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 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.

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.

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.

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 transitions in myosin heavy chain isoforms of the rabbit superficial masseter and digastric muscle

We investigated the early (< 8 weeks) and late (> 8 weeks) postnatal development of the fibre type composition and fibre cross-sectional area in the superficial masseter and digastric muscle of male rabbits. It was hypothesized, first, that due to the transition between suckling and chewing, during early postnatal development the increase in the proportion of slow fibre types and in fibre cross-sectional areas would be larger in the masseter than in the digastric; and second, that due to the supposed influence of testosterone during late postnatal development, the proportion of slow fibre types in both muscles would decrease. Fibre types were classified by immunostaining according to their myosin heavy chain (MyHC) content. The proportion of slow fibre types significantly increased in the masseter, from 7% at week 1 to 47% at week 8, and then decreased to 21% at week 20, while in the digastric it increased from 5% in week 1 to 19% at week 8 and remained the same thereafter. The changes in the proportion of fast fibre types were the opposite. The remarkable increase and decrease in the proportion of slow fibre types in the masseter was attributed predominantly to MyHC-cardiac alpha fibres. During early development, the cross-sectional area of all fibres in both muscles increased. However, only the fast fibre types in the masseter continued to grow further after week 8. Before weaning, the fast fibre types in the digastric were larger than those in the masseter, but after week 8, they became larger in the masseter than in the digastric. In adult animals, masseter and digastric had the same percentage of fast fibre types, but these fibres were almost twice as large in masseter as in digastric.

Daily activity of the rabbit jaw muscles during early postnatal development

Early postnatal development of the jaw muscles is characterized by the transition from suckling to chewing behavior. As chewing develops the jaw closing muscles become more powerful compared with the jaw openers. These changes are likely to affect the amount of daily muscle activity. Therefore, the purpose of this study was to characterize for a jaw opener (digastric) and jaw closer (masseter) the total duration of daily muscle activity (i.e. the duty time), and the daily burst numbers and lengths during early postnatal development. Using radiotelemetry the activity of these muscles was recorded in 10 young New Zealand White rabbits between three and eight weeks of age. Fiber-type composition was analyzed at eight weeks of age by determining the myosin heavy chain content of the fibers. During postnatal development both muscles showed no significant decrease or increase in their daily activity. However, the interindividual variation of the duty time and burst number significantly decreased. There were no significant differences between the digastric and masseter except for the most powerful activities at eight weeks of age, where the masseter showed a significantly higher duty time and burst number than the digastric. The masseter contained a higher number of slow-type fibers expressing myosin heavy chain-I and myosin heavy chain-cardiac alpha than the digastric. The present results suggest that the amount of jaw muscle activation is already established early during postnatal development, before the transition from suckling to chewing behavior. This amount of activation seems to be related to the number of slow-type fibers.

Ontogeny of suction feeding capacity in snook,Centropomus undecimalis

The ontogeny of suction feeding performance, as measured by peak suction generating capacity, was studied in the common snook, Centropomus undecimalis. Suction pressure inside the buccal cavity is a function of the total expansive force exerted on the buccal cavity distributed across the projected area of the buccal cavity. Thus, the scaling exponent of peak suction pressure with fish standard length was predicted to be equal to the scaling exponent of sternohyoideus muscle cross-sectional area, found to be 1.991, minus the scaling exponent for the projected buccal cavity area, found to be 2.009, equal to -0.018. No scaling was found in peak suction pressure generated by 12 snook ranging from 94 to 314 mm SL, supporting the prediction from morphology. C. undecimalis are able to generate similar suction pressures throughout ontogeny.

Embryonic and postnatal development of masticatory and tongue muscles

This review summarizes findings concerning the unique developmental characteristics of mouse head muscles (mainly the masticatory and tongue muscles) and compares their characteristics with those of other muscles. The developmental origin of the masticatory muscles is the somitomeres, whereas the tongue and other muscles, such as the trunk (deep muscles of the back, body wall muscles) and limb muscles, originate from the somites. The program controlling the early stages of masticatory myogenesis, such as the specification and migration of muscle progenitor cells, is distinctly different from those in trunk and limb myogenesis. Tongue myogenesis follows a similar regulatory program to that for limb myogenesis. Myogenesis and synaptogenesis in the masticatory muscles are delayed in comparison with other muscles and are not complete even at birth, whereas the development of tongue muscles proceeds faster than those of other muscles and ends at around birth. The regulatory programs for masticatory and tongue myogenesis seem to depend on the developmental origins of the muscles, i.e., the origin being either a somite or somitomere, whereas myogenesis and synaptogenesis seem to progress to serve the functional requirements of the masticatory and tongue muscles.

Muscle Influence on Postnatal Craniofacial Development and Diagnostics

The postnatal craniofacial development is determined by exogenous and endogenous factors that may result in morphological and functional muscle changes and influence the dentoskeletal region in terms of a physiologic or dysgnathic development. Using functional appliances, efforts are made to treat skeletal malocclusions through targeted exercise and to prevent an undesirable development of the dentition and the craniofacial structures. However, the success of the treatment and the stability of the outcome are not always adequate. To illustrate the treatment processes, clinically relevant measures for diagnosing muscle function and morphology have been developed in recent years. Electromyographic investigations and bite-force measurements show an excessively high variability and the histologic examinations applied to date are restricted in their suitability for analysis of the human masticatory muscles. Animal experimental studies have meanwhile succeeded in simulating functional jaw orthopedics and in demonstrating muscle remodeling processes at the genetic level. Despite some invasiveness, the time and the small quantity of muscle tissue involved permit molecular biological measuring in the orofacial system.

Changes in jaw muscles activity with age: effects on food bolus properties

Bolus formation depends on chewing process that evolves with age. This study aims to analyze the effect of age on chewing behavior (recorded by EMG) and the consequences on bolus formation for meat products. Twenty-five young adults (age range: 25-30) and 20 healthy elderly (age range: 68-73) having at least six pairs of natural postcanine teeth participated. From two different textures of bovine meat, boli were characterized by shear force measurements. Saliva incorporated into the bolus was quantified. Chewing duration was significantly longer in the elderly group for both textures, but muscle activity was significantly lower for the toughest texture only. Moreover, muscle activity was less accurately adapted to food texture in elderly than in young. In order to control that changes in EMG reflect changes in bite force, EMG was recorded during static bite forces. Slopes were fairly similar for both groups suggesting that EMG/bite force relationships do not vary with age. Elderly subjects could partly compensate for a weaker chewing efficiency by increasing the number of chewing cycles before swallowing. It is hypothesized that lengthening of chewing duration results from a decrease in muscle activity during healthy aging. After chewing, the mechanical resistance of the bolus was always higher for the elderly than for the young subjects. No significant age effect was found on the amount of saliva incorporated in the bolus. Elderly subjects, despite the lengthening of the chewing sequence, were less efficient to comminute a meat bolus than young subjects and swallowed less comminuted boli.

