Fetal myosin heavy chain increases in human masseter muscle during aging

Satellite cells in ageing: use it or lose it

Individuals that maintain healthy skeletal tissue tend to live healthier, happier lives as proper muscle function enables maintenance of independence and actuation of autonomy. The onset of skeletal muscle decline begins around the age of 30, and muscle atrophy is associated with a number of serious morbidities and mortalities. Satellite cells are responsible for regeneration of skeletal muscle and enter a reversible non-dividing state of quiescence under homeostatic conditions. In response to injury, satellite cells are able to activate and re-enter the cell cycle, creating new cells to repair and create nascent muscle fibres while preserving a small population that can return to quiescence for future regenerative demands. However, in aged muscle, satellite cells that experience prolonged quiescence will undergo programmed cellular senescence, an irreversible non-dividing state that handicaps the regenerative capabilities of muscle. This review examines how periodic activation and cycling of satellite cells through exercise can mitigate senescence acquisition and myogenic decline.

The ancient sarcomeric myosins found in specialized muscles

Striated muscles express an array of sarcomeric myosin motors that are tuned to accomplish specific tasks. Each myosin isoform found in muscle fibers confers unique contractile properties to the fiber in order to meet the demands of the muscle. The sarcomeric myosin heavy chain (MYH) genes expressed in the major cardiac and skeletal muscles have been studied for decades. However, three ancient myosins, MYH7b, MYH15, and MYH16, remained uncharacterized due to their unique expression patterns in common mammalian model organisms and due to their relatively recent discovery in these genomes. This article reviews the literature surrounding these three ancient sarcomeric myosins and the specialized muscles in which they are expressed. Further study of these ancient myosins and how they contribute to the functions of the specialized muscles may provide novel insight into the history of striated muscle evolution.

Functional and molecular outcomes of the human masticatory muscles

The masticatory muscles achieve a broad range of different activities such as chewing, sucking, swallowing, and speech. In order to accomplish these duties, masticatory muscles have a unique and heterogeneous structure and fiber composition, enabling them to produce their strength and contraction speed largely dependent on their motor units and myosin proteins that can change in response to genetic and environmental factors. Human masticatory muscles express unique myosin isoforms, including a combination of thick fibers, expressing myosin light chains (MyLC) and myosin class I and II heavy chains (MyHC) -IIA, -IIX, α-cardiac, embryonic and neonatal and thin fibers, respectively. In this review, we discuss the current knowledge regarding the importance of fiber-type diversity in masticatory muscles versus supra- and infrahyoid muscles, and versus limb and trunk muscles. We also highlight new information regarding the adaptive response and specific genetic variations of muscle fibers on the functional significance of the masticatory muscles, which influences craniofacial characteristics, malocclusions, or asymmetry. These findings may offer future possibilities for the prevention of craniofacial growth disturbances.

Fiber-type Composition of the Human Jaw Muscles—(Part 1) Origin and Functional Significance of Fiber-type Diversity

This is the first of two articles on the fiber-type composition of the human jaw muscles. The present article discusses the origin of fiber-type composition and its consequences. This discussion is presented in the context of the requirements for functional performance and adaptation that are imposed upon the jaw muscles. The human masticatory system must perform a much larger variety of motor tasks than the average limb or trunk motor system. An important advantage of fiber-type diversity, as observed in the jaw muscles, is that it optimizes the required function while minimizing energy use. The capacity for adaptation is reflected by the large variability in fiber-type composition among muscle groups, individual muscles, and muscle regions. Adaptive changes are related, for example, to the amount of daily activation and/or stretch of fibers. Generally, the number of slow, fatigue-resistant fibers is relatively large in muscles and muscle regions that are subjected to considerable activity and/or stretch.

