Histochemical enzyme profile of the masseter muscle in different mammalian species

Morphological variability is greater at developing than mature mouse neuromuscular junctions

The neuromuscular junction (NMJ) is the highly specialised peripheral synapse formed between lower motor neuron terminals and muscle fibres. Post‐synaptic acetylcholine receptors (AChRs), which are found in high density in the muscle membrane, bind to acetylcholine released into the synaptic cleft of the NMJ, thereby enabling the conversion of motor action potentials to muscle contractions. NMJs have been studied for many years as a general model for synapse formation, development and function, and are known to be early sites of pathological changes in many neuromuscular diseases. However, information is limited on the diversity of NMJs in different muscles, how synaptic morphology changes during development, and the relevance of these parameters to neuropathology. Here, this crucial gap was addressed using a robust and standardised semi‐automated workflow called NMJ‐morph to quantify features of pre‐ and post‐synaptic NMJ architecture in an unbiased manner. Five wholemount muscles from wild‐type mice were dissected and compared at immature (post‐natal day, P7) and early adult (P31−32) timepoints. The inter‐muscular variability was greater in mature post‐synaptic AChR morphology than that of the pre‐synaptic motor neuron terminal. Moreover, the developing NMJ showed greater differences across muscles than the mature synapse, perhaps due to the observed distinctions in synaptic growth between muscles. Nevertheless, the amount of nerve to muscle contact was consistent, suggesting that pathological denervation can be reliably compared across different muscles in mouse models of neurodegeneration. Additionally, mature post‐synaptic endplate diameters correlated with fibre type, independently of muscle fibre diameter. Altogether, this work provides detailed information on healthy pre‐ and post‐synaptic NMJ morphology from five anatomically and functionally distinct mouse muscles, delivering useful reference data for future comparison with neuromuscular disease models.

Fiber Types in Mammalian Skeletal Muscles

Mammalian skeletal muscle comprises different fiber types, whose identity is first established during embryonic development by intrinsic myogenic control mechanisms and is later modulated by neural and hormonal factors. The relative proportion of the different fiber types varies strikingly between species, and in humans shows significant variability between individuals. Myosin heavy chain isoforms, whose complete inventory and expression pattern are now available, provide a useful marker for fiber types, both for the four major forms present in trunk and limb muscles and the minor forms present in head and neck muscles. However, muscle fiber diversity involves all functional muscle cell compartments, including membrane excitation, excitation-contraction coupling, contractile machinery, cytoskeleton scaffold, and energy supply systems. Variations within each compartment are limited by the need of matching fiber type properties between different compartments. Nerve activity is a major control mechanism of the fiber type profile, and multiple signaling pathways are implicated in activity-dependent changes of muscle fibers. The characterization of these pathways is raising increasing interest in clinical medicine, given the potentially beneficial effects of muscle fiber type switching in the prevention and treatment of metabolic diseases.

Histoenzymology and morphometry of the masticatory muscles of tufted capuchin monkey (Cebus apella Linnaeus, 1758)

Samples of the anterior and posterior regions of the masseter and temporal muscles and of the anterior belly of the digastric muscle of 4 adult male tufted capuchin monkeys (Cebus apella) were removed and stained with HE and submitted to the m-ATPase reaction (with alkaline and acid preincubation) and to the NADH-TR and SDH reactions. The results of the histoenzymologic reactions were similar, except for acid reversal which did not occur in fibers of the fast glycolytic (FG) type in the mandibular locomotor muscles. FG fibers had a larger area and were more frequent in all regions studied. No significant differences in frequency or area of each fiber type were detected, considering the anterior and posterior regions of the masseter and temporal muscles. The frequency of fibers of the fast oxidative glycolytic (FOG) and slow oxidative (SO) types and of FOG area differed significantly between the anterior belly of the digastric muscle and the mandibular locomotor muscle. The predominance of fast twitch (FG and FOG) fibers and the multipenniform and bipenniform internal architecture of the masseter and temporal muscles, respectively, are characteristics that permit the powerful bite typical of tufted capuchin monkeys.