Morphogenesis of parrot jaw muscles: understanding the development of an evolutionary novelty

Parrots have developed novel head structures in their evolutionary history. The appearance of two new muscles for strong jaw adduction is especially fascinating in developmental and evolutionary contexts. However, jaw muscle development of parrots has not been described, despite its uniqueness. This report first presents the normal developmental stages of the cockatiel (Nymphicus hollandicus), comparable to that of the chick. Next, the peculiar skeletal myogenesis in the first visceral arch of parrots is described, mainly focusing on the development of two new jaw muscles. One of the parrot-specific muscles, M. ethmomandibularis, was initially detected at Nymphicus Stage 28 (N28) as the rostral budding of M. pterygoideus. After N32, the muscle significantly elongates rostrodorsally toward the interorbital septum, following a course lateral to the palatine bone. Another parrot-specific muscle, M. pseudomasseter, was first recognized at N36. The muscle branches off from the posteromedial M. adductor mandibulae externus and grows in a dorsolateral direction, almost covering the lateral surface of the jugal bar. The upper tip of the muscle is accompanied by condensed mesenchyme, which seems to be derived from cephalic neural crest cells.

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.

Condyle-fossa modifications and muscle interactions during Herbst treatment, Part 2. Results and conclusions

Herbst appliances were activated progressively in growing nonhuman primates, and the results were compared with primate and human controls. The methods and materials of this research are explained in Part 1 of this study. The results are discussed here in Part 2. All experimental subjects developed large super Class I malocclusions, the result of many factors including posterior movement of the maxilla and the maxillary teeth, an increased horizontal component of condylar growth, and anterior displacement of the mandible and the mandibular teeth. The growth modification measured in the glenoid fossa was in an inferior and anterior direction. Restriction of the downward and backward growth of the fossa observed in the control subjects might additionally contribute to the overall super Class I malocclusion. Clinically, these combined effects could be significant at the fossa. The restriction of local temporal bone (fossa) growth cannot be observed clinically; thus, these results might also clarify some Class II correction effects that cannot be explained with functional appliances. Differences in the area and maximum thickness of new bone formation in the glenoid fossa and in condylar growth were statistically significant. The bony changes in the condyle and the glenoid fossa were correlated with decreased postural electromyographic activity during the experimental period. Results from permanently implanted electromyographic sensors demonstrated that lateral pterygoid muscle hyperactivity was not associated with condyle-glenoid fossa growth modification with functional appliances, and that other factors, such as reciprocal stretch forces and subsequent transduction along the fibrocartilage between the displaced condyle and fossa, might play a more significant role in new bone formation. These results support the growth relativity concept.

Condyle-fossa modifications and muscle interactions during herbst treatment, part 1. New technological methods

Changes in the condyle, the glenoid fossa, and the muscles of mastication were investigated in subjects undergoing continuous orthopedic advancement of the mandible with a Herbst-block appliance. The total sample consisted of 56 subjects and included 15 nonhuman primates (in the middle mixed, early permanent, and permanent dentitions), 17 human Herbst patients in the early permanent dentition, and 24 human controls from the Burlington Growth Center. The 8 nonhuman primates in the middle mixed dentition were the focus of this study. Mandibular advancement was obtained progressively in 5 animals by adding stops to the telescopic arms of fixed functional Herbst appliances with occlusal coverage; activations of 5.0 mm, 7.0 mm, and 8.0 mm were achieved. Two primates served as controls, and the third was a sham control. Two experimental animals and the 2 controls also wore surgically implanted electromyographic electrodes in the superior and inferior heads of the lateral pterygoid muscles and in the superficial masseter and anterior digastric muscles. Changes in condylar growth direction and amount were assessed with the Björk method from measurements made on serial cephalometric tracings superimposed on metallic implants. Undecalcified sections, treated with intravenous tetracycline vital staining, were viewed with fluorescence microscopy to examine histologic changes in the condyle and the glenoid fossa. New bone formation in the fossa associated with continuous mandibular protrusion was quantified by using computerized histomorphometric analysis of decalcified histological sections and polarized light. The unique combination of permanently implanted electromyographic electrodes, tetracycline vital staining, and histomorphometry represents a significant technological advancement in methods and materials. Together, they demonstrated different muscle-bone interaction results for functional appliances than those reported in previous studies. In Part 1 of this study, we describe and discuss the techniques used in this research and give a brief overview of the findings; in Part 2 (to be published next month), we offer a more in-depth discussion of the results and the implications of our findings.

[Contraction peculiarity of the muscles that control the moving of the jaw of rat in different growing time]

OBJECTIVE

This study was aimed to evaluate the effect of functional change in oral cavity on the contraction peculiarity of the muscles that control the moving of the jaw in different time of growth.

METHODS

Fifteen male Wistar rats 4, 6 and 8 weeks old were used in this experiment. After deep anesthesia, the masseter muscles(MM), digastric muscles(DM) and lateral pterygoid muscles(LPM) were stimulated in the apartment made by the present authors. The curves that express the relations of force with electric stimulation were recorded, and the relationships between the speed and force of twitch were analyzed.

RESULTS

The maximal forces of twitch of the 8-week-old rats' MM, DM, LPM were 61.7 +/- 14.4 g, 86.4 +/- 25.1 g and 175.3 +/- 39.4 g respectively. When a single electric stimulus stimulated the muscles, the speed of twitch increased with the growth of the rats. As for MM and LPM, the speed markedly increased with the growing of rats from 4 weeks old to 6 weeks old; as for DM, it increased in rats growing from 6 weeks old to 8 weeks old.

CONCLUSION

The quickest time of growth of the muscles that control the moving of the jaw of rat is a period from 6 weeks to 8 weeks old and the forces of contraction and twitch increase markedly.

