Architecture of the masticatory apparatus in eastern raccoons (Procyon lotor lotor)

The structure and function of the masticatory apparatus of raccoons resemble those found in carnivores. In this study, the architecture of the skull, dentition, and masticatory apparatus is described, and a model is proposed that suggests a mechanism used by raccoons to reduce different foods. The model suggests that jaw movements are similar to those of cats, the posterior regions of the superficial and deep parts of the temporalis and the anterior region of the medial pterygoid generate horizontal jaw movements, and the anterior portions of the superficial and deep temporalis as well as portions of the masseteric complex generate vertical closing movement. The distributions of slow, fast fatigable, and fast fatigue-resistant fibers for the temporalis and masseteric complex are related to the possible actions of these muscles during mastication, as are the regional cross-sectional areas of the masticatory muscles.     

Zebrafish dentition in comparative context

Studies of the zebrafish (Danio rerio) promise to contribute much to an understanding of the developmental genetic mechanisms underlying diversification of the vertebrate dentition. Tooth development, structure, and replacement in the zebrafish largely reflect the primitive condition of jawed vertebrates, providing a basis for comparison with features of the more extensively studied mammalian dentition. A distinctive derived feature of the zebrafish dentition is restriction of teeth to a single pair of pharyngeal bones. Such reduction of the dentition, characteristic of the order Cypriniformes, has never been reversed, despite subsequent and extensive diversification of the group in numbers of species and variety of feeding modes. Studies of the developmental genetic mechanism of dentition reduction in the zebrafish suggest a potential explanation for irreversibility in that tooth loss seems to be associated with loss of developmental activators rather than gain of repressors. The zebrafish and other members of the family Cyprinidae exhibit species-specific numbers and arrangements of pharyngeal teeth, and extensive variation in tooth shape also occurs within the family. Mutant screens and experimental alteration of gene expression in the zebrafish are likely to yield variant tooth number and shape phenotypes that can be compared with those occurring naturally within the Cyprinidae. Such studies may reveal the relative contribution to trends in dental evolution of biases in the generation of variation and sorting of this variation by selection or drift.     

Report of inaccuracies in the study of the dentition of mammals

After discussing the existing view points and on the base of author's own studies, the designation of premolars and molars is proposed in the deciduous and permanent dentition of placental mammals, corresponding to the function, form, and structure of the teeth. With the admittance of the molars in the deciduous dentition of mammals, the discrepancy between milk dentition of mammals and humans is eliminated. In this way, the molars of deciduous dentition appeared to be the precursors of premolars and molars of permanent dentition. The separate groups of teeth are differentiated depending on the way of life and kind of consumed food, according to position and function of the teeth in the dentition. Lacerating teeth, premolars are formed in carnivora mammals for tearing off the food, and in case of intensive masticatory function of herbivora and omnivora, very likely, from the same germs of premolars, the molars are formed.     

Cranial ontogeny, diet, and ecogeographic variation in African lorises

A series of 22 craniodental measurements were obtained for the three subspecies of potto (Perodicticus) and angwantibos (Arctocebus). To describe patterns of variation in Perodicticus, a discriminant function analysis (DFA) was performed with adult data. To investigate the ecogeographic correlates of size variation in Perodicticus, adult cranial dimensions were compared with field data on latitudinal and longitudinal coordinates for available specimens as well as altitudinal data for a more limited sample. Ontogenetic series for larger-bodied Perodicticus and smaller-bodied Arctocebus were compared to test the hypothesis that inter- and intrageneric variation in skull form results from the differential extension/truncation of shared patterns of relative growth, and to assess morphological variation in the masticatory complex of sister taxa with differing dietary habits. Analyses of relative growth indicate that skull proportions in Perodicticus subspecies are largely ontogenetically scaled. In comparisons between Perodicticus and Arctocebus, most facial dimensions also are ontogenetically scaled, with all but one of the seven divergent comparisons (interorbital breadth) representing a feature of the masticatory apparatus. The DFA provided independent support for prior classifications of Perodicticus into three taxa. Size differentiation in African lorises appears to be correlated with altitudinal variation (Bergmann's Rule) as well as character displacement. The smallest pottos, P. p. potto, occupy low-lying coastal habitats in western Africa, whereas the larger, eastern forms inhabit higher, presumably colder elevations. The largest potto, P. p. edwardsi, is sympatric throughout most of its range with the smallest and most insectivorous African lorises (Arctocebus). A basis for intrageneric taxonomic variation in Perodicticus is supported by such nonclinal size variation, as well as divergences in the ontogeny of masticatory proportions corresponding to interspecific variation in dietary proclivities.     

