A 3D analysis of growth trajectory and integration during early human prenatal facial growth.

Significant shape changes in the human facial skeleton occur in the early prenatal period, and understanding this process is critical for studying a myriad of congenital facial anomalies. However, quantifying and visualizing human fetal facial growth has been challenging. Here, we applied quantitative geometric morphometrics (GM) to high-resolution magnetic resonance images of human embryo and fetuses, to comprehensively analyze facial growth. We utilized non-linear growth estimation and GM methods to assess integrated epigenetic growth between masticatory muscles and associated bones. Our results show that the growth trajectory of the human face in the early prenatal period follows a curved line with three flexion points. Significant antero-posterior development occurs early, resulting in a shift from a mandibular prognathic to relatively orthognathic appearance, followed by expansion in the lateral direction. Furthermore, during this time, the development of the zygoma and the mandibular ramus is closely integrated with the masseter muscle.

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.

Differing structural properties of foods affect the development of mandibular control and muscle coordination in infants and young children.

The development of chewing is an essential motor skill that is continually refined throughout early childhood. From a motor control perspective, the advancement of textures is dependent upon the fit between a child's oral anatomic and motor system and food properties. The purpose of this exploratory study is to identify age-related changes in chewing motor coordination and control and to determine if these changes are associated with the differing structural properties of solid foods, as well as to explore the role of explanatory variables such as the emergence of teeth and bite force. The masticatory muscle coordination (i.e., coupling of synergistic and antagonistic muscle pairs) and control (i.e., speed, displacement, chewing rate, duration, and number of chews) of fifty children were assessed cross-sectionally at five ages: 9-, 12-, 18-, 24-, and 36-months using electromyography (EMG) and 3D optical motion capture while children ate three foods that had differing structural properties. The results of this study found that children made gains in their chewing motor control (decreased duration of chewing sequences and lateral jaw displacement) and coordination (improved jaw muscle coupling) throughout this period. The structural differences in foods also affected chewing performance at all ages. These preliminary findings suggest that some solid textures are better adapted for immature mandibular control than others and that the development of chewing is a protracted process that may be impacted by the emergence of teeth and changes to bite force.

Morphological evolution of the mammalian jaw adductor complex.

The evolution of the mammalian jaw during the transition from non-mammalian synapsids to crown mammals is a key event in vertebrate history and characterised by the gradual reduction of its individual bones into a single element and the concomitant transformation of the jaw joint and its incorporation into the middle ear complex. This osteological transformation is accompanied by a rearrangement and modification of the jaw adductor musculature, which is thought to have allowed the evolution of a more-efficient masticatory system in comparison to the plesiomorphic synapsid condition. While osteological characters relating to this transition are well documented in the fossil record, the exact arrangement and modifications of the individual adductor muscles during the cynodont-mammaliaform transition have been debated for nearly a century. We review the existing knowledge about the musculoskeletal evolution of the mammalian jaw adductor complex and evaluate previous hypotheses in the light of recently documented fossils that represent new specimens of existing species, which are of central importance to the mammalian origins debate. By employing computed tomography (CT) and digital reconstruction techniques to create three-dimensional models of the jaw adductor musculature in a number of representative non-mammalian cynodonts and mammaliaforms, we provide an updated perspective on mammalian jaw muscle evolution. As an emerging consensus, current evidence suggests that the mammal-like division of the jaw adductor musculature (into deep and superficial components of the m. masseter, the m. temporalis and the m. pterygoideus) was completed in Eucynodontia. The arrangement of the jaw adductor musculature in a mammalian fashion, with the m. pterygoideus group inserting on the dentary was completed in basal Mammaliaformes as suggested by the muscle reconstruction of Morganucodon oehleri. Consequently, transformation of the jaw adductor musculature from the ancestral ('reptilian') to the mammalian condition must have preceded the emergence of Mammalia and the full formation of the mammalian jaw joint. This suggests that the modification of the jaw adductor system played a pivotal role in the functional morphology and biomechanical stability of the jaw joint.