Ontogeny of feeding function in the gray short-tailed opossum Monodelphis domestica: empirical support for the constrained model of jaw biomechanics

The constrained model of masticatory function enables specific predictions of bite force potentials in skulls of differing craniodental configurations. In this study, empirical support for the constrained model is provided using maximum voluntary bite force data along Region I and II of the jaws of gray short-tailed opossums Monodelphis domestica. Then, growth series of M. domestica are used to assess how bite force potential changes with growth by evaluating craniodental changes using longitudinal sets of dorsoventral radiographs and by assessing maximal bite force potential at the Region I-II boundary of the jaw in juveniles (aged 70-80 days) and adults. Our findings show that, while juveniles and adults alike enclose at least three molariform teeth within Region II (the area of highest bite force potential along the jaw), age-dependent elongation of the masticatory muscle resultant lever arm and narrowing of the palate relative to jaw length especially enhance the mechanical advantage of the adductor muscle resultant in adults. While maximal bite forces at the Region I-II boundary are absolutely greater in adults, these bite forces scale isometrically with body mass, which suggests that mass-specific forces exerted by jaw adductor muscles of larger (adult) opossums are disproportionately smaller than those exerted by smaller (juvenile) opossums.

Control of facial muscle development by MyoR and capsulin

Members of the MyoD family of basic helix-loop-helix (bHLH) transcription factors control the formation of all skeletal muscles in vertebrates, but little is known of the molecules or mechanisms that confer unique identities to different types of skeletal muscles. MyoR and capsulin are related bHLH transcription factors expressed in specific facial muscle precursors. We show that specific facial muscles are missing in mice lacking both MyoR and capsulin, reflecting the absence of MyoD family gene expression and ablation of the corresponding myogenic lineages. These findings identify MyoR and capsulin as unique transcription factors for the development of specific head muscles.

Sagittal occlusal relationships and asymmetry in prematurely born children

The aim of this investigation was to examine the effect of pre-term birth on sagittal occlusal relationships. The subjects were 328 prematurely born white and black children and 1804 control children who participated in the cross-sectional study of the Collaborative Perinatal Project (USA) in the early 1960s and 1970s. Dental examinations, including dental casts and photographs, were performed at the age of 6-12 years. The sagittal occlusion of the permanent molars and the canine relationship was recorded by examining and measuring the hard stone casts. The pre-term and comparison groups were divided by sex and race. A significantly greater prevalence of pre-normal canine relationships was found in the pre-term group than in the controls (P < 0.001). The incidence of a bilateral symmetrical canine relationship was 60.3 per cent in both the pre-term and control groups, but in the pre-term group the girls had better symmetry than the boys. Asymmetry occurred significantly more often on the left side (P < 0.001), especially in the control boys, but this was not so clear in the pre-term group. The prevalence of mesial molar occlusion was greater in the pre-term group. These results suggest that premature birth and the consequent exceptional adaptation from intra- to extra-uterine nutrition may influence dental occlusal development. This emphasizes the importance of early functional activity and differences in masticatory muscle activity and the largely unknown phenomenon of early catch-up growth. Individual differences in neonatal factors, in the need for intubation and other medical care are also of importance. Pre-term birth may also interfere with the development of symmetry and lateralization.

Effects of masticatory muscle function and bite-raising on mandibular morphology in the growing rat

The aim of this series of investigations was to study the effect of masticatory muscle function on the growth pattern and on the internal structure of the mandible during growth. The muscular and dentoskeletal growth adaptation to prolonged bite-raising and the role of the functional state of the masticatory muscles in this adaptation were also to be elucidated. Differences in masticatory muscle function were induced in young rats by altering the consistency of the diet. Bite-raising was produced by the insertion of posterior bite blocks. Morphometric analysis of the internal bone structures of the mandible was performed on microradiographs, and videodensitometric analysis was performed on lateral radiographs and microradiographs. The effect of muscle function and bite-raising on mandibular growth was studied on a series of lateral cephalograms, superimposed on bone markers. Muscle belly, sarcomere and aponeurosis length adaptation to bite-raising was studied in situ with a digital caliper and under a microscope after fluorescent vital staining of the deep masseter muscle. A soft diet altered the pattern of growth of the mandible and reduced bone growth in the angular region. Transversal dimensions and cross-sectional area of the dentoalveolar process were smaller. Bone mass in areas possibly subjected to direct loads or bending forces was smaller. This was due to either less trabecular bone or thinner cortical bone. Only a few sites showed lower bone density. Posterior bite-blocks affected the size of the mandible as well as its growth pattern, and intruded lower molars. The soft diet influenced the effect of bite-blocks and caused less intrusion of upper molars and less inhibition of bone growth at the angular process. The deep masseter muscle adapted to bite-raising by elongation of the aponeurosis, but less in rats on a soft diet. Changes in masticatory muscle function affected the growth of the mandible in both the sagittal and transversal plane. Reduced loads on molars and condyle and smaller bending forces in other regions of the mandible possibly reduced the levels of stimulation of the osteocyte network and osteoblasts, thus inducing less trabecular bone and cortical bone formation in specific areas. In rats fed the soft diet, smaller increase in bone density represented an adaptation process in areas characterised by a lower bone apposition rate. The forces produced by the passive stretching of the masseter muscle affected the skeletal growth pattern and dental eruption. Weaker forces possibly produced by passive stretching of hypofunctional muscles resulted in more eruption of the upper molars and less inhibition of periosteal bone apposition in the angular region. Length adaptation in the masseter muscle through lengthening of the aponeurosis and dentofacial growth adaptation possibly decreased passive forces applied to teeth and skeletal structures, particularly in rats with higher functional demands. This may have caused a gradually decreasing effect of the appliance.

[Comparative study of masticatory muscles in the suncus and the mouse]