Spatial distribution of myosin heavy-chain isoforms in mouse masseter

There is a paucity of information regarding the anatomy and muscle fiber phenotype of the masseter. The objective of this study was to characterize the distribution of each myosin heavy-chain (MyHC) isoform within different anatomical regions of male and female mouse masseters. Masseters from male and female CD-1 mice (2-4 months old) were examined for description of the anatomical partitioning of muscle fibers and endplate distribution. The spatial distribution of MyHC isoforms--embryonic, neonatal, slow, alpha-cardiac, IIa, and IIb--was determined within the defined masseter partitions by means of Western blot analysis and immunofluorescent localization. Types IIa, IIx, and IIb were the predominant MyHC isoforms observed. Distinct differences in the spatial distribution of these MyHC isoforms were found between muscle regions and varied between sexes. The regionalization of muscle fiber types in the mouse masseter is consistent with the functional compartmentalization of the masseter observed in other species.

Muscle fibre types in the suprahyoid muscles of the rat

Five muscle fibre types (I, IIc, IIa, IIx and IIb) were found in the suprahyoid muscles (mylohyoid, geniohyoid, and the anterior and posterior bellies of the digastric) of the rat using immuno and enzyme histochemical techniques. More than 90% of fibres in the muscles examined were fast contracting fibres (types IIa, IIx and IIb). The geniohyoid and the anterior belly of the digastric had the greatest number of IIb fibres, whilst the mylohyoid was almost exclusively formed by aerobic fibres. The posterior belly of the digastric contained a greater percentage of aerobic fibres (83.4%) than the anterior belly (67.8%). With the exception of the geniohyoid, the percentage of type I and IIc fibres, which have slow myosin heavy chain (MHCbeta), was relatively high and greater than has been previously reported in the jaw-closing muscles of the rat, such as the superficial masseter. The geniohyoid and mylohyoid exhibited a mosaic fibre type distribution, without any apparent regionalisation, although in the later MHCbeta-containing fibres (types I and IIc) were primarily located in the rostral 2/3 region. In contrast, the anterior and posterior bellies of the digastric revealed a clear regionalisation. In the anterior belly of the digastric 2 regions were observed: both a central region, which was almost exclusively formed by aerobic fibres and where all of the type I and IIc fibres were located, and a peripheral region, where type IIb fibres predominated. The posterior belly of the digastric showed a deep aerobic region which was greater in size and where type I and IIc fibres were confined, and a superficial region, where primarily type IIx and IIb fibres were observed.

Ontogeny of histochemical fiber types and muscle function in the masseter muscle of miniature swine

In this study of masticatory maturation, the ontogeny of the histochemical fiber type composition of musculus masseter is examined in the omnivorous miniature swine (Sus scrofa). Fiber type characteristics are interpreted by comparison with electromyography (EMG) recorded during feeding behavior. Similar to locomotion studies, the results suggest a correspondence between the composition and arrangement of motor units and their recruitment pattern. Serial sections of masseter muscles from 10 minipigs, ranging from 2 weeks to slightly over 1 year of age, were stained for myosin adenosine triphosphatase (mATPase) activity to distinguish slow-twitch from fast-twitch fibers, and for nicotinamide adenosine dehydrogenase-tetrazolium reductase to assess the aerobic capacity of the same fibers. Although maintaining a uniformly high aerobic capacity throughout ontogeny and in adult animals, a transition is observed in the relative proportions of fast- and slow-twitch fibers. The primarily fast-twitch neonatal pig masseter eventually comprises approximately 25-30% slow-twitch fibers in adults, with a higher predominance of slow fibers in the deep (vs. superficial) and anterior (vs. posterior) regions of the muscle. Furthermore, while individual fibers of adult masseters generally stain for either alkaline- or acid-stable mATPase activity, a substantial proportion of cells in developing animals exhibits the presence of both isozymes. EMG results indicate functional heterogeneity within the masseter of adult pigs. During chewing, when pig chow is replaced by cracked corn, EMG activity in the deep portion of the muscle either decreases or increases slightly. In the superficial portion, however, muscle amplitudes become dramatically higher for corn, surpassing levels generated for chewing the less obdurate chow. These results are consistent with a behavioral transition from neonatal suckling to sustained mastication of foods of more complex textures eaten by adult pigs. The relationship between these fiber type and EMG results for pig masseter corresponds to those pertaining to motor unit recruitment in the extensor muscles of locomotion. Implications of this work for the evolutionary morphology of mastication also are discussed.

