication measurement: acoustic recordings of vermicelli pasta biting from 17 old and 17 young adult rats, randomized into exercise and control groups.

RESULTS

We found the following: 1) Mastication characteristics were impacted by age. Specifically in older rats, there was an increase in time to eat and number of bites and intervals between bites decreased, suggesting increased oral motor-processing requirements for bolus formation. 2) tongue exercise did not impact mastication behaviors in young adult or old rats.

CONCLUSION

Tongue exercise may not have been specific enough to result in behavioral changes in mastication or exercise dose may not have been sufficient. Nevertheless, results were noteworthy in expanding the established rat model of aging and have relevant clinical implications for future translation to human populations.

LEVEL OF EVIDENCE

NA Laryngoscope, 127:E29-E34, 2017.

Deterioration of mechanical properties of discs in chronically inflamed TMJ

Temporomandibular joint (TMJ) discs frequently undergo degenerative changes in arthritis. However, the biomechanical properties of pathogenic discs remain to be explored. In this study, we evaluated the effects of chronic inflammation on the biomechanical properties of TMJ discs in rats. Chronic inflammation of TMJs was induced by double intra-articular injections of complete Freund's adjuvant for 5 weeks, and biomechanical properties and ultrastructure of the discs were examined by mechanical testing, scanning electron microscopy, and transmission electron microscopy. The instantaneous compressive moduli of the anterior and posterior bands of discs in inflamed TMJs were decreased significantly compared with those in the control group. The instantaneous tensile moduli of the discs of inflamed TMJs also showed significant decreases in both the anterior-posterior and mesial-lateral directions. The relaxation moduli of the discs of inflamed TMJs showed nearly the same tendency as the instantaneous moduli. The surfaces of the discs of inflamed TMJs became rough and porous due to the loss of the superficial gel-like stratum, with many collagen fibers exposed and degradation of the sub-superficial collagen fibrils. Our results suggested that chronic inflammation of TMJ could lead to deterioration of mechanical properties and alteration of disc ultrastructure, which might contribute to TMJ disc displacement.

The morphology of the mouse masticatory musculature

The mouse has been the dominant model organism in studies on the development, genetics and evolution of the mammalian skull and associated soft-tissue for decades. There is the potential to take advantage of this well studied model and the range of mutant, knockin and knockout organisms with diverse craniofacial phenotypes to investigate the functional significance of variation and the role of mechanical forces on the development of the integrated craniofacial skeleton and musculature by using computational mechanical modelling methods (e.g. finite element and multibody dynamic modelling). Currently, there are no detailed published data of the mouse masticatory musculature available. Here, using a combination of micro-dissection and non-invasive segmentation of iodine-enhanced micro-computed tomography, we document the anatomy, architecture and proportions of the mouse masticatory muscles. We report on the superficial masseter (muscle, tendon and pars reflecta), deep masseter, zygomaticomandibularis (anterior, posterior, infraorbital and tendinous parts), temporalis (lateral and medial parts), external and internal pterygoid muscles. Additionally, we report a lateral expansion of the attachment of the temporalis onto the zygomatic arch, which may play a role in stabilising this bone during downwards loading. The data presented in this paper now provide a detailed reference for phenotypic comparison in mouse models and allow the mouse to be used as a model organism in biomechanical and functional modelling and simulation studies of the craniofacial skeleton and particularly the masticatory system.