The external forms of head of the adult suncus and mouse resemble each other, but those of their newborns differ. In the newborn suncus, the upper view of external form of the head is narrow in width and long antero-posteriorly as is the cranial bone, but the adult form is almost triangular in shape based on the posterior portion of the head. In contrast, the external forms of the newborn and the adult are similar in the mouse. The postnatal changes of external form seem to be related to the development of the masticatory muscles in the suncus. Therefore, the present report comparatively studied the development of the masticatory muscles in suncus and mouse. To examine the developmental change of the volume of muscles on two animals, serial sections from fetus through adult were prepared by a commonly used staining method. Muscle volume was determined by the number of voxcels obtained from an image processor. Endplates stained by AchE staining and the course of muscle fibers of the masseter and temporal muscle (especially the MT1-temporal muscle in suncus) were also studied in the two animals. Ten measuring points on each craniofacial bone related with muscle growth were selected and the lengths between each point were measured to confirm the development of the musculoskeletal system in suncus. In suncus, the suncus masticatory muscles, both MT1-temporal and masseter muscle, were shown to have a multipinnate structure. This type of structure increases the physiological cross-sectional area to increase the masticatory force. The direction of these muscles, as a whole, run anteroposteriorly in this animal. Comparing the growth patterns between newborn and adult, the volumes of temporal and masseter muscle increase in the suncus more than those of in the mouse. The diameters of the temporal and masseter muscle fibers increase greatly after birth in the suncus in comparison with those of the mouse, though the fetus and newborn of both animals have approximately the same diameters. The differences in external forms of the head between newborn and adult in the suncus in comparison with those in the mouse may be caused by the greater increase in the volumes of temporal and masseter muscle, resulting in a remarkable increase in the forces of muscular contractions in the suncus. Postnatal dry weight of each masticatory muscle was measured in the suncus. The weights of the MT1-temporal and masseter muscle increased more than those of other muscles, especially from 14 days through 28 days. The results obtained here thus may be closely correlated to the growth of the diameters of muscle fibers. The whole parietal bone was not covered with the MT1-temporal muscle at 7 days after birth, but was covered at 14 days. Sagittal and nuchal crests were not observed at the former stage but was clearly evident at the latter in the suncus. In the suncus, the length and width of the MT1-temporal muscle became larger in the second week after birth than in the later weeks. In the suncus, during from 7 days through 14 days after birth the growth rate of the portion between Etf and Dwp was enhanced compared to that of other portions. In this term the length of the MT1-temporal muscle also grew noticeably. The distance between Etf and Dwp corresponds to the length of MT1-temporal muscle at the ventral portion. The growth between Etf and Dwp precedes that of muscle volume in the suncus. In conclusion, the developmental change of the cranial bone between Etf and Dwp precedes that of the masticatory muscles in the suncus. The formation of the well-developed sagittal and nuchal crests and the expansion of muscle attachment may well have led to the increase of muscle volume. As a result, the difference of external forms of head between newborn and adult in the suncus has been shown clearly to be due to the developmental changes of the volume of each masticatory muscle from 14 days through adult after birth, while such a difference is not seen in mouse.

A longitudinal electromyographic study of the postnatal maturation of mastication in the rabbit

At 2 weeks of age, infant rabbits show chewing movements that resemble those of the adult animal. Previous studies have shown that, at that stage, the accompanying masticatory motor pattern is clearly similar to the suckling motor pattern. As early as 4 weeks, chewing muscle activity is indistinguishable from the adult chewing motor pattern. These reports suggest that the adult chewing motor pattern is developed from the suckling motor pattern. In this study, the chewing motor pattern in the intermediate period (between 2 and 4 weeks of age) was investigated by means of fine-wire electromyography and jaw tracking. Maturation of masticatory movements was found to have two phases. Maximum gape increased in the first few days and was followed by strong development of transverse jaw excursions after the age of 17 days. The increase in jaw excursions was brought about by changes in motor behaviour and facilitated by the development of smooth occlusal surfaces. The changes in motor behaviour were: (1) the level of activity of the balancing-side muscles became more equal to that of the working side; (2) the timing of digastric muscle activity became asymmetrical at the age of 17 days; (3) the peak activity of masseter, temporalis, medial pterygoid and lateral pterygoid muscle portions was gradually shifted or prolonged into the power-stroke phase. It can be concluded that the masticatory contraction pattern shifts from one derived from the suckling contraction pattern at the age of 14 days to one almost similar to the adult chewing pattern at the age of 23 days.

Why fuse the mandibular symphysis? A comparative analysis

Fused symphyses, which evolved independently in several mammalian taxa, including anthropoids, are stiffer and stronger than unfused symphyses. This paper tests the hypothesis that orientations of tooth movements during occlusion are the primary basis for variations in symphyseal fusion. Mammals whose teeth have primarily dorsally oriented occlusal trajectories and/or rotate their mandibles during occlusion will not benefit from symphyseal fusion because it prevents independent mandibular movements and because unfused symphyses transfer dorsally oriented forces with equal efficiency; mammals with predominantly transverse power strokes are predicted to benefit from symphyseal fusion or greatly restricted mediolateral movement at the symphysis in order to increase force transfer efficiency across the symphysis in the transverse plane. These hypotheses are tested with comparative data on symphyseal and occlusal morphology in several mammals, and with kinematic and EMG analyses of mastication in opossums (Didelphis virginiana) and goats (Capra hircus) that are compared with published data on chewing in primates. Among mammals, symphyseal fusion or a morphology that greatly restricts movement correlates significantly with occlusal orientation: species with more transversely oriented occlusal planes tend to have fused symphyses. The ratio of working- to balancing-side adductor muscle force in goats and opossums is close to 1:1, as in macaques, but goats and opossums have mandibles that rotate independently during occlusion, and have predominantly vertically oriented tooth movements during the power stroke. Symphyseal fusion is therefore most likely an adaptation for increasing the efficiency of transfer of transversely oriented occlusal forces in mammals whose mandibles do not rotate independently during the power stroke.

Cellular and molecular aspects of muscle growth, adaptation and ageing

Although major advances have been made over the past few decades in prosthetic dentistry, deterioration in oral function and altered facial appearance are still common accompaniments of ageing. Molecular biology methods now allow us to understand these age-related changes at the level of gene expression. Muscle loss as well as bone loss still present major problems, the magnitude of which increases as the age profile of our society changes. Both muscle and bone tissue respond to mechanical signals for which bone depends on muscle and for muscle, stretch has been shown to be important as it induces protein synthesis and an increase in girth as well as length of the muscle fibres. The latter involves the production of more sarcomeres in series so that the jaw muscles adapt to a new functional length following changes in vertical dimension of occlusion. It also determines the postural position of the lower jaw. In our investigations into the control of muscle mass we have recently cloned a growth factor which is expressed in exercised and/or overloaded muscles. This comes in two forms: an autocrine or local form and a paracrine or systemic form. Both growth factors influence muscle growth markedly and it is probable that the systemic type is also involved in maintenance of bone. The discovery of these growth factors provides the mechanisms by which mechanical signals are transduced into chemical signals that in turn regulate gene expression and protein synthesis.