Sexually dimorphic expression of myosin heavy chains in the adult mouse masseter

Little is known regarding the role of androgenic hormones in the maintenance of myosin heavy chain (MHC) composition of rodent masticatory muscles. Because the masseter is the principal jaw closer in rodents, we felt it was important to characterize the influence of androgenic hormones on the MHC composition of the masseter. To determine the extent of sexual dimorphism in the phenotype of masseter muscle fibers of adult (10-mo-old) C57 mice, we stained tissue sections with antibodies specific to type IIa and IIb MHC isoforms. Females contain twice as many fibers containing the IIa MHC as males, and males contain twice as many fibers containing the IIb MHC as females. There is a modest amount of regionalization of MHC phenotypes in the mouse masseter. The rostral portions of the masseter are composed mostly of type IIa fibers, whereas the midsuperficial and caudal regions contain mostly type IIb fibers. Using immunoblots, we showed that castration results in an increase in the expression of type IIa MHC fibers in males. Ovariectomy has no effect on the fiber type composition in females. We conclude that testosterone plays a role in the maintenance of MHC expression in the adult male mouse masseter.

Automated cytometry of fibre size and spatial distribution in the superficial masseter muscle of the rat at three ages

An image cytometry program was applied to sections of the superficial masseter muscle of female and male 21-, 42- and 105-day-old rats. Lesser diameter and spatial distribution of more than 6000 muscle fibres were automatically measured in digital images from muscle sections stained for myofibrillar ATPase activity. In this muscle, only type 2A, 2B and 2C fibres were observed. At the three ages and in both sexes, 2A fibres were found to have the largest diameter and were the most frequent (> 54%). In the 21-day-old animals, females presented larger diameters than did males; in contrast, in the 105-day-old animals, the three fibre types were larger in males than in females. At all ages and in both sexes, type 2A occupied 32 to 80% more cross-sectional area than type 2B. Most images (98%) showed a random spatial distribution of their fibre types. All fibre types grew in diameter with age. The coefficient of variation of the diameter was age-independent and remained under 23%. The finding of an age-independent variable could have a practical application: an increase of the coefficient of variation (> 25%) can be considered as pathological, even without a perfect age-matched control.

Fiber types and diameters in the porcine masseter muscle. A histochemical study

Type I, I:B, II:A, and II:C fibers were identified by adenosine triphosphatase histochemistry in masseter muscles from 22 female pigs (1 year old, 70-90 kg body weight). Type II:B fibers were not found. This was in contrast to the findings of five fiber types in the porcine soleus muscles. In the porcine masseter the most prominent fiber type was II:A (75%). Type I fibers constituted 15% of the fiber types on average. Type I:B and II:C fibers were less frequent (4-6%). No significant difference was found between various biopsy locations, but there was a tendency towards more type I fibers in the deeper part of the masseter muscle. The mean fiber diameters were larger in the masseter muscles than in the soleus; however, the differences were significant only for fiber type I:B.