Evolutionary Developmental Biology and Human Language Evolution: Constraints on Adaptation

A tension has long existed between those biologists who emphasize the importance of adaptation by natural selection and those who highlight the role of phylogenetic and developmental constraints on organismal form and function. This contrast has been particularly noticeable in recent debates concerning the evolution of human language. Darwin himself acknowledged the existence and importance of both of these, and a long line of biologists have followed him in seeing, in the concept of "descent with modification", a framework naturally able to incorporate both adaptation and constraint. Today, the integrated perspective of modern evolutionary developmental biology ("evo-devo") allows a more subtle and pluralistic approach to these traditional questions, and has provided several examples where the traditional notion of "constraint" can be cashed out in specific, mechanistic terms. This integrated viewpoint is particularly relevant to the evolution of the multiple mechanisms underlying human language, because of the short time available for novel aspects of these mechanisms to evolve and be optimized. Comparative data indicate that many cognitive aspects of human language predate humans, suggesting that pre-adaptation and exaptation have played important roles in language evolution. Thus, substantial components of what many linguists call "Universal Grammar" predate language itself. However, at least some of these older mechanisms have been combined in ways that generate true novelty. I suggest that we can insightfully exploit major steps forward in our understanding of evolution and development, to gain a richer understanding of the principles that underlie human language evolution.

Functional evolution of the feeding system in rodents

The masticatory musculature of rodents has evolved to enable both gnawing at the incisors and chewing at the molars. In particular, the masseter muscle is highly specialised, having extended anteriorly to originate from the rostrum. All living rodents have achieved this masseteric expansion in one of three ways, known as the sciuromorph, hystricomorph and myomorph conditions. Here, we used finite element analysis (FEA) to investigate the biomechanical implications of these three morphologies, in a squirrel, guinea pig and rat. In particular, we wished to determine whether each of the three morphologies is better adapted for either gnawing or chewing. Results show that squirrels are more efficient at muscle-bite force transmission during incisor gnawing than guinea pigs, and that guinea pigs are more efficient at molar chewing than squirrels. This matches the known diet of nuts and seeds that squirrels gnaw, and of grasses that guinea pigs grind down with their molars. Surprisingly, results also indicate that rats are more efficient as well as more versatile feeders than both the squirrel and guinea pig. There seems to be no compromise in biting efficiency to accommodate the wider range of foodstuffs and the more general feeding behaviour adopted by rats. Our results show that the morphology of the skull and masticatory muscles have allowed squirrels to specialise as gnawers and guinea pigs as chewers, but that rats are high-performance generalists, which helps explain their overwhelming success as a group.

Finite element modelling of squirrel, guinea pig and rat skulls: using geometric morphometrics to assess sensitivity

Rodents are defined by a uniquely specialized dentition and a highly complex arrangement of jaw-closing muscles. Finite element analysis (FEA) is an ideal technique to investigate the biomechanical implications of these specializations, but it is essential to understand fully the degree of influence of the different input parameters of the FE model to have confidence in the model's predictions. This study evaluates the sensitivity of FE models of rodent crania to elastic properties of the materials, loading direction, and the location and orientation of the models' constraints. Three FE models were constructed of squirrel, guinea pig and rat skulls. Each was loaded to simulate biting on the incisors, and the first and the third molars, with the angle of the incisal bite varied over a range of 45°. The Young's moduli of the bone and teeth components were varied between limits defined by findings from our own and previously published tests of material properties. Geometric morphometrics (GMM) was used to analyse the resulting skull deformations. Bone stiffness was found to have the strongest influence on the results in all three rodents, followed by bite position, and then bite angle and muscle orientation. Tooth material properties were shown to have little effect on the deformation of the skull. The effect of bite position varied between species, with the mesiodistal position of the biting tooth being most important in squirrels and guinea pigs, whereas bilateral vs. unilateral biting had the greatest influence in rats. A GMM analysis of isolated incisor deformations showed that, for all rodents, bite angle is the most important parameter, followed by elastic properties of the tooth. The results here elucidate which input parameters are most important when defining the FE models, but also provide interesting glimpses of the biomechanical differences between the three skulls, which will be fully explored in future publications.