The function of the disco-muscular apparatus in the human temporomandibular joint

The morphology and function of the disco-muscular apparatus of the human TMJ is a controversial subject. Connections between the muscles which move the mandible and the "disco-capsular complex" have been described in a contradictory way. The disco-muscular apparatus is also described as being more extensive than that of the M. pterygoideus alone to include to the Mm. temporalis and masseter. However, the involvement of the latter is considered to be a peripheral variation of the normal anatomy and of little, if any, functional significance. The existence of independent relationships between the deep portions of the masseter and temporal muscles and the disco-capsular apparatus of the human TMJ is rarely discussed or explained. The morphologic findings were derived from fixed and unfixed human temporomandibular joints (TMJ) of varying ages and both sexes, whereby the functional maturity of the masticatory apparatus was taken into consideration. The results of the study show that aside from fibers originating from the superior venter of the M. pterygoideus lateralis, additional muscle or connective tissue fibers from the perimysium of the M. masseter are inserted to varying extents into the disc. The same is true for the M. temporalis, which is also directly connected to the disc via muscular or fibrous elements, or indirectly via fibers from the M. masseter. The insertion of the M. pterygoideus lateralis is always in the medial portion of the Discus articularis and those of the Mm. temporalis and masseter in the middle and lateral portions of the disc respectively. It is highly probable that a direct force transfer through the Mm. temporalis and masseter to the articular disc takes place, and that these muscles contribute to the movement of the disc during jaw movement, whereas the size and form of the muscle insertions are subject to a great deal of individual variation.

Developmental changes in enzyme activities and in structural features of rat masticatory muscle mitochondria

The functional ability of a muscle is closely related to the activities of the mitochondria, which are energy-producing organelles in muscle cells. The development of the mammalian masticatory muscle progresses dramatically when feeding behavior changes from suckling to mastication, but it is unclear how the energy-producing systems of the mitochondria change. In this paper, the development of rat masticatory muscle mitochondria was investigated in terms of enzyme activities of the mitochondrial respiratory chain and the structural and numerical development of mitochondria, especially regarding the change in feeding behavior from suckling to mastication. Using isolated mitochondria from the masticatory muscle, we measured succinate dehydrogenase, NADH dehydrogenase, succinate-O2 oxidoreductase, and NADH-O2 oxidoreductase. These were found to be increased in the 15-day postnatal rat compared with the 0- to 10-day postnatal rat. The structural development of mitochondria was gradual in the 0- to 15-day postnatal rat. However, a notable increase was found in the cross-sectional area of mitochondria between 10 and 15 days postnatally. The number of mitochondria per muscle fiber was apparently constant during the same period. We demonstrated that the change in feeding behavior was well-correlated with an increase in mitochondrial enzyme activity, also supported by the early structural development of mitochondria.

Skeletal fiber types and spindle distribution in limb and jaw muscles of the adult and neonatal opossum, Monodelphis domestica

The South American opossum, Monodelphis domestica, is very immature at birth, and we wished to assess its potential for studies of jaw muscle development. Given the lack of prior information about any Monodelphis fiber types or spindles, our study aimed to identify for the first time fiber types in both adult and neonatal muscles and the location of spindles in the jaw muscles. Fiber types were identified in frozen sections of adult and 6-day-old jaw and limb muscles by using myosin ATPase and metabolic enzyme histochemistry and by immunostaining for myosin isoforms. The distribution of fiber types and muscle spindles throughout the jaw-closer muscles was identified by immunostaining of sections of methacarnoy-fixed, wax-embedded heads. Most muscles contained one slow (type I) and two fast fiber types (equivalent to types IIA and IIX), which were similar to those in eutherian muscle, and an additional (non-IIB) fast type. In jaw-closer muscles, the main extrafusal fiber type was IIM (characteristic of these muscles in some eutherians), and almost all spindles were concentrated in four restricted areas: one in masseter and three in temporalis. Six-day neonatal muscles were very immature, but future spindle-rich areas were revealed by immunostaining and corresponded in position to the adult areas. Extrafusal and spindle fiber types in Monodelphis share many similarities with eutherian mammalian muscle. This finding, along with the immaturity of myosin isoform expression observed 6 days postnatally, indicates that Monodelphis could provide a valuable model for studying early developmental events in the jaw-closer muscles and their spindles.

Jaw muscle development as evidence for embryonic repatterning in direct-developing frogs

The Puerto Rican direct-developing frog Eleutherodactylus coqui (Leptodactylidae) displays a novel mode of jaw muscle development for anuran amphibians. Unlike metamorphosing species, several larval-specific features never form in E. coqui; embryonic muscle primordia initially assume an abbreviated, mid-metamorphic configuration that is soon remodelled to form the adult morphology before hatching. Also lacking are both the distinct population of larval myofibres and the conspicuous, larval-to-adult myofibre turnover that are characteristic of muscle development in metamorphosing species. These modifications are part of a comprehensive alteration in embryonic cranial patterning that has accompanied life history evolution in this highly speciose lineage. Embryonic 'repatterning' in Eleutherodactylus may reflect underlying developmental mechanisms that mediate the integrated evolution of complex structures. Such mechanisms may also facilitate, in organisms with a primitively complex life cycle, the evolutionary dissociation of embryonic, larval, and adult features.

Craniofacial and TMJ effects after glutamate and TRH microsphere implantation in proximity to trigeminal motoneurons of growing rats

The sequelae of sustained, in vivo delivery of two important neurotransmitter substances, glutamate and thyrotropin-releasing hormone (TRH), upon craniofacial growth and development have previously not been investigated. Our purpose was to document and compare the relative effects of glutamate and TRH microspheres stereotactically placed in proximity to trigeminal motoneurons within the trigeminal motor nucleus. The following null hypotheses were tested: (1) TRH microspheres in proximity to trigeminal motoneurons have no significant effect upon the craniofacial skeleton, and (2) there are no significant differences between the relative effects of chronic, long-term delivery of glutamate and TRH upon the neuromusculoskeletal system of growing rats. Forty male Sprague-Dawley rats were divided into 4 experimental groups (glutamate microspheres, TRH microspheres, blank microspheres, sham surgeries) and underwent stereotactic neurosurgery at 35 days; 5 rats of each group were killed at 14 and 21 days for data collection. Histology revealed that implants were clustered in the pontine reticular formation, close to the ventrolateral tegmental nucleus. Both glutamate and TRH rats had implant-side deviation of their facial skeleton and snout regions; 4 x 2 ANOVA and post hoc t-tests revealed significant (P < or = 0.05, 0.01) differences between groups and sides for motoneuron count, muscle weight, and osteometric data. TRH rats also demonstrated larger implant-side TMJ discs and mandibular fossae in comparison with the other groups. The stated null hypotheses were therefore rejected.