Dietary correlates of temporomandibular joint morphology in New World primates

Previous analyses of the masticatory apparatus have demonstrated that the shape of the temporomandibular joint (TMJ) is functionally and adaptively linked to variation in feeding behavior and diet in primates. Building on previous research, this study presents an analysis of the link between diet and TMJ morphology in the context of functional and dietary differences among New World primates. To evaluate this proposed relationship, I used three-dimensional morphometric methods to quantify TMJ shape across a sample of 13 platyrrhine species. A broad interspecific analysis of this sample found strong relationships among TMJ size, TMJ shape, and diet, suggesting that both size and diet are significant factors influencing TMJ morphology in New World primates. However, it is likely that at least some of these differences are related to a division of dietary categories along clade lines.A series of hypotheses related to load resistance capabilities and range of motion in the TMJ were then tested among small groups of closely related taxa with documented dietary differences. These pairwise analyses indicate that some aspects of TMJ morphology can be used to differentiate among closely related species with different diets. However, not all of my predictions were upheld. The anteroposterior dimensions of the TMJ were most strongly consistent with hypothesized differences in ingestive/masticatory behaviors and jaw gape, whereas the predictions generated for variation in entoglenoid and articular tubercle height were not upheld. These results imply that while some features can be reliably associated with increased load resistance and facilitation of wider jaw gapes in the masticatory apparatus, other features are less strongly correlated with masticatory function.     

Molecular strategies in the evolution of mammalian dental patterning

The acquisition of masticatory capability by mammals allowed a better processing of food and a consequent increase in the efficiency of nutrients intake by the digestive system. The development of tooth classes and variations in tooth number can be considered intrinsic characteristics of mammalian dentition. These features allowed species to develop specialized dentitions, creating new adaptive zones. Comparative developmental data from knockout mutant mice and human tooth agenesis present new insights on the molecular strategies that permitted rapid phenotypic differentiation, adaptation and speciation of mammalian dentition.     

Anatomo-anthropological typology of the jaws in man

Basing on the morphological signs selected in the morphologically contrast samples of the facial skeleton, 27 variants of the upper and lower jaws have been obtained for each, characterizing interconnection and interdependence of several morphological signs. Into each of these variants people of various races and sex, with massive or gracile masticatory apparatus can get. However, the reduction degree, directness, temps and successiveness of reorganizations together with exogenic influence within the limits of the normal reaction in various parts of the masticatory apparatus of the individuals, that belong to the same variant, is identical. Thus, an anatomo-anthropological typology of the dento-maxillary apparatus is obtained, owing to which it is possible to study and compare: 1--individual morphological peculiarities, 2--regional and ethnic norms in morphology of the jaws, 3--within the 27 variants of the structure selected it is possible to study functional, biochemical and other factors. The results demonstrate, that 27 morphological variants of the jaw structure can serve as anatomical marker for distinguishing 27 risk groups for caries.     

Unification of Criteria for Distinguishing Morphotypes of Cheek Teeth in Lemmings (Lemmini,Arvicolinae, Rodentia)

Biology Bulletin - This paper summarizes the data on morphotype dental patterns in recent lemmings (Lemmini) and their earliest presumed ancestors based on analysis of both the existing literature...     

Homoplasy and the early hominid masticatory system: inferences from analyses of extant hominoids and papionins