Subglottal pressure, tracheal airflow, and intrinsic laryngeal muscle activity during rat ultrasound vocalization

Vocal production requires complex planning and coordination of respiratory, laryngeal, and vocal tract movements, which are incompletely understood in most mammals. Rats produce a variety of whistles in the ultrasonic range that are of communicative relevance and of importance as a model system, but the sources of acoustic variability were mostly unknown. The goal was to identify sources of fundamental frequency variability. Subglottal pressure, tracheal airflow, and electromyographic (EMG) data from two intrinsic laryngeal muscles were measured during 22-kHz and 50-kHz call production in awake, spontaneously behaving adult male rats. During ultrasound vocalization, subglottal pressure ranged between 0.8 and 1.9 kPa. Pressure differences between call types were not significant. The relation between fundamental frequency and subglottal pressure within call types was inconsistent. Experimental manipulations of subglottal pressure had only small effects on fundamental frequency. Tracheal airflow patterns were also inconsistently associated with frequency. Pressure and flow seem to play a small role in regulation of fundamental frequency. Muscle activity, however, is precisely regulated and very sensitive to alterations, presumably because of effects on resonance properties in the vocal tract. EMG activity of cricothyroid and thyroarytenoid muscle was tonic in calls with slow or no fundamental frequency modulations, like 22-kHz and flat 50-kHz calls. Both muscles showed brief high-amplitude, alternating bursts at rates up to 150 Hz during production of frequency-modulated 50-kHz calls. A differentiated and fine regulation of intrinsic laryngeal muscles is critical for normal ultrasound vocalization. Many features of the laryngeal muscle activation pattern during ultrasound vocalization in rats are shared with other mammals.

Inferences of diplodocoid (Sauropoda: Dinosauria) feeding behavior from snout shape and microwear analyses

BACKGROUND

As gigantic herbivores, sauropod dinosaurs were among the most important members of Mesozoic communities. Understanding their ecology is fundamental to developing a complete picture of Jurassic and Cretaceous food webs. One group of sauropods in particular, Diplodocoidea, has long been a source of debate with regard to what and how they ate. Because of their long lineage duration (Late Jurassic-Late Cretaceous) and cosmopolitan distribution, diplodocoids formed important parts of multiple ecosystems. Additionally, fortuitous preservation of a large proportion of cranial elements makes them an ideal clade in which to examine feeding behavior.

METHODOLOGY/PRINCIPAL FINDINGS

Hypotheses of various browsing behaviors (selective and nonselective browsing at ground-height, mid-height, or in the upper canopy) were examined using snout shape (square vs. round) and dental microwear. The square snouts, large proportion of pits, and fine subparallel scratches in Apatosaurus, Diplodocus, Nigersaurus, and Rebbachisaurus suggest ground-height nonselective browsing; the narrow snouts of Dicraeosaurus, Suuwassea, and Tornieria and the coarse scratches and gouges on the teeth of Dicraeosaurus suggest mid-height selective browsing in those taxa. Comparison with outgroups (Camarasaurus and Brachiosaurus) reinforces the inferences of ground- and mid-height browsing and the existence of both non-selective and selective browsing behaviors in diplodocoids.

CONCLUSIONS/SIGNIFICANCE

These results reaffirm previous work suggesting the presence of diverse feeding strategies in sauropods and provide solid evidence for two different feeding behaviors in Diplodocoidea. These feeding behaviors can subsequently be tied to paleoecology, such that non-selective, ground-height behaviors are restricted to open, savanna-type environments. Selective browsing behaviors are known from multiple sauropod clades and were practiced in multiple environments.

Mandibular corpus bone strain in goats and alpacas: implications for understanding the biomechanics of mandibular form in selenodont artiodactyls