Estudo do efeito analgésico do laser de baixa potência na mialgia dos músculos mastigatórios: estudo clínico randomizado duplo-cego / Study of low power laser analgesic effect in myalgia of the masticatory muscles: Clinical randomized double-blind

O presente estudo avaliou a eficácia do laser de baixa potência (LBP) e do placebo no tratamento da mialgia dos músculos mastigatórios. Sessenta pacientes (média de idade de 38,83 anos ± 14,2) com mialgia dos músculos mastigatórios foram selecionados e alocados em 2 grupos, de forma randomizada: Grupo A: pacientes que foram submetidos à aplicação de laser de baixa potência (LBP) na primeira fase e à aplicação de laser placebo na segunda fase e Grupo B: pacientes que foram submetidos à aplicação de laser placebo na primeira fase e à aplicação de LBP na segunda fase. O laser e o placebo foram aplicados pontualmente nos músculos masseter e temporal bilateralmente. A presença e intensidade de dor espontânea (através da escala visual analógica - EVA), dor à palpação (através do algômetro de pressão) e máxima abertura bucal (com paquímetro) foram mensurados. Os dados foram analisados usando os testes Exato de Fisher, teste t e de Mann-Witney, adotando-se p<0,05 como nível de significância. Melhoras estatisticamente significativas foram encontradas para os fatores dor espontânea, dor à palpação total e dor no masseter direito na análise intragrupo e entre grupos. Não foram encontrados efeitos residuais diferentes em nenhum dos indicadores ao compararmos os grupos que receberam ou não o laser. Apenas o masseter direito apresentou diferença estatisticamente significativa quando foi comparada a ordem das aplicações nos grupos que receberam primeiro o laser e depois o placebo versus o grupo que recebeu primeiro o placebo e depois o laser. Esse resultado indica que não existem diferenças entre a ordem de aplicação do laser. Os resultados obtidos sugerem que houve uma melhora significativa na dor medida através da escala de pontos álgicos nos pacientes que receberam o laser, entretanto a auto avaliação de dor medida pelo EVA não foi estatisticamente diferente. Os efeitos residuais e de ordem também não foram estatisticamente significativos.

This study evaluated the effectiveness of low power laser (LBP) and placebo in the treatment of myalgia of the masticatory muscles. Sixty patients (average age of 38.83 years ± 14.2) with myalgia of the masticatory muscles were selected and allocated into 2 groups randomly: Group A: patients who underwent low-power laser application (LBP) in the first phase and the application of laser placebo in the second phase and Group B: patients who were submitted to laser placebo in the first phase and the implementation of LBP in the second phase. The laser and the placebo were applied punctually in the masseter and temporalis muscles bilaterally. The presence and intensity of spontaneous pain (by visual analogue scale - VAS), pain on palpation (through pressure algometer) and maximal mouth opening (with caliper) were measured. Data were analyzed using the Fisher Test, T test and Mann-Whitney, adopting p <0.05 significance level. Statistically significant improvements were found for spontaneous pain, pain on palpation and complete pain in the right masseter in the intra-group analysis and between groups. There were no different residual effects in any of the indicators when comparing the groups with or without the laser. Just the right masseter showed a statistically significant difference when compared with the order of applications in the groups that received first laser and then the placebo versus the group that received placebo first and then the laser. This result indicates that there are no differences between the laser application order. The results suggests that there was a significant improvement in pain measured by the scale of nociceptive points in patients who received the laser, though the self assessment of pain measured by VAS was not statistically different. Residual and order effects were not statistically significant.

γ-Aminobutyric acid-, glycine-, and glutamate-immunopositive boutons on mesencephalic trigeminal neurons that innervate jaw-closing muscle spindles in the rat: ultrastructure and development.

Unlike other primary sensory neurons, the neurons in the mesencephalic trigeminal nucleus (Vmes) receive most of their synaptic input onto their somata. Detailed description of the synaptic boutons onto Vmes neurons is crucial for understanding the synaptic input onto these neurons and their role in the motor control of masticatory muscles. For this, we investigated the distribution of γ-aminobutyric acid (GABA)-, glycine-, and glutamate-immunopositive (+) boutons on Vmes neurons and their ultrastructural parameters that relate to transmitter release: Vmes neurons that innervate masseteric muscle spindles were identified by labeling with horseradish peroxidase injected into the muscle, and immunogold staining and quantitative ultrastructural analysis of synapses onto these neurons were performed in adult rats and during postnatal development. The bouton volume, mitochondrial volume, and active zone area of the boutons contacting labeled somata (axosomatic synapses) were similar to those of boutons forming axoaxonic synapses with Vmes neurons but smaller than those of boutons forming axodendritic or axosomatic synapses with most other neurons. GABA+ , glycine+ , and glutamate+ boutons constituted a large majority (83%) of all boutons on labeled somata. A considerable fraction of boutons (28%) was glycine(+) , and all glycine+ boutons were also GABA+ . Bouton size remained unchanged during postnatal development. These findings suggest that the excitability of Vmes neurons is determined to a great extent by GABA, glycine, and glutamate and that the relatively lower synaptic strength of axosomatic synapses may reflect the role of the Vmes neurons in modulating orofacial motor function.