Early hominid masticatory characters are widely considered to be more prone to homoplasy than characters from other regions of the early hominid skull and therefore less reliable for phylogenetic reconstruction. This hypothesis has important implications for current reconstructions of early hominid phylogeny, but it has never been tested. In this paper we evaluate the likely veracity of the hypothesis using craniometric data from extant primate groups for which reliable consensus molecular phylogenies are available.Datasets representing the extant large-bodied hominoid genera and the extant papionin genera were compiled from standard measurements. The data were adjusted to minimise the confounding effects of body size, and then converted into discrete character states using divergence coding. Each dataset was divided into four regional character groups: (1) palate and upper dentition, (2) mandible and lower dentition, (3) face and (4) cranial vault and base. Thereafter, the regional character groups were analysed using cladistic methods and the resulting phylogenetic hypotheses judged against the consensus molecular phylogenies for the hominoids and papionins.The analyses indicated that the regions dominated by masticatory characters-the palate and upper dentition, and the mandible and lower dentition-are no less reliable for phylogenetic reconstruction than the other regions of the skull. The four regions were equally affected by homoplasy and were, therefore, equally unreliable for phylogenetic reconstruction. This finding challenges the recent suggestion that Paranthropus is polyphyletic, which is based on the assumption that masticatory characters are especially prone to homoplasy. Our finding also suggests that, contrary to current practice, there is no a priori reason to de-emphasise the phylogenetic significance of the masticatory similarities between Homo rudolfensis and the australopiths. The corollary of this is that H. rudolfensis is unlikely to be a member of the Homo clade and should therefore be allocated to another genus.     

A unique form of dental bilophodonty and a functional interpretation of peculiarities in the masticatory system of Arsinoitherium (mammalia,Embrithopoda)

The highly autapomorphic upper molar bilophodonty of the Oligocene mammal, Arsinoitherium (Embrithopoda) is an extreme form of dilambdodonty effected by lingual positioning of normally buccally situated cusps with reduction of lingual cusps. This effectively limits the molar dentition to a single phase shearing occlusal motion. Molar and premolar morphology is very different, premolars exhibiting high longitudinal ectolophs and typical two phase occlusal morphology. A double faceted mandibular condyle and angular discontinuity between lower molar and premolar dentitions is interpreted as a means of separating premolar from molar occlusion. A bifunctional masticatory system is proposed whereby efficient premolar occlusion is achieved only after a repositioning of the temporomandibular joint. Loss of phase II occlusion in the molars is compensated by maintenance of a crushing/grinding mode in the premolars. This coupled with the ability to maintain high occlusal pressures along the length of the mandible explains the unbroken dental arcade. Arsinoitheres therefore possess an extremely specialised masticatory apparatus and are interpreted as highly selective browsing herbivores.     

The Golgi apparatus and cell polarity: Roles of the cytoskeleton,the Golgi matrix,and Golgi membranes

Membrane trafficking plays a crucial role in cell polarity by directing lipids and proteins to specific subcellular locations in the cell and sustaining a polarized state. The Golgi apparatus, the master organizer of membrane trafficking, can be subdivided into three layers that play different mechanical roles: a cytoskeletal layer, the so-called Golgi matrix, and the Golgi membranes. First, the outer regions of the Golgi apparatus interact with cytoskeletal elements, mainly actin and microtubules, which shape, position, and orient the organelle. Closer to the Golgi membranes, a matrix of long coiled–coiled proteins not only selectively captures transport intermediates but also participates in signaling events during polarization of membrane trafficking. Finally, the Golgi membranes themselves serve as active signaling platforms during cell polarity events. We review here the recent findings that link the Golgi apparatus to cell polarity, focusing on the roles of the cytoskeleton, the Golgi matrix, and the Golgi membranes.     

A MODULAR FRAMEWORK CHARACTERIZES MICRO‐ AND MACROEVOLUTION OF OLD WORLD MONKEY DENTITIONS

The study of modularity can provide a foundation for integrating development into studies of phenotypic evolution. The dentition is an ideal phenotype for this as it is developmentally relatively simple, adaptively highly significant, and evolutionarily tractable through the fossil record. Here, we use phenotypic variation in the dentition to test a hypothesis about genetic modularity. Quantitative genetic analysis of size variation in the baboon dentition indicates a genetic modular framework corresponding to tooth type categories. We analyzed covariation within the dentitions of six species of Old World monkeys (OWMs) to assess the macroevolutionary extent of this framework: first by estimating variance–covariance matrices of linear tooth size, and second by performing a geometric morphometric (GM) analysis of tooth row shape. For both size and shape, we observe across OWMs a framework of anterior and postcanine modules, as well as submodularity between the molars and premolars. Our results of modularity by tooth type suggest that adult variation in the OWM dentition is influenced by early developmental processes such as odontogenesis and jaw patterning. This study presents a comparison of genotypic modules to phenotypic modules, which can be used to better understand their action across evolutionary time scales.     