The goal of this study is to clarify the functional and biomechanical relationship between jaw morphology and in vivo masticatory loading in selenodont artiodactyls. We compare in vivo strains from the mandibular corpus of goats and alpacas to predicted strain patterns derived from biomechanical models for mandibular corpus loading during mastication. Peak shear strains in both species average 600-700 microepsilon on the working side and approximately 450 microepsilon on the balancing side. Maximum principal tension in goats and alpacas is directed at approximately 30 degrees dorsocaudally relative to the long axis of the corpus on the working side and approximately perpendicular to the long axis on the balancing side. Strain patterns in both species indicate primarily torsion of the working-side corpus about the long axis and parasagittal bending and/or lateral transverse bending of the balancing-side corpus. Interpretation of the strain patterns is consistent with comparative biomechanical analyses of jaw morphology suggesting that in goats, the balancing-side mandibular corpus is parasagittally bent whereas in alpacas it experiences lateral transverse bending. However, in light of higher working-side corpus strains, biomechanical explanations of mandibular form also need to consider that torsion influences relative corpus size and shape. Furthermore, the complex combination of loads that occur along the selenodont artiodactyl mandibular corpus during the power stroke has two implications. First, added clarification of these loading patterns requires in vivo approaches for elucidating biomechanical links between mandibular corpus morphology and masticatory loading. Second, morphometric approaches may be limited in their ability to accurately infer masticatory loading regimes of selenodont artiodactyl jaws.

Mosaic convergence of rodent dentitions

Background

Understanding mechanisms responsible for changes in tooth morphology in the course of evolution is an area of investigation common to both paleontology and developmental biology. Detailed analyses of molar tooth crown shape have shown frequent homoplasia in mammalian evolution, which requires accurate investigation of the evolutionary pathways provided by the fossil record. The necessity of preservation of an effective occlusion has been hypothesized to functionally constrain crown morphological changes and to also facilitate convergent evolution. The Muroidea superfamily constitutes a relevant model for the study of molar crown diversification because it encompasses one third of the extant mammalian biodiversity.

Methodology/Principal Findings

Combined microwear and 3D-topographic analyses performed on fossil and extant muroid molars allow for a first quantification of the relationships between changes in crown morphology and functionality of occlusion. Based on an abundant fossil record and on a well resolved phylogeny, our results show that the most derived functional condition associates longitudinal chewing and non interlocking of cusps. This condition has been reached at least 7 times within muroids via two main types of evolutionary pathways each respecting functional continuity. In the first type, the flattening of tooth crown which induces the removal of cusp interlocking occurs before the rotation of the chewing movement. In the second type however, flattening is subsequent to rotation of the chewing movement which can be associated with certain changes in cusp morphology.

Conclusion/Significance

The reverse orders of the changes involved in these different pathways reveal a mosaic evolution of mammalian dentition in which direction of chewing and crown shape seem to be partly decoupled. Either can change in respect to strong functional constraints affecting occlusion which thereby limit the number of the possible pathways. Because convergent pathways imply distinct ontogenetic trajectories, new Evo/Devo comparative studies on cusp morphogenesis are necessary.

Myofiber properties of mouse mylohyoid muscle in the growth period

The mouse mylohyoid muscle belongs to the mastication-related suprahyoid muscle group. It shows a plate-like morphology and forms the mouth floor. There have been no reports on the characteristics of the mouse mylohyoid muscle fibers, and especially on their functional role during ingestion action, and many points remain unclear. We examined the mouse mylohyoid muscle at both the transcriptional and protein levels by RT-PCR, immunohistochemistry, and Western Blotting. MyHC-2b, which is expressed in almost all head and neck muscles and is thought to play a role in rapid mastication movement, was not detected in the mouse mylohyoid muscle. This result suggests that the mouse mylohyoid muscle has a special function and does not directly function during ingestion.

