Morphological and Functional Questions on the Growth and Plasticity of Mammalian and Avian Jaw Muscles

SYNOPSIS: Growth studies of mammalian jaw muscles and studiesdetermining the degree of plasticity of these muscles are few.There are questions concerning the degrees and types of morphologicaland physiological change occurring in these muscles during normalgrowth and the ability of external forces to modify this growthprocess. There are also questions on the effects of sensoryloss on muscle growth and on changes in the motor system asthese muscles change. In this report, questions are proposedand hypotheses presented that deal with these areas relativeto the jaw muscles. It is suggested that (1) changes in themorphology and function of jaw muscles during growth vary regionallyand are related to the action of the muscle and the loadingforces imposed; (2) the degree of muscle plasticity varies regionallyand varies depending on the type of loading forces and age ofthe animal; (3) loss of sensory input during the growth of thejaw muscles would produce marked changes in the histochemicalprofile, the distribution of motor neurons, and the activitypattern of these muscles; and (4) organization of the trigeminalmotor nucleus changes as the jaw muscles grow and may also changein response to sensory loss or application of different typesof loading forces. In addition, possible approaches to thesequestions are discussed.     

Functional characteristics of rat gastrocnemius and tibialis anterior muscles during growth

Several morphological and functional characteristics of the rat gastrocnemius medialis and tibialis anterior muscle were studied in young, adult, and old rats to assess the influence of growth. Antagonist muscles were studied to determine how changes of muscle architecture and functional characteristics are influenced by the demands of increased body weight and by the specific roles of these muscles in locomotion. Both muscles change drastically, for instance, in muscle length, volume, physiological cross-sectional area aponeurosis length, and their muscular geometry changes allometrically for both muscles. The relationships between muscle length, distance between origin and insertion, tendon length, and tibial length also change with growth. Both muscles are rather pennate, so that the increase of physiological cross-sectional area is a major factor in the determination of muscle length. No significant difference could be shown for fundamental physiological characteristics (i.e., functional characteristics normalized for muscular dimensions such as maximal work per unit volume). The changes of morphological and functional variables of both muscles parallel each other as is apparent from the index of antagonist characteristics, which is constant for all variables studied with the exception of muscle volume and tendon length. Consequently, the considerable and similar changes of TA and GM morphology and functional characteristics that take place during growth from approximately four weeks postnatally is not caused by changes of muscular material but by changes of the amount and architectural arrangement of the material involved.     

Mesenchymal and mechanical mechanisms of secondary cartilage induction

Secondary cartilage occurs at articulations, sutures, and muscle attachments, and facilitates proper kinetic movement of the skeleton. Secondary cartilage requires mechanical stimulation for its induction and maintenance, and accordingly, its evolutionary presence or absence reflects species-specific variation in functional anatomy. Avians illustrate this point well. In conjunction with their distinct adult mode of feeding via levered straining, duck develop a pronounced secondary cartilage at the insertion (i.e., enthesis) of the mandibular adductor muscles on the lower jaw skeleton. An equivalent cartilage is absent in quail, which peck at their food. We hypothesized that species-specific pattern and a concomitant dissimilarity in the local mechanical environment promote secondary chondrogenesis in the mandibular adductor enthesis of duck versus quail. To test our hypothesis we employed two experimental approaches. First, we transplanted neural crest mesenchyme (NCM) from quail into duck, which produced chimeric “quck” with a jaw complex resembling that of quail, including an absence of enthesis secondary cartilage. Second, we modified the mechanical environment in embryonic duck by paralyzing skeletal muscles, and by blocking the ability of NCM to support mechanotransduction through stretch-activated ion channels. Paralysis inhibited secondary cartilage, as evidenced by changes in histology and expression of genes that affect chondrogenesis, including members of the FGF and BMP pathways. Ion channel inhibition did not alter enthesis secondary cartilage but caused bone to form in place of secondary cartilage at articulations. Thus, our study reveals that enthesis secondary cartilage forms through mechanisms that are distinct from those regulating other secondary cartilage. We conclude that by directing the musculoskeletal patterning and integration of the jaw complex, NCM modulates the mechanical forces and molecular signals necessary to control secondary cartilage formation during development and evolution.     