Human masseter muscle fibers from the elderly express less neonatal Myosin than those of young adults

In contrast to limb muscles where neonatal myosin (MyHC-neo) is present only shortly after birth, adult masseter muscles contain a substantial portion of MyHC-neo, which is coexpressed with mature MyHC isoforms. Changes in the numerical and area proportion of muscle fibers containing MyHC-neo in masseter muscle with aging could be expected, based on previously reported findings that (i) developmental MyHC-containing muscle fibers exhibit lower shortening velocities compared to fibers with exclusively fast MyHC isoforms and (ii) transformation toward faster phenotype occurs in elderly compared to young masseter muscle. In this study, we detected MyHC isoforms in the anterior superficial part of the human masseter muscle in a sufficiently large sample of young, middle-aged, and elderly subjects to reveal age-related changes in the coexpression of MyHC-neo with adult MyHC isoforms. MyHC isoforms were visualized with immunoperoxidase method and the results were presented by (i) the area proportion of fibers containing particular MyHC isoforms and (ii) the numerical proportion of fiber types defined by MyHC-1, -2a, -2x, and -neonatal isoform expression from a successive transverse sections. We found a lower numerical and area proportion of fibers expressing MyHC-neo as well as a lower area proportion of fibers containing MyHC-1 in elderly than in young subjects. We conclude that the diminished expression of MyHC-neo with age could point to a lower regeneration capacity of masseter muscle in the elderly.

Masseter myosin heavy chain composition varies with mandibular asymmetry

Human jaw dysmorphologies are frequent and often affect young patients, resulting in malocclusion of teeth and inappropriate jaw relationships. Treatment is performed by means of orthodontics with orthognathic surgery as required. Mandibular asymmetry is one of the most frequent dysmorphologies, but in many cases, the specific cause is unknown.In healthy patients who were undergoing orthognathic surgery for correction of malocclusion, we tested the hypothesis that masseter muscle phenotype composition, which determines contractile properties, was different between sides in patients with mandibular asymmetry but not in those without mandibular asymmetry. After cephalometric analysis, 50 patients from whom we obtained samples of both right and left masseter muscles were separated into 2 groups: with or without mandibular lateral deviation. Samples were immunostained with myosin-isoform-specific antibodies to identify 4 skeletal muscle fiber types, and their fiber areas and proportions were measured. Two-tailed Wilcoxon test for paired samples was used to compare the 4 fiber-type compositions by means of percent occupancy and mean fiber area on both sides. Patients with mandibular asymmetry were associated with a significant increase of type II fiber occupancy (P = 0.0035) on the same side as the deviation. This finding that masseter muscle phenotype is significantly linked to mandibular asymmetry is of relevance to physiotherapeutic and surgical managements of jaw discrepancies and merits further investigation in the light of its possible role in the etiology of this condition.

Differences in fibre type composition between human masseter and biceps muscles in young and adults reveal unique masseter fibre type growth pattern

The human jaw system is different from those of other primates, carnivores, ruminants, and rodents in temporomandibular joint and muscle anatomy. In adults, jaw muscles also differ markedly from limb and trunk muscles in composition and distribution of fibre types. It can be assumed that age-related changes between young age to adulthood in terms of craniofacial growth, teeth eruption, and improvement of jaw functions are paralleled by alterations also in composition and distribution of jaw muscle fibre types. To address this question, we have examined the fibre type composition of the human masseter, a jaw closing muscle, at young age. For comparison, the young biceps brachii was examined. The results were compared with previous data for adult masseter and biceps muscles. Young masseter and biceps were similar in that type I fibres outnumbered other fibre types and were of the same diameter. However, they differed in composition of other fibre types. Young masseter contained fibre types I, IM, IIC, IIAB, IIB, and scarce IIA, with regional differences, whereas young biceps showed types I, IIA, IIAB, and few IIB. Young masseter differed from young biceps also by smaller type II fibre diameter and by containing fetal MyHC. In addition, the masseter and biceps differed in age-related changes of composition and distribution of fibre types between young age and adulthood. We conclude that the human masseter is specialized in fibre types already at young age and shows a unique fibre type growth pattern, in concordance with being a separate allotype of muscle.

The myosin light chain 1 isoform associated with masticatory myosin heavy chain in mammals and reptiles is embryonic/atrial MLC1