Morphotypic variability of masticatory surface pattern of molars in the recent and Pleistocene Lemmus and Myopus (Rodentia,Cricetidae) of Europe and Western Siberia

In this article, we present the results of morphological study, applying multivariate statistical analysis, of seven samples of recent Lemmini species (Lemmus lemmus, L. sibiricus, Myopus schisricolor) and eight samples of the ancient Siberian lemmings from the Middle and Late Pleistocene localities, mainly in northeastern Europe. On the basis of studying variability of masticatory surface pattern of molars, 24 morphotypes have been described. The differences in morphotypes frequencies were found between the recent Myopus and Lemmus and between the two Norwegian lemming samples. There are some indications of an evolutionary trend in molar morphology of the Siberian lemmings of the Middle, Late Pleistocene, and the recent of Eastern Europe. The distinctions, at least, between the Middle Pleistocene and modern lemmings exceed a species level.     

Evaluation of a self-assessed screening test for masticatory ability of Taiwanese older adults

doi: 10.1111/j.1741‐2358.2012.00632.x Evaluation of a self‐assessed screening test for masticatory ability of Taiwanese older adults Objectives: The aim of this study was to evaluate a screening test based on a food intake questionnaire to discriminate the masticatory ability of Taiwanese older adults with 20 and more natural teeth and at least 8 functional tooth units (FTUs) from the masticatory ability of those with fewer than 20 natural teeth and 8 FTUs. Materials and Methods: The subjects were 2244 patients (mean age, 60.4±10.0 years) recruited from 23 counties and cities in Taiwan. Information about their demographic data, dentition, and masticatory ability was collected. Masticatory ability was measured for 23 food groups comprising 35 common Taiwanese foods, and receiver operation characteristic curve analysis was performed. Results: The results showed that the final questionnaire included 14 food groups and a subject choosing ’difficult to eat‘ responses for 4 and more of these food groups had the same masticatory ability as individuals with fewer than 20 natural teeth and 8 FTUs. Conclusion: In conclusion, foods that are the most difficult to eat are not necessarily good discriminatory indicators. Hence, the 14‐food group questionnaire can be considered the best screening test for masticatory ability of Taiwanese older adults in terms of the presence of 20 and more natural teeth and at least 8 FTUs.     

Masticatory ability in 80-year-old subjects and its relation to intake of energy,nutrients and food items

Objective: The aim of the present study was to analyse the relationship between masticatory ability (self‐assessed masticatory ability and bite force) and intake of energy, nutrients and food items in a population sample of elderly subjects. Design and Subjects: From a population sample of 80‐year‐old people, 160 individuals (74 men and 86 women) took part in an odontological study. Main Outcome Measures: A dental examination including bite force recording, a questionnaire focusing on self‐assessed masticatory ability, and a dietary interview. Setting: Department of Geriatric Medicine, Göteborg University, Sweden. Results: The dental status among the participants varied much (from edentulous in both jaws ‐ 22% ‐ to more than 20 natural teeth ‐ 30%). The mean maximum bite force was higher in men (165 N) than in women (105 N). Bite force was significantly correlated to the Eichner index and to the number of teeth. One third of the subjects reported no masticatory problem, whereas 18% identified 3 such problems. The intake of energy and nutrients varied much but the means were well above recommended values. The correlations between dental status and bite force on one side and dietary intake on the other side were in general weak and most often statistically non‐significant. Impaired general health and reduced dentition were both associated with more masticatory problems. Conclusion: The examined sample of 80‐year‐old subjects had a great variation in dental status, bite force and self‐assessed masticatory ability, but these factors had only a minor influence on dietary selection and intake, which on average were well above recommended values.     