Presence of cardiac alpha-myosin correlates with histochemical myosin Ca2+ ATPase activity in rabbit masseter muscle

A combined enzyme-histochemical (ATPase reactivity) and immunohistochemical study has been performed on sections of rabbit masseter muscle. The majority of the fibres previously designated as type IIC and/or type I according to their ATPase activity were found to contain 'cardiac' alpha-myosin heavy chain in addition to other myosin heavy chains. All alpha-myosin heavy chain-containing fibres reveal ATPase activity after pre-incubation at pH 4.2-4.6 similar to that of the classical type I fibres, while, in that pH range, limb type IIC fibres show intermediate ATPase activity. One group of these fibres reveal ATPase activity after pre-incubation at pH 10.1-10.3 as well, but not at pH 10.4-10.5. These fibres contain exclusively either alpha- or alpha- and I-myosin heavy chains but do not contain the IIA-myosin heavy chain. The second part of the fibres reveals ATPase activity after treatment within the whole alkaline pre-incubation range (pH 10.1-10.5) and these fibres contain alpha-myosin and IIA-myosin but no I-myosin heavy chain. It is concluded that the classical IIC fibre type is not present in the rabbit masseter muscle. Furthermore, ATPase reactivity does not allow us to distinguish fibres on their myosin heavy chain content in rabbit masseter muscle.

Jaw protruder muscles and condylar cartilage growth in the rat

Many studies have explored the role of the protrusive musculature in promoting growth at the condylar cartilage and the overall lengthening of the lower jaw, with emphasis on the lateral pterygoid muscle (LPM). The largely anteroposterior orientation of the superficial part of the masseter muscle (SM) in the rat suggests that it may also function as a protruder of the lower jaw. Accordingly, it is possible that the action of the SM may play a part in the regulation of growth of the condylar cartilage and the lower jaw. To examine this hypothesis, bilateral resection of the superficial portion of the masseter muscle was performed in male Sprague-Dawley rats at 26 days of age. At 5 days after surgery, [3H]-thymidine incorporation in the condylar cartilage was increased (F = 6.93, p less than or equal to 0.01) in the SM myectomy group relative to the surgical control and unoperated control groups. However, by 20 days after surgery no differences were present. At this sacrifice interval, lower jaw dimensions relating to areas of muscle attachment, as well as ramus height, were significantly reduced in the SM myectomy group, but overall jaw length (mental foramen to condyle) was unaffected. In contrast, myotomy of the LPM resulted in a significant decrease in mitotic activity of the cartilage 4 days after surgery. This decrease was present, but not more pronounced, in animals subjected to both SM myectomy and LPM myotomy. Hence, myotomy or myectomy of these two muscles, each with a protrusive orientation, produces opposite effects on proliferative activity at the condylar cartilage.

An animal model for human masseter muscle: histochemical characterization of mouse, rat, rabbit, cat, dog, pig, and cow masseter muscle

The masseter muscle of several animal species was investigated by use of a histochemical method for the demonstration of acid-stable and alkali-stable myosin adenosine triphosphatase (ATPase). The following subdivisions of fiber types were used: Type I fibers show weak ATPase activity at pH 9.4, type IM fibers react moderately, and type II fibers react strongly. Rat and mouse masseter muscles contained type II fibers only, as did some rabbit masseter muscles, whereas other rabbit masseter muscles possessed equal amounts of type I and II fibers. Cat and dog masseter muscles possessed both type II and I fibers, with type II predominating. Cow masseter muscle consisted mainly of type I fibers, although some cow masseter muscles contained a very small number of type II fibers. Pig masseter muscle had both type I, II, and IM fibers. One of the characteristics of human masseter muscle is type IM fibers, which are rarely seen in muscles other than the masticatory muscles. Therefore, pig masseter muscle might be a suitable animal model for experimental studies, such as an investigation of the distribution and diameter of fiber types in the masticatory muscles before and after orthognathic surgery.