Postnatal histomorphogenesis of the mandible in the house mouse.

The mandible of the house mouse, Mus musculus, is a model structure for the study of the development and evolution of complex morphological systems. This research describes the histomorphogenesis of the house mouse mandible and analyses its biological significance from the first to the eighth postnatal weeks. Histological data allowed us to test a hypothesis concerning modularity in this structure. We measured the bone growth rates by fluorescent labelling and identified the bone tissue types through microscopic analysis of histological cross-sections of the mandible during its postnatal development. The results provide evidence for a modular structure of the mouse mandible, as the alveolar region and the ascending ramus show histological differences throughout ontogeny. The alveolar region increases in length during the first two postnatal weeks by bone growth in the posterior region, while horizontally positioned incisors preclude bone growth in the anterior region. In the fourth postnatal week, growth dynamics shows a critical change. The alveolar region drifts laterally and the ramus becomes more vertical due to the medial growth direction of the coronoid region and the lateral growth of the ventral region of the ramus. Diet changes after weaning are probably involved in these morphological changes. In this way, the development of the masticatory muscles that insert on the ascending ramus may be particularly related to this shape modeling of the house mouse mandible.

A kinematic description of the temporal characteristics of jaw motion for early chewing: preliminary findings.

PURPOSE: The purpose of this investigation was to describe age- and consistency-related changes in the temporal characteristics of chewing in typically developing children between the ages of 4 and 35 months and adults using high-resolution optically based motion capture technology. METHOD: Data were collected from 60 participants (48 children, 12 adults) across 5 age ranges (beginners, 7 months, 12 months, 35 months, and adults); each age group included 12 participants. Three different food consistencies were trialed as appropriate. The data were analyzed to assess changes in chewing rate, chewing sequence duration, and estimated number of chewing cycles. RESULTS: The results revealed both age- and consistency-related changes in chewing rate, sequence duration, and estimated number of chewing cycles, with consistency differences affecting masticatory timing in children as young as 7 months of age. Chewing rate varied as a function of age and consistency, and chewing sequence duration was shorter for adults than for children regardless of consistency type. In addition, the results from the estimated number of chewing cycles measure suggest that chewing effectiveness increased with age; this measure was also dependent on consistency type. CONCLUSIONS: The findings suggest that the different temporal chewing variables follow distinct developmental trajectories and are consistency dependent in children as young as 7 months of age. Clinical implications are detailed.

Influence of food consistency on growth and morphology of the mandibular condyle.

The objective of this study was to determine if variation in the shape and mineralization of the mandibular condyle are the result of natural adaptation in response to different functional loading demands. Eight female Kuni Kuni piglets were randomly assigned to two groups of four, receiving either a soft or hard diet. Each animal was given three separate doses of vital stains intravenously at set time points during the study. At 8.5 months, animals were euthanized and temporomandibular joints (TMJs) were excised. Histological analysis was used to measure the amount of new bone deposition in the anterior, central, and posterior regions of the mandibular condyle. Backscatter electron (BSE) imaging was used as a semiquantitative estimate of bone mineralization in these two diet groups. Histology revealed that the degree of new bone deposition in the hard-diet group was significantly (n = 4, P < 0.001, paired t-test) higher than that of the soft-diet group. Also, the majority (87%) of animals fed a hard diet tended to show greater new bone deposition on the leftside in comparison to the right, indicating a chewing preference for the left side. In both groups, the degree of new bone deposition was significantly (P < 0.01) higher in the posterior area than in other regions. BSE imaging corroborated basic histology results, with significantly (P < 0.01) higher mineralization levels detected in the hard-diet group. These findings indicate that diet consistency has a small but significant effect on the rate of bone deposition in the mandibular condyle.

Masticatory hypermuscularity is not related to reduced cranial volume in myostatin-knockout mice.