We recently reported that masticatory myosin heavy chain (MHC-M) is expressed as the exclusive or predominant MHC isoform in masseter and temporalis muscles of several rodent species, contrary to the prevailing dogma that rodents express almost exclusively MHC isoforms that are typically found in fast limb muscles and not masticatory myosin. We also reported that the same rodent species express the embryonic/atrial isoform of myosin light chain 1 (MLC1E/A) in jaw-closing muscles and not a unique masticatory MLC1 isoform that others have reported as being expressed in jaw-closing muscles of carnivores that express MHC-M. The objective of this study was to test the hypothesis that MLC1E/A is consistently expressed in jaw-closing muscles whenever MHC-M is expressed as the predominant or exclusive MHC isoform. Jaw-closing muscles, fast and slow limb muscles, and cardiac atria and ventricles of 19 species (six Carnivora species, one Primates species, one Chiroptera species, five marsupial species, an alligator and five turtle species) were analyzed using protein gel electrophoresis, immunoblotting, mass spectrometry and RNA sequencing. Gel electrophoresis and immunoblotting indicate that MHC-M is the exclusive or predominant MHC isoform in the jaw-closing muscles of each of the studied species. The results from all of the approaches collectively show that MLC1E/A is exclusively or predominantly expressed in jaw-closing muscles of the same species. We conclude that MLC1E/A is the exclusive or predominant MLC1 isoform that is expressed in jaw-closing muscles of vertebrates that express MHC-M, and that a unique masticatory isoform of MLC1 probably does not exist.

Expression of myosin heavy chain isoforms in the postnatal mouse masseter muscle

We investigated the properties of the masseter muscle in mice from five to seven weeks of age. Myosin heavy chain (MyHC) isoforms were measured in the masseter muscle. The three types of muscle fibers (Type I, strong reaction; Type IIA, intermediate reaction; and Type IIB, weak reaction) were all present in the masseter muscle in five-weeks-old mice and seven-weeks-old mice, the three types could be clearly distinguished by their enzyme activity. The percentage of Type IIB fibers (above 50%) was the highest among all fiber types both 5- and 7-weeks-old mice. The mRNA levels for myosin slow and myosin IIb increased significantly between 5 and 7 weeks. These observations suggest that muscle fiber size, muscle fiber types and mRNA levels of the MyHC isoforms all contribute to the diminished functional adaptability of enzyme activity in the masseter muscle.

Masticatory (;superfast') myosin heavy chain and embryonic/atrial myosin light chain 1 in rodent jaw-closing muscles

Masticatory myosin is widely expressed among several vertebrate classes. Generally, the expression of masticatory myosin has been associated with high bite force for a carnivorous feeding style (including capturing/restraining live prey), breaking down tough plant material and defensive biting in different species. Masticatory myosin expression in the largest mammalian order, Rodentia, has not been reported. Several members of Rodentia consume large numbers of tree nuts that are encased in very hard shells, presumably requiring large forces to access the nutmeat. We, therefore, tested whether some rodent species express masticatory myosin in jaw-closing muscles. Myosin isoform expression in six Sciuridae species was examined, using protein gel electrophoresis, immunoblotting, mass spectrometry and RNA analysis. The results indicate that masticatory myosin is expressed in some Sciuridae species but not in other closely related species with similar diets but having different nut-opening strategies. We also discovered that the myosin light chain 1 isoform associated with masticatory myosin heavy chain, in the same four Sciuridae species, is the embryonic/atrial isoform. We conclude that rodent speciation did not completely eliminate masticatory myosin and that its persistent expression in some rodent species might be related to not only diet but also to feeding style.

The adaptive response of jaw muscles to varying functional demands

Jaw muscles are versatile entities that are able to adapt their anatomical characteristics, such as size, cross-sectional area, and fibre properties, to altered functional demands. The dynamic nature of muscle fibres allows them to change their phenotype to optimize the required contractile function while minimizing energy use. Changes in these anatomical parameters are associated with changes in neuromuscular activity as the pattern of muscle activation by the central nervous system plays an important role in the modulation of muscle properties. This review summarizes the adaptive response of jaw muscles to various stimuli or perturbations in the orofacial system and addresses general changes in muscles as they adapt, specific adaptive changes in jaw muscles under various physiologic and pathologic conditions, and their adaptive response to non-surgical and surgical therapeutic interventions. Although the jaw muscles are used concertedly in the masticatory system, their adaptive changes are not always uniform and vary with the nature, intensity, and duration of the stimulus. In general, stretch, increases neuromuscular activity, and resistance training result in hypertrophy, elicits increases in mitochondrial content and cross-sectional area of the fibres, and may change the fibre-type composition of the muscle towards a larger percentage of slow-type fibres. In contrast, changes in the opposite direction occur when neuromuscular activity is reduced, the muscle is immobilized in a shortened position, or paralysed. The broad range of stimuli that affect the properties of jaw muscles might help explain the large variability in the anatomical and physiological characteristics found among individuals, muscles, and muscle portions.