Adaptation of the lateral pterygoid and superficial masseter muscles to mandibular protrusion in the rat

It has been suggested that protrusion of the mandible results in an alteration of the functional activity of the lateral pterygoid muscle. If this is true, however, it is unclear whether this altered muscle function is a transient phenomenon with no long-term effect or whether it results in structural and functional adaptation of the involved musculature. The purpose of this study was to determine whether or not physiologic and metabolic changes take place within two jaw-protruder muscles--the lateral pterygoid muscle and the superficial masseter muscle--in rats after treatment with a protrusive appliance. Thirty 45-day-old male Sprague-Dawley rats were divided equally into experimental and control groups. The experimental animals wore bonded protrusive-type appliances for 2 weeks. Histochemical analysis of muscle fiber types and in vivo whole-muscle contractile-property analysis were used to evaluate structural and functional muscle adaptations. Mandibular length was slightly but significantly greater in the experimental group, indicating that the protrusive appliance had the expected positive effect on mandibular growth. Histochemically, the lateral pterygoid muscle in the experimental group exhibited a significantly greater area occupied by type I fibers at the expense of type IIb fibers. The superficial masseter muscle exhibited a significantly greater percentage of areas for both type IIa and type IIb fibers in the experimental group. Contraction time (TPT) increased in both muscles; that is, the muscles became slower. The histochemical and contractile-properties data indicate that the protrusive appliance caused the lateral pterygoid muscle to become more active with respect to tonic (postural) activity, whereas the superficial masseter muscle became more active phasically.

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.

Comparison of myosins from the masseter muscle of adult rat, mouse and guinea-pig. Persistence of neonatal-type isoforms in the murine muscle

Adult rat, mouse, and guinea-pig masseter muscles display distinct myosin electrophoretic patterns. The rat muscle contains four main forms which by reference to the myosins of the IIB tensor fasciae latae, of the IIA mylohyoid, and of the red and white portions of the sternomastoid muscles, correspond respectively to the intermediate-type and to the three fast-type isoforms. The mouse masseter muscle contains only three main myosins, the intermediate-type and two fast-type isoforms. The guinea-pig muscle also displays only three bands, whose assignment is, however, less straightforward than in the murine species; their electrophoretic mobilities are not strictly the same as those of their homologous forms in rat and mouse. Comparison with the myosins of the tensor fasciae latae and of the sternomastoid muscles of guinea-pig allows their identification as intermediate and fast-type myosins. In addition to these typical adult-type forms, adult murine masseter muscles are observed to contain between zero and 30% of neonatal-type myosins. The comparison of the developmental transitions of myosins in the rat masseter with those in the skeletal muscles of the same animal indicates a delay in the appearance of the adult as well as in the disappearance of the neonatal-type myosins in the masseter muscle. Both the variability in myosin types with the animal species and the atypical presence of neonatal forms in the murine adults suggest that myosin expression in the masseter muscle is subjected to unusual regulations.

Histochemical enzyme profile of the masseter, temporal and lateral pterygoid muscles of the European hedgehog (Erinaceus europeaus)

In man, there are large differences in histochemical fibre-type composition, distribution and size between jaw and trunk muscles, probably related to the special functions of the human stomatognathic system. In the hedgehog, the influence of alkaline and acid pre-incubations on the reaction for myofibrillar ATPase was different from that in man, suggesting a different myosin structure; the fibre composition was different also. The masseter, the superficial portion of the temporal and the lateral pterygoid muscles all showed a homogeneous fibre type profile with almost 100 per cent alkali-stable fibres. In two animals, the deep temporal muscles showed an apparent heterogeneous fibre pattern with 81 per cent alkali-stable fibres, 4 per cent alkali-labile fibres and 15 per cent ATPase-intermediate fibres; in one animal 87 per cent alkali-stable fibres and 13 per cent ATPase-intermediate fibres. There was no difference in cross-sectional area between the three fibre types within each muscle, but the fibres of the lateral pterygoid were smaller than the alkali-stable and the alkali-labile fibres of the masseter and temporal muscles. The limb and trunk muscles showed reactions for myofibrillar ATPase similar to the jaw muscles, but had a heterogeneous fibre-type profile. There was no significant difference in cross-sectional fibre area between the jaw and the limb muscles. Thus the jaw and limb muscles of the hedgehog have similar fibre types and about equal fibre size.