It has been suggested recently that masticatory muscle size reduction in humans resulted in greater encephalization through decreased compressive forces on the cranial vault. Following this logic, if masticatory muscle size were increased, then a reduction in brain growth should also occur. This study was designed to test this hypothesis using a myostatin (GDF-8) knockout mouse model. Myostatin is a negative regulator of skeletal muscle growth, and individuals lacking this gene show significant hypermuscularity. Sixty-two [32 wild-type (WT) and 30 GDF-8 -/- knockout], 1, 28, 56, and 180-day-old CD-1 mice were used. Body and masseter muscle weights were collected following dissection and standardized lateral and dorsoventral cephalographs were obtained. Cephalometric landmarks were identified on the radiographs and cranial volume was calculated. Mean differences were assessed using a two-way ANOVA. KO mice had significantly greater body and masseter weights beginning at 28 days compared with WT controls. No significant differences in cranial volumes were noted between KO and WT. Muscle weight was not significantly correlated with cranial volume in 1, 28, or 180-day-old mice. Muscle weights exhibited a positive correlation with cranial volume at 56 days. Results demonstrate that masticatory hypermuscularity is not associated with reduced cranial volume. In contrast, there is abundant data demonstrating the opposite, brain growth determines cranial vault growth and masticatory apparatus only affects ectocranial morphology. The results presented here do not support the hypothesis that a reduction in masticatory musculature relaxed compressive forces on the cranial vault allowing for greater encephalization.

Developmental plasticity links local adaptation and evolutionary diversification in foraging morphology.

Developmental plasticity is thought to reconcile the constraining role of natural selection in maintaining local adaptation with evolutionary diversification under novel conditions, but empirical documentations are rare. In vertebrates, growth and development of bones is partially guided by contractions of attached musculature and such muscle activity changes progressively through embryonic development from sporadic motility to direct functional effects. In species with short generation times, delayed skull maturation extends the guiding effects of muscle activity on formation of foraging morphology into adulthood, providing an opportunity to directly examine the links between plasticity of bone development, ecological adaptations, and evolutionary diversification in foraging morphology. In this case, the morphological consequences of inputs due to local functional requirements should be evident in adaptive divergence across taxa. Here we provide evidence that epigenetic regulation of bone growth in Soricid shrews may enable both development of local adaptations and evolutionary divergence in mandibular morphology. We contrast the effects of muscle stimulation on early- vs. late-maturing components of, foraging apparatus to show that the morphology of late-maturing components is more affected by functional requirements than are early-ossifying traits. Further, the divergence in foraging morphology across shrew species occurs along the directions delineated by inductive effects of muscle loading and bite force on bone formation in late-maturing but not early-maturing mandible components within species. These results support the hypothesis that developmental plasticity can link maintenance of local adaptations with evolutionary diversification in morphology.

Adaptation of rat jaw muscle fibers in postnatal development with a different food consistency: an immunohistochemical and electromyographic study.

The development of the craniofacial system occurs, among other reasons, as a response to functional needs. In particular, the deficiency of the proper masticatory stimulus affects the growth. The purpose of this study was to relate alterations of muscle activity during postnatal development to adaptational changes in the muscle fibers. Fourteen 21-day-old Wistar strain male rats were randomly divided into two groups and fed on either a solid (hard-diet group) or a powder (soft-diet group) diet for 63 days. A radio-telemetric device was implanted to record muscle activity continuously from the superficial masseter, anterior belly of digastric and anterior temporalis muscles. The degree of daily muscle use was quantified by the total duration of muscle activity per day (duty time), the total burst number and their average length exceeding specified levels of the peak activity (5, 20 and 50%). The fiber type composition of the muscles was examined by the myosin heavy chain content of fibers by means of immunohistochemical staining and their cross-sectional area was measured. All muscle fibers were identified as slow type I and fast type IIA, IIX or IIB (respectively, with increasing twitch contraction speed and fatigability). At lower activity levels (exceeding 5% of the peak activity), the duty time of the anterior belly of the digastric muscle was significantly higher in the soft-diet group than in the hard-diet group (P < 0.05). At higher activity levels (exceeding 20 and 50% of the peak activity), the duty time of the superficial masseter muscle in the soft-diet group was significantly lower than that in the hard-diet group (P < 0.05). There was no difference in the duty time of the anterior temporalis muscle at any muscle activity level. The percentage of type IIA fibers of the superficial masseter muscle in the soft-diet group was significantly lower than that in the hard-diet group (P < 0.01) and the opposite was true with regard to type IIB fibers (P < 0.05). The cross-sectional area of type IIX and type IIB fibers of the superficial masseter muscle was significantly smaller in the soft-diet group than in the hard-diet group (P < 0.05). There was no difference in the muscle fiber composition and the cross-sectional area of the anterior belly of the digastric and anterior temporalis muscles. In conclusion, for the jaw muscles of male rats reared on a soft diet, the slow-to-fast transition of muscle fiber was shown in only the superficial masseter muscle. Therefore, the reduction in the amount of powerful muscle contractions could be important for the slow-to-fast transition of the myosin heavy chain isoform in muscle fibers.