Using "Mighty Mouse" to understand masticatory plasticity: myostatin-deficient mice and musculoskeletal function

Knockout mice lacking myostatin (Mstn), a negative regulatorof the growth of skeletal muscle, develop significant increasesin the relative mass of masticatory muscles as well as the abilityto generate higher maximal muscle forces. Wild-type and Mstn-deficientmice were compared to investigate the postnatal influence ofelevated masticatory loads due to increased jaw-adductor andbite forces on the biomineralization of mandibular articularand cortical bone, the internal structure of the jaw joints,and the composition of temporomandibular joint (TMJ) articularcartilage. To provide an interspecific perspective on the long-termresponses of mammalian jaw joints to altered loading conditions,the findings on mice were compared to similar data for growingrabbits subjected to long-term dietary manipulation. Statisticallysignificant differences in joint proportions and bone mineraldensity between normal and Mstn-deficient mice, which are similarto those observed between rabbit loading cohorts, underscorethe need for a comprehensive analysis of masticatory tissueplasticity vis-à-vis altered mechanical loads, one inwhich variation in external and internal structure are considered.Differences in the expression of proteoglycans and type-II collagenin TMJ articular cartilage between the mouse and rabbit comparisonssuggest that the duration and magnitude of the loading stimuluswill significantly affect patterns of adaptive and degradativeresponses. These data on mammals subjected to long-term loadingconditions offer novel insights regarding variation in ontogeny,life history, and the ecomorphology of the feeding apparatus.     

Effects of colchicine- and cytochalasin B-treatment on the intracellular distribution of yolk droplets, lipid bodies, and Golgi apparatus of the chick neuroepithelial cells

Treatment with colchicine (antimicrotubular agent) and cytochalasin B (antimicrofilamentous agent) has been used to investigate the possible role played by the cytoskeleton in the maintenance of intracellular distribution of yolk droplets, lipid bodies, and Golgi apparatus of the chick neuroepithelial cells. On the one hand, embryos treated with colchicine showed modifications in their distribution patterns of yolk droplets and lipid bodies, which suggests the involvement of the microtubular integrity of neuroepithelial cells in the maintenance of normal distribution patterns. On the other hand, the close relationships between vitelline and lipid inclusions and Golgi apparatus observed in untreated embryos seems to be kept in the embryos treated with colchicine and cytochalasin B. Moreover, from the effects of colchicine on Golgi apparatus position a possible functional role for the microtubular system in the maintenance of Golgi apparatus polarity in the chick neuroepithelial cells can be proposed. The results provided here constitute new information about the cellular mechanisms involved in chick neurulation.     

Electromyography and mechanics of mastication in the albino rat.

The masticatory apparatus in the albino rat was studied by means of electromyography and subsequent estimation of muscular forces. The activity patterns of the trigeminal and suprahyoid musculature and the mandibular movements were recorded simultaneously during feeding. The relative forces of the individual muscles in the different stages of chewing cycles and biting were estimated on the basis of their physiological cross sections and their activity levels, as measured from integrated electromyograms. Workinglines and moment arms of these muscles were determined for different jaw positions. In the anteriorly directed masticatory grinding stroke the resultants of the muscle forces at each side are identical; they direct anteriorly, dorsally and slightly lingually and pass along the lateral side of the second molar. Almost the entire muscular resultant force is transmitted to the molars while the temporo-mandibular joint remains unloaded. A small transverse force, produced by the tense symphyseal cruciate ligaments balances the couple of muscle resultant and molar reaction force in the transverse plane. After each grinding stroke the mandible is repositioned for the next stroke by the overlapping actions of three muscle groups: the pterygoids and suprahyoids produce depression and forward shift, the suprahyoids and temporal backward shift and elevation of the mandible while the subsequent co-operation of the temporal and masseter causes final closure of the mouth and starting of the forward grinding movement. All muscles act in a bilaterally symmetrical fashion. The pterygoids contract more strongly, the masseter more weakly during biting than during chewing. The wide gape shifts the resultant of the muscle forces more vertically and moreposteriorly. The joint then becomes strongly loaded because the reaction forces are applied far anteriorly on the incisors. The charateristic angle between the almost horizontal biting force and the surface of the food pellet indicates that the lower incisors produce a chisel-like action. Tooth structure reflects chewing and biting forces. The transverse molar lamellae lie about parallel to the chewing forces whereas perpendicular loading of the occlusal surfaces is achieved by their inclination in the transverse plane. The incisors are loaded approximately parallel to their longitudinal axis, placement that avoids bending forces during biting. It is suggested that a predominantly protrusive musculature favors the effective force transmission to the lower incisors, required for gnawing. By grinding food across transversely oriented molar ridges the protrusive components of the muscles would be utilized best. From the relative weights of the masticatory muscles in their topographical relations with joints, molars and incisors it may be concluded that the masticatory apparatus is a construction adapted to optimal transmission of force from muscles to teeth.