Functional morphology of the masticatory musculature of the rodent subfamily microtinae

Postnatal histomorphogenesis of the mandible in the house mouse

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

Mammalian masticatory muscles: homology, nomenclature, and diversification

There is a deep and rich literature of comparative studies of jaw muscles in mammals but no recent analyses employ modern phylogenetic techniques to better understand evolutionary changes that have occurred in these muscles. In order to fully develop and utilize the Feeding Experiments End-user Database (FEED), we are constructing a comprehensive ontology of mammalian jaw muscles. This process has led to a careful consideration of nomenclature and homologies of the muscles and their constituent parts. Precise determinations of muscle attachments have shown that muscles with similar names are not necessarily homologous. Using new anatomical descriptions derived from the literature, we defined character states for the jaw muscles in diverse mammalian species. We then mapped those characters onto a recent phylogeny of mammals with the aid of the Mesquite software package. Our data further elucidate how muscle groups associated with the feeding apparatus differ and have become highly specialized in certain mammalian orders, such as Rodentia, while remaining conserved in other orders. We believe that careful naming of muscles and statistical analyses of their distributions among mammals, in association with the FEED database, will lead to new, significant insights into the functional, structural, and evolutionary morphology of the jaw muscles.

Jaw muscle functional anatomy in northern grasshopper mouse,Onychomys leucogaster, a carnivorous murid

The jaw muscle anatomy of the northern grasshopper mouse, Onychomys leucogaster, was observed and the mechanical basis of the insectivorous/carnivorous adaptations were examined. Compared with Peromyscus maniculatus, a granivorous relative of Onychomys, there is a reduction of some aponeuroses within the masseter deep layer. This characteristic indicates that shearing meat or crushing arthropod exoskeletons requires less occlusal pressure than does grinding plant material. In Onychomys both the anterior and posterior portions of the masseter deep layer are more anterodorsally inclined, so that the line of action of the masseter lies further from the jaw joint than in Peromyscus. A strong incisal bite for killing vertebrates such as other rodents can be produced by a jaw mechanism with the high lever advantage of this muscle, which compensates for the decline in muscle mass. Our quantitative analysis suggests that the disappearance of an aponeurosis along the zygomatic plate in Onychomys decreases the stretch of the corresponding muscle, i.e., the anterior fibers of the masseter deep layer, accompanying jaw opening, and increases the maximum gape necessary for hunting large prey.

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

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

Mechanical advantage of area of origin for the external pterygoid muscle in two murid rodents, Apodemus speciosus and Clethrionomys rufocanus

The actions of masticatory muscles in relation to transverse grinding, associated with forward masticatory movement of the mandible, were investigated by using a mechanical model in the two murid rodents, the Japanese field mouse (Apodemus speciosus: subfamily Murinae) and the gray red-backed vole (Clethrionomys rufocanus: subfamily Arvicolinae). Furthermore, statics of the masticatory system on a sagittal plane while chewing is taking place were also analyzed in these rodents. The inward grinding movements of hemimandibles are generated by the posterior temporalis and internal and external pterygoids in both species. In addition to these muscles, the anterior temporalis also moves the hemimandibles lingually in Apodemus speciosus. The area of origin of the external pterygoid seems more advantageous for transverse grinding in A. speciosus than in Clethrionomys rufocanus. On the basis of the static analysis, the anterodorsal area of origin of the external pterygoid to the upper second and third molars in Clethrionomys rufocanus appears to be an adaptive character to prevent the jaw joints from dislocation during occlusion at a posterior point on the elongated row of cheek teeth.

Comparative functional morphology of mandibular forward movement during mastication of two murid rodents, Apodemus speciosus (Murinae) and Clethrionomys rufocanus (Arvicolinae)

The anatomy of the masticatory apparatus, the direction in which masticatory muscles act during mastication, and jaw muscle forces as estimated by muscle dry weight are compared between two murid rodents, the Japanese field mouse (Apodemus speciosus, subfamily Murinae) and the gray red-backed vole (Clethrionomys rufocanus; subfamily Arvicolinae). The occlusal forces exerted by the deep masseter and the anterior temporalis are large in C. rufocanus. Furthermore, in this species, the angle between the sagittal plane and the occlusal plane of the cheek teeth is larger than in A. speciosus. Therefore, a relatively large occlusal force can be generated in C. rufocanus. The estimated line of action of the anterior temporalis differs markedly between these two species. The functional significance of this difference is discussed relative to the adaptive dental characteristics for food processing, the forces required to masticate different types of food, and the forces that control mandibular forward movement.

Muscle-fibre architecture of the rat medial pterygoid muscle

The detailed fibre architecture of the rat medial pterygoid muscle, including the courses and attachment points of muscle fibres, was investigated histologically in 10 micron thick serial sections in the horizontal, coronal and parasagittal planes. Four extramuscular tendinous sheets (external aponeuroses) and four intramuscular tendinous sheets (internal aponeuroses) were found. Three of the internal aponeuroses were arranged parallel to the rostro-caudal axis; the other was oblique to the rostro-caudal axis. Muscle fibres were located between internal aponeuroses, between external aponeuroses, between internal and external aponeuroses, between an internal aponeurosis and the periosteum, and between an external aponeurosis and the periosteum. The courses of muscle fibres were divided into three main groups: vertical, rostro-caudal and medio-lateral. Eight compartments were distinguished. The multiple movement of the medial pterygoid muscle seemed to be due to this compartmentalization. These findings suggested that the rat medial pterygoid muscle, like the rat masseter muscle, is mainly composed of multipennate muscles and compartmentalized into many muscle-fibre bundles running in different directions.