Expression of nuclear and mitochondrial thyroid hormone receptors in postnatal rat tongue muscle

In this quantitative study, a competitive RT-PCR analysis was used to measure the level of the thyroid hormone receptors (TRs) in rat tongue muscle during the development of male Wistar rats aged 0, 5, 10, 15 and 21 postnatal days. There were differences between the expression of TR-alpha1 mRNA and the mRNAs for TR-beta1 and TR-beta2 in rat tongue muscle. Using Western blot analysis, a difference in expression between TR-alpha1 protein (c-ErbAalpha1 protein) and 43-kD c-ErbAalpha1 protein (T(3)-binding 43-kD mitochondrial protein) was detected during the development of the rat tongue muscle. Immunohistochemical examination using electron microscopy showed that TR-alpha1 was found in the mitochondria and nuclei in contrast to TR-beta1 detected in rat tongue muscle. In mitochondrial fractions from rat tongue muscle, the expression of 43-kD c-ErbAalpha1 protein was increased dramatically at 15 and 21 days, and a similar tendency was seen in cytochrome c proteins using Western blot analysis. We presume that the 43-kD c-ErbAalpha1 protein plays a role in regulating mitochondrial RNA synthesis during the postnatal development of rat tongue. The mRNA and protein myosin heavy chain isoforms of muscle also had a different expression during development. The slow myosin isoform protein was not found from day 10 in contrast to fast myosin isoforms. It is likely that the expression of TR-alpha1 mRNA from the rat tongue muscle may be related to a specific phase in muscle phenotype during the development.

Developmental and functional considerations of masseter muscle partitioning

The masseter muscle participates in a wide variety of activities including mastication, swallowing and speech. The functional demands for accurate mandibular positioning and generation of forces during incising or a power stroke require a diverse set of forces that are determined by the innate muscle form. The complex internal tendon architecture subdivides the masseter into multiple partitions that can be further subdivided into neuromuscular compartments representing small motor unit territories. Individual masseter compartments have unique biomechanical properties that, when activated individually or in groups, can generate a wide range of sagittal and off-sagittal torques about the temporomandibular joint. The myosin heavy chain (MyHC) fibre-type distribution in the adult masseter is sexually dimorphic and is influenced by hormones such as testosterone. These testosterone-dependent changes cause a phenotype switch from slower to faster fibre-types in the male. The development of the complex organization of the masseter muscle, the MyHC fibre-type message and protein expression, and the formation of endplates appear to be pre-programmed and not under control of the muscle nerve. However, secondary myotube generation and endplate maturation are nerve dependent. The delayed development of the masseter muscle compared with the facial, tongue and jaw-opening muscles may be related to the delayed functional requirements for chewing. In summary, masseter muscle form is pre-programmed prior to birth while muscle fibre contractile characteristics are refined postnatally in response to functional requirements. The motor control mechanisms that are required to coordinate the activation of discrete functional elements of this muscle remain to be determined.

Fiber-type composition of the human jaw muscles--(part 2) role of hybrid fibers and factors responsible for inter-individual variation

This is the second of two articles about fiber-type composition of the human jaw muscles. It reviews the functional relationship of hybrid fibers and the adaptive properties of jaw-muscle fibers. In addition, to explain inter-individual variation in fiber-type composition, we discuss these adaptive properties in relation to environmental stimuli or perturbations. The fiber-type composition of the human jaw muscles is very different from that of limb and trunk muscles. Apart from the presence of the usual type I, IIA, and IIX myosin heavy-chains (MyHC), human jaw-muscle fibers contain MyHCs that are typical for developing or cardiac muscle. In addition, much more frequently than in limb and trunk muscles, jaw-muscle fibers are hybrid, i.e., they contain more than one type of MyHC isoform. Since these fibers have contractile properties that differ from those of pure fibers, this relatively large quantity of hybrid fibers provides a mechanism that produces a very fine gradation of force and movement. The presence of hybrid fibers might also reflect the adaptive capacity of jaw-muscle fibers. The capacity for adaptation also explains the observed large inter-individual variability in fiber-type composition. Besides local influences, like the amount of muscle activation and/or stretch, more general influences, like aging and gender, also play a role in the composition of fiber types.