Maturation of static sensitivity is related to expansion of the capsular space in rat buccal stretch receptors

The developmental relationship between static sensitivity and structure of the buccal stretch receptor (BSR) in rats was investigated. When responses to ramp-and-hold stretches were recorded from isolated BSRs, their static sensitivities suddenly increased between 2 and 4 weeks after birth. However, no apparent change was observed throughout other developmental stages examined. Electron microscopic examination revealed a conspicuous expansion of the fluid-filled capsular space caused by completion of the outer capsule during the same postnatal period. These findings suggest that an increase in the capsular space of BSR may be involved in increasing the static sensitivity of this receptor.

Distribution of the macromolecular components of masticatory muscles during differentiation of the muscle fibers in the postnatal rat

The distribution of collagen fibers of rat masticatory muscles during the postnatal period (two weeks), was investigated by electrophoresis and immunohistochemistry. At these stages, the myosin of rat masticatory muscles displays specific electrophoretic patterns. Comparison of the myosin patterns of these muscles allows their identification. 1) Analysis by SDS-PAGE indicated that one of three weakly reactive stainable proteins with lower mobility than the heavy chain of myosin disappeared from the temporal muscle on day 13, as compared with other masticatory muscles. However, in histochemical analysis of the muscle fibers, the reaction specific for succinic dehydrogenase (SDH) activity was strong, and the fibers on day 13 could be classified into two types with respect to SDH activity. By contrast, on day 0, the fibers were classified into two types with respect to myosin ATPase activity. 2) Immunohistochemical analysis indicated that the distribution of the components of the extracellular matrix in the epimysium (type I collagen), perimysium (type I collagen, fibronectin, and laminin) and endomysium (type III collagen, fibronectin, laminin, and tenascin) was related to the metabolic capacity on days 12 to 13. The variability in the types of myosin and in proteins of the extracellular matrix might be important during the development of rat masticatory muscles.

Age changes in mastication in the pig

A comparative study of chewing in miniature pigs at three stages of dental development was carried out using electromyography and movement analysis. The mastication of the youngest pigs was characterized by longer burst durations of jaw muscles, particularly closing muscles, and greater relative jaw opening. Chewing side alternated irregularly and burst durations were variable. Between the intermediate and oldest ages, the parameters remained almost unchanged. Because the amount of dental development that occurred between the second two stages was just as extensive as that between the first two, progressive eruption cannot account for the age changes observed in mastication.

Morphology and analysis of the development of the human temporomandibular joint and masticatory muscle

The calcification levels of the mandible and the temporal bone of human fetuses, which ranged from 12 to 32 weeks of gestation, were systematically investigated with a soft X-ray analyzer linked to an image analyzer. The profile of the condylar process (head) revealed high levels of calcification, in contrast to that in the mandibular fossa of the temporal bone. The basal portion of the condylar process and the mandibular notch exhibited moderate calcification from 12 weeks of gestation. The weight and the cross-sectional areas of the muscle and the muscle fibers in masticatory muscles (masseter, temporal, medial, and lateral pterygoid muscles) are all increased gradually during development from 12 to 32 weeks of gestation. These changes in calcification and in cross-sectional area of muscle suggest that muscle development may be related to bone calcification during formation of the mandible.

Mandibular elevator muscles: physiology, action, and effect of dental occlusion

In spite of differences in embryologic origin, central nervous organization, and muscle fiber distribution, the physiology and action of mandibular elevator muscles are comparable to those of skeletal muscles of the limbs, back, and shoulder. They also share the same age-, sex-, and activity-related variations of muscular strength. With respect to pathogenesis, the type of muscular performance associated with the development of fatigue, discomfort, and pain in mandibular elevators seems to be influenced by the dental occlusion. Clinical research comparing the extent of occlusal contact in patients and controls as well as epidemiologic studies have shown reduced occlusal support to be a risk factor in the development of craniomandibular disorders. In healthy subjects with full natural dentition, occlusal support in the intercuspal position generally amounts to 12-14 pairs of contacting teeth, with predominance of contact on first and second molars. The extent of occlusal contact clearly affects electric muscle activity, bite force, jaw movements, and masticatory efficiency. Neurophysiologic evidence of receptor activity and reflex interaction with the basic motor programs of craniomandibular muscles tends to indicate that the peripheral occlusal control of the elevator muscles is provided by feedback from periodontal pressoreceptors. With stable intercuspal support, especially from posterior teeth, elevator muscles are activated strongly during biting and chewing with a high degree of force and masticatory efficiency, and with relatively short contractions, allowing for pauses. These variables of muscle contraction seem, in general, to strengthen the muscles and prevent discomfort. Therefore, occlusal stability keeps the muscles fit, and enables the masticatory system to meet its functional demands.

Anatomical and electrotonic coupling in developing genioglossal motoneurons of the rat

Dye-, tracer- and electrotonic coupling were studied independently in genioglossal (GG) motoneurons using intracellular recordings in in vitro brainstem slices from rats postnatal ages 1-30 days. The subpopulation of GG motoneurons were retrogradely labeled after an injection of dextran-rhodamine into the posterior tongue. Dye-coupling was studied with Lucifer yellow injected into 55 motoneurons and tracer-coupling with neurobiotin injected into 89 presumptive GG motoneurons. Of the motoneurons injected with Lucifer yellow, only 6 of 41 cells (16.2%) exhibited dye-coupling; all occurred in animals less than 9 days old. In all but one instance, dye-coupling was restricted to only one other cell. No evidence of dye-coupling was found in the 14 cells injected in animals older than 8 days. Tracer-coupling (neurobiotin) was demonstrated in 12 of 30 cells (40%) from animals 1-2 days old and in 6 of 21 cells (28.6%) from animals 3-8 days old. Of the remaining 38 cells from animals 10 days of age and older, only one cell was found to be tracer-coupled. Cells injected with neurobiotin were coupled to an average of two other cells. Electrotonic coupling, as demonstrated with a short latency depolarization (SLD) in response to stimulation of hypoglossal axons, was found in developing GG motoneurons. These SLDs were revealed in 17 of 40 GG motoneurons (42.5%) examined in 1-8-day-old animals. There were no SLDs recorded in the 10 cells examined from animals of 10 days and older. The significance of coupling relative to patency of the newborn upper airways is discussed.