[Influence of a liquid diet on the jaw-closing muscle spindles in growing rats].

Recently, it has been shown that prolonged feeding of a liquid diet after being weaned impedes the functional development and leads to immature mastication in growing rats. Since the jaw muscle spindles play an important role in the control of the jaw movement during the normal masticatory function, in this study we investigated the effects of prolonged feeding of a liquid diet after being weaned on the functional development of the jaw-closing muscle spindles in growing rats. Soon after weaning, 40 female Wistar rats were divided into two equal groups. The control group was fed a solid diet and the experimental group was fed a liquid diet. At 5, 7, 9 and 11 weeks, the rats were anesthetized and the response of the masseter muscle spindles to ramp-and-hold jaw stretches were recorded from the mesencephalic trigeminal nucleus. In the experimental groups, both the dynamic and the static indices were significantly lower than those of the control groups at the age of 5, 7, 9 and 11 weeks old. There was no significant change within the same group during the experimental period in both indices. These results suggest that the long-term masticatory functional change due to feeding of a liquid diet may impede the maturation of the functional properties of the jaw-closing muscle spindles, leading to immature mastication in growing rats.

Synaptic transmission from the supratrigeminal region to jaw-closing and jaw-opening motoneurons in developing rats.

The supratrigeminal region (SupV) receives abundant orofacial sensory inputs and descending inputs from the cortical masticatory area and contains premotor neurons that target the trigeminal motor nucleus (MoV). Thus it is possible that the SupV is involved in controlling jaw muscle activity via sensory inputs during mastication. We used voltage-sensitive dye, laser photostimulation, patch-clamp recordings, and intracellular biocytin labeling to investigate synaptic transmission from the SupV to jaw-closing and jaw-opening motoneurons in the MoV in brain stem slice preparations from developing rats. Electrical stimulation of the SupV evoked optical responses in the MoV. An antidromic optical response was evoked in the SupV by MoV stimulation, whereas synaptic transmission was suppressed by substitution of external Ca2+ with Mn2+. Photostimulation of the SupV with caged glutamate evoked rapid inward currents in the trigeminal motoneurons. Gramicidin-perforated and whole cell patch-clamp recordings from masseter motoneurons (MMNs) and digastric motoneurons (DMNs) revealed that glycinergic and GABAergic postsynaptic responses evoked in MMNs and DMNs by SupV stimulation were excitatory in P1-P4 neonatal rats and inhibitory in P9-P12 juvenile rats, whereas glutamatergic postsynaptic responses evoked by SupV stimulation were excitatory in both neonates and juveniles. Furthermore, the axons of biocytin-labeled SupV neurons that were antidromically activated by MoV stimulation terminated in the MoV. Our results suggest that inputs from the SupV excite MMNs and DMNs through activation of glutamate, glycine, and GABAA receptors in neonates, whereas glycinergic and GABAergic inputs from the SupV inhibit MMNs and DMNs in juveniles.

Is fiber-type composition related to daily jaw muscle activity during postnatal development?

AIM: Muscles containing large numbers of slow-contracting fibers are generally more active than muscles largely composed of fast fibers. This relationship between muscle activity and phenotype suggests that (1) changes in fiber-type composition during postnatal development are accompanied by changes in daily activity and (2) individual variations in fiber-type composition are related to similar variations in daily muscle activity. METHODS: The masseter and digastric muscles of 23 New Zealand White rabbits (young, juvenile and adult) were examined for their phenotype (myosin heavy chain content) and their daily activity (total daily number of activity bursts). RESULTS: During development, the masseter showed a strong increase in the number of fast-type fibers compared to the number of slow-type fibers. During development, also the number of powerful bursts in the masseter increased. The digastric showed no significant changes in fiber types or burst numbers. Within each muscle, across individual animals, no significant correlations (R < 0.70) were found between any of the fiber types and daily burst numbers in any of the age groups. CONCLUSIONS: The results suggest that activity-related influences are of relatively minor importance during development and that other factors are dominant in determining fiber-type composition.