Selective expression of the type 3 isoform of ryanodine receptor Ca2+ release channel (RyR3) in a subset of slow fibers in diaphragm and cephalic muscles of adult rabbits

The expression pattern of the RyR3 isoform of Ca2+ release channels was analysed by Western blot in neonatal and adult rabbit skeletal muscles. The results obtained show that the expression of the RyR3 isoform is developmentally regulated. In fact, RyR3 expression was detected in all muscles analysed at 2 and 15 days after birth while, in adult animals, it was restricted to a subset of muscles that includes diaphragm, masseter, pterygoideus, digastricus, and tongue. Interestingly, all of these muscles share a common embryonic origin being derived from the somitomeres or from the cephalic region of the embryo. Immunofluorescence analysis of rabbit skeletal muscle cross-sections showed that RyR3 staining was detected in all fibers of neonatal muscles. In contrast, in those adult muscles expressing RyR3 only a fraction of fibers was labelled. Staining of these muscles with antibodies against fast and slow myosins revealed a close correlation between expression of RyR3 and fibers expressing slow myosin isoform.

Human single masseter muscle fibers contain unique combinations of myosin and myosin binding protein C isoforms

Striated craniofacial and limb muscles differ in their embryological origin, regulatory program during myogenesis, and innervation. In an attempt to explore the effects of these differences on the striated muscle phenotype in humans, the expression of myosin and myosin-associated thick filament proteins were studied at the single fiber level both in the human jaw-closing masseter muscle and in two limb muscles (biceps brachii and quadriceps femoris muscles). In the masseter, unique combinations of myosin heavy chain (MyHC) and myosin binding protein C (MyBP-C) isoforms were observed at the single fiber level. Compared to the limb muscles, the MyHC isoform expression was more complex in the masseter while the opposite was observed for MyBP-C. In limb muscles, a coordinated expression of three MyHC and three MyBP-C isoforms were observed, i.e., single fibers contained one or two MyHC isoforms, and up to three MyBP-C isoforms. Also, the relative content of the different MyBP-C isoforms correlated with the MyHC isoform expression. In the masseter, on the other hand, up to five different MyHC isoforms could be observed in the same fiber, but only one MyBP-C isoform was identified irrespective MyHC isoform expression. This MyBP-C isoform had a migration rate similar to the slow MyBP-C isoform in limb muscle fibers. In conclusion, a unique myofibrillar protein isoform expression was observed in the human masseter muscle fibers, suggesting significant differences in structural and functional properties between muscle fibers from human masseter and limb muscles.

Specialized cranial muscles: how different are they from limb and abdominal muscles?

Mammalian skeletal muscle fibers can be classified into functional types by the heavy chain (MyHC) and light chain (MyLC) isoforms of myosin (the primary motor protein) that they contain. Most human skeletal muscle contains fiber types and myosin isoforms I, IIA and IIX. Some highly specialized muscle fibers in human extraocular and jaw-closing muscles express either novel myosins or unusual combinations of isoforms of unknown functional significance. Extrinsic laryngeal muscles may express the extraocular MyHC isoform for rapid contraction and a tonic MyHC isoform for slow tonic contractions. In jaw-closing muscles, fiber phenotypes and myosin expression have been characterized as highly unusual. The jaw-closing muscles of most carnivores and primates have tissue-specific expression of the type IIM or 'type II masticatory' MyHC. Human jaw-closing muscles, however, do not contain IIM myosin. Rather, they express myosins typical of developing or cardiac muscle in addition to type I, IIA and IIX myosins, and many of their fibers are hybrids, expressing two or more isoforms. Fiber morphology is also unusual in that the type II fibers are mostly of smaller diameter than type I. By combining physiological and biochemical techniques it is possible to determine the maximum velocity of unloaded shortening (V(o)) of an individual skeletal muscle fiber and subsequently determine the type and amount of myosin isoform. When analyzed, some laryngeal fibers shorten at much faster rates than type II fibers from limb and abdominal muscle. Yet some type I fibers in masseter show an opposite trend towards speeds 10-fold slower than type I fibers of limb muscle. These unusual shortening velocities are most probably regulated by MyHC isoforms in laryngeal fibers and by MyLC isoforms in masseter. For the jaw-closing muscles, this finding represents the first case in human muscle of physiological regulation of kinetics by light chains. Together, these results demonstrate that, compared to other skeletal muscles, cranial muscles have a wider repertoire of contractile protein expression and function. Molecular techniques for reverse transcription of mRNA and amplification by polymerase chain reaction have been applied to typing of single fibers isolated from limb muscles, successfully identifying pure type I, IIA and IIX and hybrid type I/IIA and IIA/IIX fibers. This demonstrates the potential for future studies of the regulation of gene expression in jaw-closing and laryngeal muscles, which have such a variety of complex fiber types fitting them for their roles in vivo.