Functional links between canine height and jaw gape in catarrhines with special reference to early hominins

This study tests the hypothesis that decreased canine crown height in catarrhines is linked to (and arguably caused by) decreased jaw gape. Associations are characterized within and between variables such as upper and lower canine height beyond the occlusal plane (canine overlap), maximum jaw gape, and jaw length for 27 adult catarrhine species, including 539 living subjects and 316 museum specimens. The data demonstrate that most adult male catarrhines have relatively larger canine overlap dimensions and gapes than do conspecific females. For example, whereas male baboons open their jaws maximally more than 110% of jaw length, females open about 90%. Humans and hylobatids are the exceptions in that canine overlap is nearly the same in both the sexes and so is relative gape (ca. 65% for humans and 110% for hylobatids). A correlation analysis demonstrates that a large portion of relative gape (maximum gape/projected jaw length) is predicted by relative canine overlap (canine overlap/jaw length). Relative gape is mainly a function of jaw muscle position and/or jaw muscle‐fiber length. All things equal, more rostrally positioned jaw muscles and/or shorter muscle fibers decrease gape and increase bite force during the power stroke of mastication, and the net benefit is to increase the mechanical efficiency during chewing. Similarly, more caudally positioned muscles and/or longer muscle fibers increase the amount of gape and decrease bite force. Overall, the data support the hypothesis that canine reduction in early hominins is functionally linked to decreased gape and increased mechanical efficiency of the jaws. Am J Phys Anthropol, 2013. © 2012 Wiley Periodicals, Inc.     

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.     

Analyzing surface EMG signals to determine relationship between jaw imbalance and arm strength loss

ABSTRACT: BACKGROUND: This study investigated the relationship between dental occlusion and arm strength; in particular, the imbalance in the jaw can cause loss in arm strength phenomenon. One of the goals of this study was to record the maximum forces that the subjects can resist against the pull-down force on their hands while biting a spacer of adjustable height on the right or left side of the jaw. Then EMG measurement was used to determine the EMG-Force relationship of the jaw, neck and arms muscles. This gave us useful insights on the arms strength loss due to the biomechanical effects of the imbalance in the jaw mechanism. METHODS: In this study to determine the effects of the imbalance in the jaw to the strength of the arms, we conducted experiments with a pool of 20 healthy subjects of both genders. The subjects were asked to resist a pull down force applied on the contralateral arm while biting on a firm spacer using one side of the jaw. Four different muscles -- masseter muscles, deltoid muscles, bicep muscles and trapezoid muscles -- were involved. Integrated EMG (iEMG) and Higuchi fractal dimension (HFD) were used to analyze the EMG signals. RESULTS: The results showed that (1) Imbalance in the jaw causes loss of arm strength contra-laterally; (2) The loss is approximately a linear function of the height of the spacers. Moreover, the iEMG showed the intensity of muscle activities decreased when the degrees of jaw imbalance increased (spacer thickness increased). In addition, the tendency of Higuchi fractal dimension decreased for all muscles. CONCLUSIONS: This finding indicates that muscle fatigue and the decrease in muscle contraction level leads to the loss of arm strength.     

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.     

Bone growth in length and width: the Yin and Yang of bone stability

Bone growth in length is primarily achieved through the action of chondrocytes in the proliferative and hypertrophic zones of the growth plate. Longitudinal growth is controlled by systemic, local paracrine and local mechanical factors. With regard to the latter, a feedback mechanism must exist which ensures that bone growth proceeds in the direction of the predominant mechanical forces. How this works is unknown at present. Bone growth in length is detrimental to bone stability, but this effect is counteracted by concomitant bone growth in width. This occurs through periosteal apposition, which is the responsibility of periosteal osteoblasts. The action of these cells is mainly controlled by local factors, with modulation by systemic agents. According to the mechanostat theory, periosteal apposition is regulated by mechanical requirements. An alternative model, called sizostat hypothesis, maintains that a master gene or set of genes regulate bone growth in width to reach a pre-programmed size, independent of mechanical requirements. The virtues of these two hypotheses have been the subject of much discussion, but experimental data are scarce. Future research will have to address the question how periosteal bone cells manage to integrate mechanical, hormonal and other input to shape bones that are as strong as they need to be.     