Biomechanical changes in the rabbit masticatory system during postnatal development

Using dissection, biometry, and two three-dimensional mechanical models, the postnatal changes of the rabbit masticatory muscles were studied by analyzing their three-dimensional orientation, their strength and fiber lengths, and certain functional consequences of these changes. The first mechanical model uses length-tension relationships of the muscles and predicts the maximum bite force as a function of mandibular position. It shows that young rabbits are able to generate large bite forces at a wider gape than adult animals and that the forces are directed more vertically. In spite of the postnatal changes the mechanical advantage of the system remains about equal. However, the muscles are reoriented so that they exert a larger degree of parallel action, suggesting a larger bite force magnitude but a smaller range of bite force directions. The second model-predicts this range. It shows that during postnatal development a relative gain occurs in the possibilities for the system to exert forces directed rostrodorsally. In all other directions the capability to exert force decreases. The results suggest that during development the possibility of the system to generate large bite forces is increased at the cost of a restriction in the range of jaw excursion and that a restriction takes place in the range of possible force directions that can be exerted at the molars.

[Masticatory muscles of domestic sheep and swine in ontogenesis]

Age changes of morphometrical parameters of the masticatory muscles have been analyzed in domestic sheep and pigs of white large breed in the following age groups: 2-, 3-, 4-month-old fetuses, newborns, 4-month-old lambs, 10-month-old pigs, 18-month-old lambs, mature she-sheep and brood-sows. Uneven weight growth of the masticatory muscles in the sheep and pigs during the prenatal ontogenesis should be considered as a consequence of recapitulation of their phylogenesis, and in the postnatal ontogenesis it depends on changes in life conditions, type of nutrition, character of food and type of life. In newborn sheep the digastric, lateral, pterygoid and temporal muscles grow intensively, and in pigs--medial pterygoid and temporal ones. When they pass to roughage, in the former the mass of the musculus masseter major and medial pterygoid muscle increases, and in the latter--that of the musculus masseter major and temporal one. The masticatory muscles of the species studied increase in their mass especially intensively during the middle of the prenatal ontogenesis and during suckling period of their development. This should be taken into consideration in stock-breeding practice. In domestic pigs there is only one muscular belly in the digastric muscle. In sheep there are two bellies, separated one from another by means of a tendinous intersection, owing to crossing of the latter by the stylohyoid muscle.

Development of masticatory muscles and oral behavior from suckling to chewing in dogs

1. At the age of 20 postnatal days, more than a half of the dogs (13/20) showed incisor eruption. At the age of 25 postnatal days, half of the dogs (10/20) showed molar eruption. 2. At the age of 24 postnatal days, half of the dogs (10/20) started chewing behavior. 3. EMG amplitudes of temporal muscle were larger than that of masseter muscle until 26.5 days after birth, but they were reversed at 26.5 days after birth. 4. This suggests that in dogs the earlier teeth erupted, the earlier the dominant oral behavior was changed from suckling to mastication, and concomitantly the dominant muscle for the oral behavior was changed from the temporal to the masseter muscle.

[The effects of solid or liquified diet on the submandibular glands of mice with age]

The influence of properties of the diets on the retardation of growth of the jaw bones and masticatory muscles has been established. As the intake of liquid foodstuffs increases in the human diet, it is considered possible that these liquid diets may retard the growth of human salivary glands. 180 newborn C3H/He strain male mice were divided into solid and liquid diet groups and then the development and aging of the submandibular glands were observed both in function and morphologically until 60 weeks of age. The following results were obtained: 1. No differences could be detected in body length and body weight of the mice between solid and liquid diet groups. The wet weight of the liver in the liquid diet group was significantly larger than the solid diet group, which suggested that the liquid diet may have accelerated metabolic functions. Additionally, masticatory muscles of the liquid diet group were significantly smaller than the solid diet group which suggested growth retardation of the masticatory organ. 2. The submandibular gland wet weight and its DNA content in the liquid diet group were lower than in the solid diet group during the experimental period. This suggested that the number of cells and their ability to divide was diminished. The wet weight of glands per unit DNA content showed an increase in cell size during the adult period. 3. The alpha-amylase activity and total protein content of the submandibular glands in the liquid diet group were lower than in the solid diet group. This suggested that the saliva secretory function and protein synthesis were depressed, in agreement with the low number of cells detected. 4. The differentiation of the acinar cells and convoluted tubule cells of the liquid diet group was retarded and the 60 weeks subgroup showed hypertrophy of the convoluted tubule cells, disappearance of the secretory granules and a decrease in the number of acinar cells. 5. Slight stromal fibrosis was observed with increasing age (60 weeks) with no difference between the two groups. 6. This suggested that when mice were fed a liquid diet, the development of salivary glands was retarded and also aging changes occurred more rapidly.

Growth patterns of the rabbit masticatory muscles

The post-natal growth of the masticatory muscles in the rabbit was examined. By means of anatomical dissection and measurement, total muscle length, muscle fiber length, and muscle weight were determined in animals varying in age between one week and 36 months and exhibiting a 50-fold weight increase. Growth data were fitted by linear regression models with facial skull length used as the independent variable. Many deviations occur from size-dependent isometric growth. The muscles can be divided into three groups, according to their pattern of weight increase: The jaw openers grow negatively allometrically, and their contribution to total muscle weight decreases with time; the temporal muscle grows negatively allometrically, but its relative weight proportion remains about the same; the masseter and medial pterygoid muscles have positively allometric growth, and their contribution to total muscle weight increases strongly. Generally, the length of the muscles and of their fibers increases at lower rates than does the length of the facial skull. After weaning, the rate of longitudinal growth drops steeply in some muscles. Total fiber area or physiological cross-section (PCS) of muscles is computed from weight and fiber length. It increases positively allometrically in the jaw closers and negatively allometrically in the jaw openers. In the lateral pterygoid muscle, the increase of PCS changes from negatively- to positively-allometric growth after weaning. The study demonstrates that individual oral muscles follow different patterns of longitudinal and cross-sectional growth, so that their functional capacities (force, range of contraction) and mutual functional relationships are age-dependent.

Effects of easily chewable diet and unilateral extraction of upper molars on the masseter muscle in developing mice

The effects of easily chewable diets and unilateral extraction of upper molars on the masseter muscle were studied in developing mice. A liquid diet requiring no mastication suppressed the development of the masseter muscles more than a fine-grained diet, and extraction of unilateral upper molars also caused inhibition of muscle development. Moreover, both unilateral extraction of upper molars and a liquid diet had an additive effect on the suppression of the postnatal development of the masseter muscle, and bilateral suppression of the development of the masseter muscle was induced following unilateral extraction of upper molars. These findings suggest that the sensory input from the sensory endings in the periodontal ligament may also play an important role in the postnatal development of the masseter muscle and that there may be some crossing pathways to convey the sensory input coming from the side of the extracted upper molars to the contralateral motor neurons via the interneuronal circuits.