Molecular and cellular mechanisms involved in the generation of fiber diversity during myogenesis

Skeletal muscles have a characteristic proportion and distribution of fiber types, a pattern which is set up early in development. It is becoming clear that different mechanisms produce this pattern during early and late stages of myogenesis. In addition, there are significant differences between the formation of muscles in head and those found in rest of the body. Early fiber type differentiation is dependent upon an interplay between patterning systems which include the Wnt and Hox gene families and different myoblast populations. During later stages, innervation, hormones, and functional demand increasingly act to determine fiber type, but individual muscles still retain an intrinsic commitment to form particular fiber types. Head muscle is the only muscle not derived from the somites and follows a different development pathway which leads to the formation of particular fiber types not found elsewhere. This review discusses the formation of fiber types in both head and other muscles using results from both chick and mammalian systems.

Human skeletal muscle satellite cells: aging, oxidative stress and the mitotic clock

Normal satellite cell cultures, isolated from human skeletal muscle, have a limited proliferative capacity and inevitably reach replicative senescence. In this study, we have focused on the consequences of a single oxidative stress by hydrogen peroxide (H(2)O(2)) on both proliferative capacity and myogenic characteristics. Treatment with 1mM H(2)O(2) for 30 min causes a small decrease in the viability and lifespan while the number of cells which are able to proliferate, decreases dramatically. This premature arrest of the cells in a non-proliferative state was not due to spontaneous differentiation since there was no increase in the number of myogenin positive cells. This stress did not affect the myogenicity of the cells or their ability to differentiate and fuse to form multinucleated myotubes. In addition, the mitotic clock does not seem to be modified by oxidative stress treatment since the rate of telomere shortening was similar in H(2)O(2)-treated and control cells. This could be the consequence of the high level of oxygen consumption with an even higher level of ROS being produced in skeletal muscle than in other tissues which would be counteracted by an increase in the antioxidant defense system.

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.

Morphological characterization of the levator veli palatini muscle in children born with cleft palates

OBJECTIVE

The aim of this study was to analyze, morphologically and biochemically, one of the soft palate muscles, the levator veli palatini (LVP), in children born with cleft palate.

SUBJECTS AND METHODS

Biopsies were obtained from nine male and three female infants in connection with the early surgical repair of the hard and soft palate. Samples from five adult normal LVP muscles were used for comparison. The muscle morphology, fiber type and myosin heavy chain (MyHC) compositions, capillary supply, and content of muscle spindles were analyzed with different enzyme-histochemical, immunohistochemical, and biochemical techniques.

RESULTS

Compared with the normal adult subjects, the LVP muscle from the infantile subjects with cleft had a smaller mean fiber diameter, a larger variability in fiber size and form, a higher proportion of type II fibers, a higher amount of fast MyHCs, and a lower density of capillaries. No muscle spindles were observed. Moreover, one-third of the biopsies from the infantile subjects with cleft LVP either lacked muscle tissue or contained only a small amount.

CONCLUSIONS

The LVP muscle from children with cleft palate has a different morphology, compared with the normal adult muscle. The differences might be related to different stages in maturation of the muscles, changes in functional demands with growth and age, or a consequence of the cleft. The lack of contractile tissue in some of the cleft biopsies offers one possible explanation to a persistent postsurgical velopharyngeal insufficiency in some patients, despite a successful surgical repair.

Histochemical studies of the masseter, the temporal and small zygomaticomandibular, and the temporomandibular masticatory muscles from aged male and female humans. Fiber types and myosin isoforms