A biomechanical model for analysis of muscle force,power output and lower jaw motion in fishes

Fish skulls are complex kinetic systems with movable components that are powered by muscles. Cranial muscles for jaw closing pull the mandible around a point of rotation at the jaw joint using a third-order lever mechanism. The present study develops a lever model for the jaw of fishes that uses muscle design and the Hill equation for nonlinear length-tension properties of muscle to calculate dynamic power output. The model uses morphometric data on skeletal dimensions and muscle proportions in order to predict behavior and force transmission mediated by lever action. The computer model calculates a range of dynamic parameters of jaw function including muscle force, torque, effective mechanical advantage, jaw velocity, bite duration, bite force, work and power. A complete list of required morphometrics is presented and a software program (MandibLever 2.0) is available for implementing lever analysis. Results show that simulations yield kinematics and timing profiles similar to actual fish feeding events. Simulation of muscle properties shows that mandibles reach their peak velocity near the start of jaw closing, peak force at the end of jaw closing, and peak power output at about 25% of the closing cycle time. Adductor jaw muscles with different mechanical designs must have different contractile properties and/or different muscle activity patterns to coordinate jaw closing. The effective mechanical advantage calculated by the model is considerably lower than the mechanical advantage estimated from morphological lever ratios, suggesting that previous studies of morphological lever ratios have overestimated force and underestimated velocity transmission to the mandible. A biomechanical model of jaw closing can be used to interpret the mechanics of a wide range of jaw mechanisms and will enable studies of the functional results of developmental and evolutionary changes in skull morphology and physiology.     

The dynamics of bone structure development during pubertal growth

The pubertal growth spurt is a time of rapid changes in bone length, mass and structure, followed by the cessation of longitudinal growth. The two best studied anatomical areas in this respect are the metaphyses and the diaphyses of peripheral long bones. A model is presented here in which the speed of longitudinal growth and the resulting age gradient in metaphyseal bone are key factors in explaining the high incidence of distal radius fractures during puberty. As growth in length accelerates, the age of the bone structural elements at a given distance to the growth plate decreases, leaving less time for cortical thickening through trabecular coalescence. This leads to a discrepancy between stagnant metaphyseal bone strength and increasing mechanical requirements in the case of accidents. In comparison to the metaphysis, diaphyseal bone develops more in line with the increasing mechanical requirements, presumably because the bone formation rates needed for diaphyseal growth in width are only a fraction of the apposition rates in the metaphysis. It remains largely unexplored how local and systemic signals are integrated to achieve site-specific changes in bone structure.     

Actin cytoskeletal dynamics in smooth muscle contraction

Smooth muscles develop isometric force over a very wide range of cell lengths. The molecular mechanisms of this phenomenon are undefined, but are described as reflecting "mechanical plasticity" of smooth muscle cells. Plasticity is defined here as a persistent change in cell structure or function in response to a change in the environment. Important environmental stimuli that trigger muscle plasticity include chemical (e.g., neurotransmitters, autacoids, and cytokines) and external mechanical signals (e.g., applied stress and strain). Both kinds of signals are probably transduced by ionic and protein kinase signaling cascades to alter gene expression patterns and changes in the cytoskeleton and contractile system. Defining the signaling mechanisms and effector proteins mediating phenotypic and mechanical plasticity of smooth muscles is a major goal in muscle cell biology. Some of the signaling cascades likely to be important include calcium-dependent protein kinases, small GTPases (Rho, Rac, cdc42), Rho kinase, protein kinase C (PKC), Src family tyrosine kinases, mitogen-activated protein (MAP) kinases, and p21 activated protein kinases (PAK). There are many potential targets for these signaling cascades including nuclear processes, metabolic pathways, and structural components of the cytoskeleton. There is growing appreciation of the dynamic nature of the actin cytoskeleton in smooth muscles and the necessity for actin remodeling to occur during contraction. The actin cytoskeleton serves many functions that are probably critical for muscle plasticity including generation and transmission of force vectors, determination of cell shape, and assembly of signal transduction machinery. Evidence is presented showing that actin filaments are dynamic and that actin-associated proteins comprising the contractile element and actin attachment sites are necessary for smooth muscle contraction.     