Plasticity of mandibular biomineralization in myostatin-deficient mice

Compared with the normal or wild-type condition, knockout mice lacking myostatin (Mstn), a negative regulator of skeletal muscle growth, develop significant increases in relative masticatory muscle mass as well as the ability to generate higher maximal muscle forces. Wild-type and myostatin-deficient mice were compared to assess the postweaning influence of elevated masticatory loads because of increased jaw-adductor muscle and bite forces on the biomineralization of mandibular cortical bone and dental tissues. Microcomputed tomography (microCT) was used to quantify bone density at a series of equidistant external and internal sites in coronal sections for two symphysis and two corpus locations. Discriminant function analyses and nonparametric ANOVAs were used to characterize variation in biomineralization within and between loading cohorts. Multivariate analyses indicated that 95% of the myostatin-deficient mice and 95% of the normal mice could be distinguished based on biomineralization values at both symphysis and corpus sections. At the corpus, ANOVAs suggest that between-group differences are due to the tendency for cortical bone mineralization to be higher in myostatin-deficient mice, coupled with higher levels of dental biomineralization in normal mice. At the symphysis, ANOVAs indicate that between-group differences are related to significantly elevated bone-density levels along the articular surface and external cortical bone in the knockout mice. Both patterns, especially those for the symphysis, appear because of the postweaning effects of increased masticatory stresses in the knockout mice versus normal mice. The greater number of symphyseal differences suggest that bone along this jaw joint may be characterized by elevated plasticity. Significant differences in bone-density levels between normal and myostatin-deficient mice, coupled with the multivariate differences in patterns of plasticity between the corpus and symphysis, underscore the need for a comprehensive analysis of the plasticity of masticatory tissues vis-à-vis altered mechanical loads.

Changes in Muscle-fiber Properties of the Murine Digastric Muscle Before and After Weaning

The digastric muscle is one of the suprahyoid muscles and consists of the anterior and posterior bellies. Because muscle fiber alignments in these two bellies are different, the functional roles are said to be different. Since the digastric muscle relates to mastication, its functions may change markedly before and after weaning, but many details remain unknown. The aim of this study was to clarify changes in muscle fiber properties of the anterior and posterior bellies of the digastric muscle in mice before and after weaning. Expressions of myosin heavy chain (MyHC) isoforms were assessed at the protein and transcriptional levels. Expression of the MyHC-2b isoform, an isoform displaying fast, strong contraction, was greater in the anterior belly than in the posterior belly after weaning. This suggests that, in mice, the anterior belly of the digastric muscle needs to move rapidly anteroposteriorly for mastication, compared with the posterior belly.

Physiological properties of molar-mechanosensitive periodontal neurons in the trigeminal ganglion of the rat

Spike discharges from periodontal mechanosensitive neurones responding to the mechanical stimulation of molar teeth were recorded from the trigeminal ganglion of rats anaesthetized with pentobarbital sodium. Maxillary molar-sensitive units were close together in a narrow, lateral area of the maxillary division of the ganglion, whereas those of mandibular molar-sensitive units were scattered throughout the mandibular division. The majority of maxillary molar-sensitive units responded only to stimulation of the first molar. They were slowly adapting and responded most strongly to pressure applied to the lingual surface and buccal cusp of the tooth or to the buccal surface and lingual cusp. By contrast, approximately one-half of the mandibular molar-sensitive units were rapidly adapting, multitooth units that responded to tooth stimulation almost equally in all directions. The other half were slowly adapting and activated most effectively by pressure applied to the lingual surface and buccal cusp of the molar tooth. These slowly adapting units consisted of first molar-sensitive, single- and multitooth units. Differences in the response characteristics of the maxillary and mandibular molar-sensitive periodontal units may reflect differences in the sensory role of individual molars.

Biomineralization and adaptive plasticity of the temporomandibular joint in myostatin knockout mice