Muscle fibre types and muscle spindles in the jaw musculature of the rat

The fibre composition and occurrence of muscle spindles was studied in the masticatory, the suprahyoid and the infrahyoid muscles of the rat. Muscle fibres were typed as fast-white, fast-intermediate, fast-red and slow-red according to their ATPase and SDH activity. Fibre type appeared to be closely related to fibre diameter. In most of the muscles, all four fibre types were found. Slow-red fibres were absent in the superficial masseter, the transverse mandibular and the omohyoid muscles; fast-white fibres were absent in the mylohyoid muscle. The masticatory muscles were mainly composed of the three fast-fibre types; the jaw-opener muscles (the anterior digastric, the posterior digastric, the posterior digastric, the stylohyoid and the lateral pterygoid muscle) showed more slow-red fibres. In the masticatory and most of the suprahyoid muscles, the slow-red fibres were restricted to an area with high SDH activity. In the infrahyoid muscles, the fibre types were evenly distributed. Many muscle spindles, often clustered, were found in the masticatory muscles, except in the lateral pterygoid. In most of the suprahyoid muscles, these sensory structures were absent. In the infrahyoid muscles, solitary muscle spindles were found.

The fibre-type composition of the first branchial arch muscles in Carnivora and Primates

A combination of standard histochemical techniques and immunohistochemical staining using myosin type-specific antisera was used to determine the fibre-type composition of the muscles of first branchial arch origin (that is, masseter, temporalis, pterygoideus medialis and lateralis, tensor veli palatini, tensor tympani, anterior digastricus and mylohyoideus) in a wide range of the Carnivora and the Primates. The rare IIM fibre type was found in the first branchial arch muscles of most of the species examined, but never in the limb muscles used as controls for this study. The jaw-closer muscles (masseter, temporalis and pterygoideus medialis) were found to contain IIM fibres in all the Carnivora except the lesser panda and in all the Primates except man. When present, the IIM fibres were usually the predominant fibre type, and the only other fibre types present were types I, II or IIC. The presence of IIM fibres in the jaw-closer muscles of most of the Carnivora and the Primates seems to be associated with an aggressive bite which is required for predation by the former and defence by the latter. In both groups of species there was the member which does not have an aggressive bite, the lesser panda and man, respectively, and these (like all other orders of mammals such as Lagomorpha, Rodentia, etc.) were found to have no IIM fibres in the jaw-closer muscles. The two muscles of the first branchial arch group which are derived from the ventral constrictor muscles of the (phylogenetically) original mandibular arch never contained IIM fibres, and were composed of type I and II fibres similar to those found in the control muscles of the limb. Tensor veli palatini and tensor tympani showed species-dependent variations in fibre-type composition and did not always reflect the composition of the jaw-closer muscles. Thus their common origin with the jaw-closers cannot be responsible for the occurrence of IIM fibres in tensor veli palatini and tensor tympani in some species. Furthermore, in tensor tympani but not in tensor veli palatini, the presence of IIM fibres was always accompanied by immunohistochemically slow-tonic fibres. Finally, the regard to the association of oxidative activity with the fibre type as defined by the myofibrillar ATPase method and by the isoform of myosin present, we suggest that in the first branchial arch muscles this is probably not directly comparable to the situation in the typical limb muscle.

Glucocorticoid-induced atrophy in different fibre types of selected rat jaw and hind-limb muscles

Six weeks of glucocorticoid treatment (triamcinolone-acetonide-21-phosphate, 1 mg/kg of body weight daily) resulted in a similar pattern of fibre type atrophy in jaw and hind-limb muscles. Fast-twitch glycolytic fibres were significantly atrophied in all muscles investigated. Fast-twitch oxidative glycolytic fibres were affected only in the fast (white) regions of the mixed muscles, while slow-twitch oxidative fibres were unaffected. The results from pair-fed controls (food intake matched to the glucocorticoid-treated animals) showed that the glucocorticoid produced an atrophic response in addition to that produced by the decreased food intake in the experimental animals. These results suggest that the jaw musculature shares the wasting effects induced by elevated circulating glucocorticoid levels often found in stress situations.