Mandibular arch muscle identity is regulated by a conserved molecular process during vertebrate development

Vertebrate head muscles exhibit a highly conserved pattern of innervation and skeletal connectivity and yet it is unclear whether the molecular basis of their development is likewise conserved. Using the highly conserved expression of Engrailed 2 (En2) as a marker of identity in the dorsal mandibular muscles of zebrafish, we have investigated the molecular signals and tissues required for patterning these muscles. We show that muscle En2 expression is not dependent on signals from the adjacent neural tube, pharyngeal endoderm or axial mesoderm and that early identity of head muscles does not require bone morphogenetic pathway, Notch or Hedgehog (Hh) signalling. However, constrictor dorsalis En2 expression is completely lost after a loss of fibroblast growth factor (Fgf) signalling and we show that is true throughout head muscle development. These results suggest that head muscle identity is dependent on Fgf signalling. Data from experiments performed in chick suggest a similar regulation of En2 genes by Fgf signalling revealing a conserved mechanism for specifying head muscle identity. We present evidence that another key gene important in the development of mouse head muscles, Tbx1, is also critical for specification of mandibular arch muscle identity and that this is independent of Fgf signalling. These data imply that dorsal mandibular arch muscle identity in fish, chick and mouse is specified by a highly conserved molecular process despite differing functions of these muscles in different lineages.     

Correlations between the cross-sectional area of the jaw muscles and craniofacial size and shape

In adult human subjects, the correlations were determined between the cross-sectional areas of the jaw muscles (measured in CT scans) and a number of facial angles and dimensions (measured from lateral radiographs). Multivariate statistical analysis of the skeletal variables in a group of 50 subjects led to the recognition of six independent factors determining facial shape, i.e., cranial base length, lower facial height, cranial base flexure and prognathism, facial width, mandibular length, and upper facial height. In 29 of these subjects, the cross-sectional areas of the jaw muscles were determined, and correlations between these areas and the scores on the above-mentioned factors were calculated. It appeared that the cross-sectional areas of temporalis and masseter muscles correlated positively with facial width, whereas the areas of masseter and both pterygoid muscles did so with mandibular length. It has been shown experimentally that a decrease in jaw muscle size in various animals likewise has an effect on facial width and mandibular length. Our results therefore support the hypothesis that in man too the jaw muscles affect facial growth and partly determine the final facial dimensions. They also hint that the role of each muscle is different.     

The control of multi-muscle systems: human jaw and hyoid movements

A model is presented of sagittal plane jaw and hyoid motion based on the model of motor control. The model, which is implemented as a computer simulation, includes central neural control signals, position- and velocity-dependent reflexes, reflex delays, and muscle properties such as the dependence of force on muscle length and velocity. The model has seven muscles (or muscle groups) attached to the jaw and hyoid as well as separate jaw and hyoid bone dynamics. According to the model, movements result from changes in neurophysiological control variables which shift the equilibrium state of the motor system. One such control variable is an independent change in the membrane potential of -motoneurons (MNs); this variable establishes a threshold muscle length () at which MN recruitment begins. Motor functions may be specified by various combinations of s. One combination of s is associated with the level of coactivation of muscles. Others are associated with motions in specific degrees of freedom. Using the model, we study the mapping between control variables specified at the level of degrees of freedom and control variables corresponding to individual muscles. We demonstrate that commands can be defined involving linear combinations of change which produce essentially independent movements in each of the four kinematic degrees of freedom represented in the model (jaw orientation, jaw position, vertical and horizontal hyoid position). These linear combinations are represented by vectors in space which may be scaled in magnitude. The vector directions are constant over the jaw/hyoid workspace and result in essentially the same motion from any workspace position. The demonstration that it is not necessary to adjust control signals to produce the same movements in different parts of the workspace supports the idea that the nervous system need not take explicit account of musculo-skeletal geometry in planning movements.This article was processed by the author using the LAT_EX style file pljour2 from Springer-Verlag.     