Mice lacking myostatin (GDF-8), a negative regulator of skeletal muscle growth, show a significant increase in muscle mass versus normal mice. We compared wild-type and myostatin deficient mice to assess the postnatal effect of elevated masticatory loads due to increased jaw-adductor muscle activity and greater bite forces on mandibular condyle morphology. Microcomputed tomography (microCT) was used to provide details of internal condylar morphology and quantify bone density in three condylar regions. Biomineralization levels, as well as external mandibular dimensions, were used to characterize within-slice, within-joint, within-group and between-group variation. Dimensions of the mandible and mandibular condyle were similar between the myostatin knockout and normal mice. Knockout mice exhibited significantly more biomineralization on the outer surface of the condylar subchondral bone and along the condylar neck, most notably on the buccal side of the condylar neck. The buccal side of the inner aspect of the condyle was significantly less biomineralized in knockout mice, both for the pooled data and for the posterior and anterior condylar slices. Whilst normal mice had symmetric subchondral bone surfaces, those of knockout mice were asymmetric, with a lower, less convex surface on the buccal side versus the lingual side. This appears related to the ontogenetic effects of increased masticatory stress in the mandibles of knockout mice as compared to normal mice. Significant differences in biomineralization between normal and myostatin knockout mice, coupled with the lack of significant differences in certain external dimensions, underscores a need for information on the external and internal morphology of mineralized tissues vis-à-vis altered or excessive mechanical loads.

Effects of masticatory muscle function on craniofacial morphology in growing ferrets (Mustela putorius furo)

Studying the effects of masticatory muscle function on craniofacial morphology in animal models with different masticatory systems is important for further understanding of related issues in humans. Forty 5-wk-old male ferrets were equally divided into two groups. One group was fed a diet of hard pellets (HDG) and the other group was fed the same diet but softened with water (SDG). Lateral and dorsoventral cephalograms were taken on each group after 6 months. Cephalometric measurements were performed by digital procedures. For SDG ferrets, the hard palate plane was more distant from the cranial base plane, and canines were more proclined compared with HDG ferrets. The SDG ferrets were also found to have smaller interfrontal and interparietal widths, and a slenderer zygomatic arch than the HDG ferrets. In the mandible, the coronoid process was generally shorter and narrower for the SDG ferrets. The effects of the altered masticatory muscle function on craniofacial morphology in growing ferrets seemed to differ from those previously reported in other animal models studied under similar experimental conditions. Such differences in the effects are presumably related to the differences in the mode of mastication, craniofacial anatomy and growth pattern in different animal models.

Internal architecture, origin-insertion site, and mass of jaw muscles in Old World hamsters

The jaw muscle (i.e., masticatory, suprahyoid, and extrinsic tongue) anatomy and mass were examined in four genera of Old World hamsters (cricetine murids), Mesocricetus, Cricetulus, Tscherskia, and Phodopus. The masseter was the largest and most complicated of the muscles examined. In the superficial layer, a few ventral fibers form a small medially turned portion with an insertion site more similar to those of sciurids than of other murids. In Mesocricetus, the superficial layer has a discrete anteroventral portion that has not been reported for other murid rodents. Examination of the fiber attachment sites indicated that the deep layer contains four parts and the medial layer contains three parts. The deep layer originates from two aponeuroses that are firmly connected to each other at their anterior ends and lie along the zygomatic arch. The aponeurosis of insertion for the deep layer is situated along the masseteric ridge and the dorsal border of the angular process, but is absent in its middle part, consistent with reports in two relatives, sigmodontine and arvicoline murids. In cricetine murids, unlike in other rodents, fibers insert on the dorsal narrow strip of the posterior mandibular aponeurosis, not on its broad medial aspect. The relative mass of some masticatory and suprahyoid muscles is related to body mass. Small species (Cricetulus and Phodopus) have relatively larger masseter and mylohyoid muscles and smaller temporalis and geniohyoid muscles than large species (Mesocricetus and Tscherskia).

New animal model for studying mastication in oral motor disorders

To identify the basic parameters of oral behavior in mice, we recorded the three-dimensional jaw movement trajectories and masseter and digastric muscle activities in freely behaving mice eating foods of various textures. Results showed that: (1) there are characteristic jaw movement patterns for food intake and mastication; (2) the pattern in a chewing cycle may be divided into opening, closing, and protruding (power) strokes; and (3) food texture affects basic patterns of jaw movement, muscle activities, and chewing rhythms. The oral motor behavior of mice appears identical to those of other experimental animals, so mice are appropriate animal models for the study of mastication.