Distinct roles of Hoxa2 and Krox20 in the development of rhythmic neural networks controlling inspiratory depth, respiratory frequency, and jaw opening.

BACKGROUND: Little is known about the involvement of molecular determinants of segmental patterning of rhombomeres (r) in the development of rhythmic neural networks in the mouse hindbrain. Here, we compare the phenotypes of mice carrying targeted inactivations of Hoxa2, the only Hox gene expressed up to r2, and of Krox20, expressed in r3 and r5. We investigated the impact of such mutations on the neural circuits controlling jaw opening and breathing in newborn mice, compatible with Hoxa2-dependent trigeminal defects and direct regulation of Hoxa2 by Krox20 in r3. RESULTS: We found that Hoxa2 mutants displayed an impaired oro-buccal reflex, similarly to Krox20 mutants. In contrast, while Krox20 is required for the development of the rhythm-promoting parafacial respiratory group (pFRG) modulating respiratory frequency, Hoxa2 inactivation did not affect neonatal breathing frequency. Instead, we found that Hoxa2-/- but not Krox20-/- mutation leads to the elimination of a transient control of the inspiratory amplitude normally occurring during the first hours following birth. Tracing of r2-specific progenies of Hoxa2 expressing cells indicated that the control of inspiratory activity resides in rostral pontine areas and required an intact r2-derived territory. CONCLUSION: Thus, inspiratory shaping and respiratory frequency are under the control of distinct Hox-dependent segmental cues in the mammalian brain. Moreover, these data point to the importance of rhombomere-specific genetic control in the development of modular neural networks in the mammalian hindbrain.

Effect of food consistency on the degree of mineralization in the rat mandible.

A switch to a soft diet, associated with reduced forces applied to the mandible during mastication, may result in an alteration of the degree of mineralization in the mandible. This alteration may be regionally different. The aim of this study was to analyze this alteration by examination of the degree of mineralization in the mandible of growing rats fed with a hard or soft diet. Fifteen Wistar male rats were used in this investigation. After weaning, six rats were fed with a hard diet and the remaining nine rats with a soft diet. After 9 weeks, three-dimensional reconstructions of the cortical and trabecular bone of their mandibles were obtained using a microCT system. The degree of mineralization was determined for the trabecular bone in the condyle and for the cortical bone in the anterior and posterior areas of the mandibular body. In both diet groups the degree of mineralization was significantly (p < 0.01) lower in the trabecular than in the cortical bone. In the mandibular body, the anterior area showed a significantly (p < 0.01) higher degree of mineralization than the posterior area in both diet groups. In both areas the soft diet group had a significantly (p < 0.05 or 0.01) higher degree of mineralization than the hard diet group. The trabecular bone in the condyle of the hard diet group showed a significantly (p < 0.01) higher degree of mineralization than in the soft diet group. The present results indicate the importance of proper masticatory muscle function for craniofacial growth and development.

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.

Postnatal changes of local neuronal circuits involved in activation of jaw-closing muscles.

Feeding behaviour in mammals changes from suckling to mastication during postnatal development and the neuronal circuits controlling feeding behaviour should change in parallel to the development of orofacial structures. In this review we discuss the location of excitatory premotor neurons for jaw-closing motoneurons (JCMNs) and postnatal changes of excitatory synaptic transmission from the supratrigeminal region (SupV) to JCMNs. We show that neurons located in SupV and the reticular formation dorsal to the facial nucleus most likely excite JCMNs. Excitatory inputs from SupV to JCMNs are mediated by activation of glutamate and glycine receptors in neonatal rats, whereas glycinergic inputs from SupV to JCMNs become inhibitory with age. We also show that the incidence of post-spike afterdepolarization increases during postnatal development, whereas the amplitude and half-duration of the medium-duration afterhyperpolarization decrease with age. Such postnatal changes in synaptic transmission from SupV to JCMNs and membrane properties of JCMNs might be involved in the transition from suckling to mastication.