Fgf and Bmp signals repress the expression of Bapx1 in the mandibular mesenchyme and control the position of the developing jaw joint

The development of the jaw joint between the palatoquadrate and proximal part Meckel's cartilage (articular) has recently been shown to involve the gene Bapx1. Bapx1 is expressed in the developing mandibular arch in two distinct caudal, proximal patches, one on either side of the head. These domains coincide later with the position of the developing jaw joint. The mechanisms that result in the restricted expression of Bapx1 in the mandibular arch were investigated, and two signaling factors that act as repressors were identified. Fibroblast growth factors (Fgfs) expressed in the oral epithelium restrict expression of Bapx1 to the caudal half of the mandibular arch, while bone morphogenetic proteins (Bmps) expressed in the distal mandibular arch restrict expression of Bapx1 to the proximal part of the mandible. Application of Fgf8 and Bmp4 beads to the proximal mesenchyme led to loss of Bapx1 expression and later fusion of the quadrate and articular as the jaw joint failed to form. In addition to fusion of the jaw joint, loss of Bapx1 lead to loss of the retroarticular process (RAP), phenocopying the defects seen after Bapx1 function was reduced in the zebrafish. By manipulating these signals, we were able to alter the expression domain of Bapx1, resulting in a new position of the jaw joint.     

A role for Engrailed-2 in determination of skeletal muscle physiologic properties

The molecular basis underlying the establishment of the myogenic lineage, subsequent differentiation, and the establishment of specific fiber types (i.e., fast versus slow) is becoming well understood. In contrast, the regulation of the general properties of a specific anatomical muscle group (e.g., leg versus jaw muscles) and the regulation of muscle-fiber properties within a particular group are less well characterized. We have investigated the potential role of the homeobox-containing gene, Engrailed-2 (En-2), in the mouse, which is specifically expressed in myoblasts in the first arch and maintained in the muscles of mastication in the adult. We have generated mice that ectopically express En-2 in all muscles during early development and primarily in fast muscles in the adult. Ectopic En-2 in nonjaw muscles leads to a decrease in fiber size, whereas overexpression in the jaw muscles leads to a shift in fiber metabolic properties as well as a decrease in fiber size. In contrast, loss of En-2 in the jaw leads to a shift in fiber metabolic properties in the jaw of female mice only. Jaw muscles are sexually dimorphic, and we propose that the function of En-2 and mechanisms guiding sexual dimorphism of the jaw muscles are integrated. We conclude that the specific expression of En-2 in the jaw therefore plays a role in specifying muscle-fiber characteristics that contribute to the physiologic properties of specific muscle groups.     

The effect of exercise and nutrition on the mechanostat

In this review, we discuss the effect of increased and decreased loading and nutrition deficiency on muscle and bone mass and strength (and bone length and architecture) independently and combined. Both exercise and nutrition are integral components of the mechanostat model but both have distinctly different roles. Mechanical strain imparted by muscle action is responsible for the development of the external size and shape of the bone and subsequently the bone strength. In contrast, immobilization during growth results in reduced growth in bone length and a loss of bone strength due to large losses in bone mass (a result of endosteal resorption in cortical bone and trabecular thinning) and changes in geometry (bone shafts do not develop their characteristic shape but rather develop a rounded default shape). The use of surrogate measures for peak muscle forces acting on bone (muscle strength, size, or mass) limits our ability to confirm a cause-and-effect relationship between peak muscle force acting on bone and changes in bone strength. However, the examples presented in this review support the notion that under adequate nutrition, exercise has the potential to increase peak muscle forces acting on bone and thus can lead to a proportional increase in bone strength. In contrast, nutrition alone does not influence muscle or bone in a dose-dependent manner. Muscle and bone are only influenced when there is nutritional deficiency--and in this case the effect is profound. Similar to immobilization, the immediate effect of malnutrition is a reduction in longitudinal growth. More specifically, protein and energy malnutrition results in massive bone loss due to endosteal resorption in cortical bone and trabecular thinning. Unlike loading however, there is indirect evidence that severe malnutrition when associated with menstrual dysfunction can shift the mechanostat set point upward, thus leading to less bone accrual for a given amount of bone strain.     