Canine muscle fiber types and susceptibility of masticatory muscles to myositis

The myofiber type composition was studied in 42 different muscles of the dog to determine if there are unique features that might explain the preferential involvement of the muscles of mastication by inflammatory myopathies. The principal myofiber types for most muscles studied were type 1 and type 2A and, to a lesser extent, type 2C, whereas the dorsal group of muscles innervated by the mandibular nerve (Mm. temporalis, and tensor veli palatini) was composed only of type 2C myofibers and a variant of the type 1 myofiber whose staining intensity was not fully reversed after preincubation in acid media. The distribution of this myofiber type composition was associated with the innervation and embryologic development of the dorsal muscles innervated by the mandibular nerve. This unique myofiber type composition could provide the basis for the preferential susceptibility of these muscles to agents (e.g., immune and/or infectious) that produce myositis; however, further studies are required to assess that possibility.

Methods for demonstration of enzyme activity in muscle fibres at the muscle/bone interface in demineralized tissue

A method for demonstration of activity for ATPase and various oxidative enzymes (succinic dehydrogenase, alpha-glycerophosphate dehydrogenase, and lactic dehydrogenase) in muscle/bone sections of fixed and demineralized tissue has been developed. It was found that it is possible to preserve considerable amounts of the above mentioned enzymes in the muscle fibres at the muscle/bone interfaces. The best results were obtained after 20 min fixation, and 2-3 weeks of storage in MgNa2EDTA containing media. As the same technique previously has been used to describe patterns of resorption and deposition with the aid of a mapping of presence of phosphomonoesterases on bone surfaces, the method may be used to study possible biochemical interactions between bone and muscle tissue at the muscle/bone interface.

Histochemical characteristics of the masseter and temporalis muscles of the rhesus monkey (Macaca mulatta)

The histochemical characteristics, cross-sectional area and capillary of the skeletal muscle fibers of the anterior and posterior regions of the superficial masseter and the temporalis muscles are described for juvenile and adult rhesus monkeys of both sexes. Slow twitch fatigue resistant (S), fast twitch fatigue resistant (FR) and fast twitch fatigable (FF) fibers were found in varying proportions throughout the muscles; however some fibers with an intermediate myofibrillar ATPase activity were observed in the anterior masseter. No significant differences for any of the variables were found between male and female juveniles for a specific muscle sample site. However, considerable variation was found between juvenile and adult and between adult male and female monkeys in the percentages of different fiber types and the cross-sectional area of fibers in specific regions of the superficial masseter and temporalis muscles. We conclude from these observations that significant differences in function exist both within and between the different masticatory muscles of rhesus monkeys. Functional differences may result from the pronounced sexual dimorphism evident in the dentofacial complex of the rhesus monkey.

A comparative histochemical study of the masseter muscle of the cattle, sheep, swine, dog, guinea pig, and rat

The masseter muscles of different mammals were studied by means of hisotchemical reactions: NADH: Nitro BT oxidoreductase (NADHOX), 3-hydroxybutyrate: NAD+ oxidoreductase (HBOX), glycerol-3-phosphate: menadione oxidoreductase (GPOX), and acid-stable and alkali-stable myosin adenosine triphosphatase (ATPase). The masseter mucles of cattle and sheep consisted only of the fibres that reacted moderately for GPOX and strongly for NADHOX, HBOX, and the acid-stable ATPase. The masseter fibres of rats and guinea pigs reacted uniformly and strongly for GPOX and the alkali-stable ATPase. The fibres of the rats showed a weak to strong reaction for NADHOX and mostly a negative reaction for HBOX, whereas those of the guinea pigs reacted uniformly and strongly for NADHOX and HBOX.The masseter fibres of swine and dogs showed a weak or strong reaction for the alkali-stable and a negative or weak reation for HBOX. The fibres of the swine were weak to strong in NADHOX activity and those of the dogs uniformly strong; the fibres of the two species gave a moderate to strong reaction for GPOX. The masseter fibres of the ruminant differed from those of the other species in histochemical properties, and appeared to have the histochemical characteristics that meed functional demands for slow, long-term exercise.