Immunocytochemical analysis of the perinatal development of cat masseter muscle using anti-myosin antibodies

The developmental changes in myosin gene expression in the masseter muscle of embryonic and juvenile kittens were examined immunocytochemically using anti-myosin heavy chain antibodies of various specificities. In the mature cat, this muscle contains only two phenotypes, the majority of fibres are superfast, the rest being slow fibres. In foetal tissues, the histological appearance of bundles of myotubes, comprising a large central myotube surrounded by a rosette of smaller myotubes, strongly suggest the existence in the jaw muscle of primary and secondary fibres during development. Immunocytochemical data are consistent with the hypothesis that there are four types of fibre; two types of primary fibre as well as two types of secondary fibre. (1) Slow primaries stain strongly with an anti-slow myosin antibody throughout the period under study. These fibres transiently express embryonic but not foetal myosin. (2) Superfast primaries stain for embryonic/foetal and slow myosins in the perinatal period but progressively replace these myosins with superfast myosin during postnatal development. (3) Superfast secondaries initially express embryonic/foetal myosins, but later, beginning around the time of birth progressively replace these myosins with superfast myosin. These fibres do not express slow myosin. (4) Slow secondaries, which initially also express embryonic/foetal myosins, but which postnatally express slow or slow and superfast myosins and express only slow myosin in the adult. These four types of fibres are homologous to the four isotypes of limb muscle fibres and may be derived from distinct lineages of myoblasts.

Length-tension relationships of masseter and digastric muscles of miniature swine during ontogeny

At incremental whole muscle lengths, active isometric and passive elastic forces were recorded from the masseter and digastric muscles of anaesthetized miniature pigs (Hanford) weighing 2.0-20.0 kg. Wet muscle mass and maximum tetanic tension values for masseter exceed those for digastric and increase more rapidly with body mass (age). At any body mass, masseter exceeds digastric in the ratio of optimum length (that length at which maximum tetanic tension is produced) to in situ muscle length (that length which corresponds to the jaw in a closed position) and the proportion of passive tension comprising total (passive plus active) tension. Passive elastic tension begins to rise in masseter at lengths as short as 87% of optimum (in younger pigs). In digastric, passive tension is absent until the muscle is stretched to a length slightly longer than optimum in younger pigs but occurs at shorter lengths in older pigs. Contractile properties explain functional differences between masseter and digastric more clearly than they explain ontogenetic changes in either muscle. The behavioural transition from infant suckling to adult mastication of solid food is best characterized by a disproportionate increase in mass (and force) of the masseter, relative to digastric, and increased reliance upon active (rather than passive) tension.

Feeding behavior in mammals: corticobulbar projection is reorganized during conversion from sucking to chewing

It is known that repetitive stimulation of the frontal cortex (cortical masticatory area, CMA) induces rhythmical jaw movements similar to chewing in adult mammals. In the present study we were able to induce rhythmical jaw movements similar to sucking by repetitive stimulation of the frontal cortex in neonatal guinea pigs. This area, which we named the cortical sucking area (CSA), was located rostral to the CMA which was later formed upon maturation. Neurons of the CSA were shown electrophysiologically and morphologically to project primarily to the dorsal part of the paragigantocellular reticular nucleus of the contralateral side. This was the site which the CMA neurons, later, projected to induce chewing. It is generally thought that tooth eruption triggers the conversion from sucking to chewing. However, guinea pigs are born with a complete permanent dentition and therefore devoid of this peripheral trigger for the conversion to chewing. Accordingly we propose that shift of the cortical projection area from the CSA to the CMA during the maturation causes the conversion of the mammalian feeding behavior. It is discussed that this transition involves extensive reorganization of the cortical efferent system including the pyramidal tract during early postnatal development.

Growth allometry of craniomandibular muscles, tendons, and bones in the laboratory rat (Rattus norvegicus): relationships to oromotor maturation and biomechanics of feeding

This study addressed the problem of how growth of craniomandibular muscles, tendons, and bones influences the acquisition of oromotor skills and biomechanics of feeding in the laboratory rat (Rattus norvegicus). Rats representing a 6.6-fold size range were dissected, and muscles, tendons, and mandibles were weighed. Cross-sectional areas of tendons and bones providing attachment surfaces for muscles were estimated. Ontogenetic scaling of craniomandibular muscles, tendons, and bones was described by using linear regression models, and departures from size-required compensations were used to characterize changes in oromotor function. A two-dimensional model was developed which permitted calculation of mechanical advantages of four masticatory muscles; the model was used to show how mandibular growth and tooth eruption influence the biomechanics of rat feeding. Relative to mandible weight, most jaw muscles scaled either isometrically or positively, tendon cross-sectional areas scaled isometrically or negatively, and bone surfaces scaled negatively. With the exception of the superficial masseter and internal pterygoid muscles, mechanical advantages did not change significantly during mandible growth. Growth patterns of craniomandibular muscles, tendons, and bones contribute significantly to changes in morphology and oromotor function.

The growth of the skull and jaw muscles and its functional consequences in the New Zealand rabbit (Oryctolagus cuniculus)

Between weaning and adulthood, the length and height of the facial skull of the New Zealand rabbit (Oryctolagus cuniculus) double, whereas much less growth occurs in the width of the face and in the neurocranium. There is a five-fold increase in mass of the masticatory muscles, caused mainly by growth in cross-sectional area. The share of the superficial masseter in the total mass increases at the cost of the jaw openers. There are changes in the direction of the working lines of a few muscles. A 3-dimensional mechanical model was used to predict bite forces at different mandibular positions. It shows that young rabbits are able to generate large bite forces at a wider range of mandibular positions than adults and that the forces are directed more vertically. In young and adult animals, the masticatory muscles differ from each other with respect to the degree of gape at which optimum sarcomere length is reached. Consequently, bite force can be maintained over a range of gapes, larger than predicted on basis of individual length-tension curves. Despite the considerable changes in skull shape and concurrent changes in the jaw muscles, the direction of the resultant force of the closing muscles and its mechanical advantage remain stable during growth. Observed phenomena suggest that during development the possibilities for generation of large bite forces are increased at the cost of a restriction of the range of jaw excursion.