Development of the trigeminal motor neurons in parrots: Implications for the role of nervous tissue in the evolution of jaw muscle morphology

Vertebrates have succeeded to inhabit almost every ecological niche due in large part to the anatomical diversification of their jaw complex. As a component of the feeding apparatus, jaw muscles carry a vital role for determining the mode of feeding. Early patterning of the jaw muscles has been attributed to cranial neural crest‐derived mesenchyme, however, much remains to be understood about the role of nonneural crest tissues in the evolution and diversification of jaw muscle morphology. In this study, we describe the development of trigeminal motor neurons in a parrot species with the uniquely shaped jaw muscles and compare its developmental pattern to that in the quail with the standard jaw muscles to uncover potential roles of nervous tissue in the evolution of vertebrate jaw muscles. In parrot embryogenesis, the motor axon bundles are detectable within the muscular tissue only after the basic shape of the muscular tissue has been established. This supports the view that nervous tissue does not primarily determine the spatial pattern of jaw muscles. In contrast, the trigeminal motor nucleus, which is composed of somata of neurons that innervate major jaw muscles, of parrot is more developed compared to quail, even in embryonic stage where no remarkable interspecific difference in both jaw muscle morphology and motor nerve branching pattern is recognized. Our data suggest that although nervous tissue may not have a large influence on initial patterning of jaw muscles, it may play an important role in subsequent growth and maintenance of muscular tissue and alterations in cranial nervous tissue development may underlie diversification of jaw muscle morphology. J. Morphol. 275:191–205, 2014. © 2013 Wiley Periodicals, Inc.     

Amphioxus and the evolution of head segmentation

Whether or not the vertebrate head is fundamentally segmented has been controversial for over 150 years. Beginning in the late 19th century, segmentalist theories proposed that the vertebrate head evolved from an amphioxus-like ancestor in which mesodermal somites extended the full length of the body with remnants of segmentation persisting as the mesodermal head cavities of sharks and lampreys. Antisegmentalists generally argued either that the vertebrate ancestors never had any mesodermal segmentation anteriorly or that they lost it before the origin of the vertebrates; in either case, the earliest vertebrates had an unsegmented head and the embryonic cranial mesoderm of vertebrates is at best pseudo-segmented, evolving independently of any pre-vertebrate segmental pattern. Recent morphologic studies have generally confirmed the accuracy of the major classical studies of head development in lampreys and sharks, yet disagree with their theoretical conclusions regarding the evolution of head segmentation. Studies of developmental genes in amphioxus and vertebrates, which have demonstrated conservation of the mechanisms of anterior-posterior patterning in the two groups, have shed new light on this controversy. Most pertinently, some homologs of genes expressed in the anterior amphioxus somites, which form as outpocketings of the gut, are also expressed in the walls of the head cavities of lampreys, which form similarly, and in their major derivatives (the velar muscles) as well as in the eye and jaw muscles of bony gnathostomes, which derive from unsegmented head mesoderm. These muscles share gene expression with the corresponding muscles of the shark, which derive from the walls of head cavities that form, not as outpocketings of the gut, but as secondary cavities within solid blocks of tissue. While molecular data that can be compared across all the relevant taxa remain limited, they are consistent with an evolutionary scenario in which the cranial paraxial mesoderm of the lamprey and shark evolved from the anterior somites of an amphioxus-like ancestor. Although, bony vertebrates have lost the mesodermal head segments present in the shark and lamprey, their remnants persist in the muscles of the eye and jaw.     

Limbs vs. Jaws: Can They Be Compared?

Current experimental research on mammalian limb muscle structureand function is compared to that on mammalian jaw muscles. Twomajor areas of comparison are stressed: structural and functional.Comparisons of limbs and jaws are made from the point of viewof the impact of recent studies on simple mechanical modelsof limb/jaw muscle function. Limb muscle structure is comparedto jaw muscles at the level of muscle architecture, muscle histochemicaland motor unit properties, and the organization of motor unitsinto neuromuscular compartments. Such comparisons reveal thatalthough limb muscles and jaw muscles might be organized insimilar ways, fundamental differences exist, both in terms ofmuscle structure and the functional conclusions which have beenbased on studies of muscle structure. The comparisons also demonstratethat much recent evidence from structural studies have had littledirect impact on simple models of muscle function but a muchlarger influence on the assumptions of the models. Comparisonsof limb/jaw muscle function from kinematic and EMG studies,indicate that many masticatory strategies are used by differentmammals but the basic problems of posture and locomotion havebeen met with essentially similar solutions, even among diversemammalian groups. The results of such comparisons demonstratethat both limb and jaw muscle function are sufficiently complexthat new or re-vitalized models are needed if the relationshipbetween structure and function are ever to be understood.