The purpose of this study was to investigate the histology of two small masticatory muscles from females and males of more than 70 years of age. By using immuno- and enzyme histochemistry the muscles were characterized by their fiber types and myosin heavy chain pattern. The observations were compared with similar studies of the masseter and temporalis muscles. Previously the two small muscles have been described based solely upon their gross anatomy. One muscle originates from the anterior, deep surface of the temporal fascia and inserts in the temporal tendon: the temporo-mandibular muscle (TM). The other muscle originates from the upper part of the temporal surface of the frontal process of the zygomatic bone and the adjacent part of the frontal bone and inserts in the temporal tendon: the zygomaticomandibular muscle (ZM). In the masseter, TM, and ZM, most of the autopsy samples contained an abundant number of fibers containing neonatal myosin heavy chains while in the temporal muscle specimens, such fibers were sparse and scattered. Electrophoresis followed by immuno-staining of Western blots supported the histochemical findings. There was no obvious correspondence between fiber typing based upon ATPase activity and the neonatal myosin heavy chain content in the muscle fibers. Neither did the fibers show accordance in their content of adult slow and fast myosin heavy chains and in their content of neonatal myosin heavy chain.

Intermuscular and intramuscular differences in myosin heavy chain composition of the human masticatory muscles

Among and within the human masticatory muscles a large number of anatomical differences exists indicating that different muscles and muscle portions are specialized for certain functions. In the present study we investigated whether such a specialization is also reflected by intermuscular and intramuscular differences in fibre type composition and fibre cross-sectional area. Fibre type compositions and fibre cross-sectional areas of masticatory muscles were determined in eight cadavers using monoclonal antibodies against myosin heavy chain (MyHC). The temporalis, masseter and pterygoid muscles could be characterized by a relatively large number of fibres containing more than one MyHC isoform (hybrid fibres). In these muscles a large number of fibres expressed MyHC-I, MyHC-fetal and MyHC-cardiac alpha. Furthermore, in these muscles type I fibres had larger cross-sectional areas than type II fibres. In contrast, the mylohyoid, geniohyoid and digastric muscle were characterized by less hybrid fibres, and by less fibres expressing MyHC-I, MyHC-fetal, and MyHC-cardiac alpha, and by more fibres expressing MyHC-IIA; the cross-sectional areas of type I and type II fibres in these muscles did not differ significantly. Compared to the masseter and pterygoid muscles, the temporalis had significantly larger fibres and a notably different fibre type composition. The mylohyoid, geniohyoid, and digastric muscles did not differ significantly in their MyHC composition and fibre cross-sectional areas. Also intramuscular differences in fibre type composition were present, i.e., a regionally higher proportion of MyHC type I fibres was found in the anterior temporalis, the deep masseter, and the anterior medial pterygoid muscle portions; furthermore, significant differences were found between the bellies of the digastric.

Characterisation of human soft palate muscles with respect to fibre types, myosins and capillary supply

Four human soft palate muscles, and palatopharyngeus, the uvula, the levator and tensor veli palatini were examined using enzyme-histochemical, immunohistochemical and biochemical methods and compared with human limb and facial muscles. Our results showed that each palate muscle had a distinct morphological identity and that they generally shared more similarities with facial than limb muscles. The palatopharyngeus and uvula muscles contained 2 of the highest proportions of type II fibres ever reported for human muscles. In contrast, the levator and tensor veli palatini muscles contained predominantly type I fibres. A fetal myosin heavy chain isoform (MyHC), not usually found in normal adult limb muscles, was present in a small number of fibres in all palate muscles. The mean muscle fibre diameter was smaller than in limb muscles and the individual and intramuscular variability in diameter and shape was considerable. All palate muscles had a high capillary density and an unusually high mitochondrial enzyme activity in the type II fibres, in comparison with limb muscles. No ordinary muscle spindles were observed. The fibre type and MyHC composition indicate that the palatopharyngeus and uvula muscles are functionally involved in quick movements whereas the levator and tensor veli palatini muscles perform slower and more continuous contractions. The high aerobic capacity and the rich capillarisation suggest that the palate muscles are relatively fatigue resistant. Absence of ordinary muscle spindles indicates a special proprioceptive control system. The special morphology of the palate muscles may be partly related to the unique anatomy with only one skeletal insertion, a feature consistent with muscle work at low load and tension and which may influence the cytoarchitecture of these muscles. Other important factors determining the special morphological characteristics might be specific functional requirements, distinct embryological origin and phylogenetic factors.

Myosin heavy chain composition of the human lateral pterygoid and digastric muscles in young adults and elderly

The myosin heavy chain (MyHC) content in different parts of, two jaw opening muscle, the human lateral pterygoid and the digastric muscles of five young adult and five elderly subjects (mean age 22 and 73 years, respectively) was determined, using gel electrophoresis and immunohistochemical methods. The lateral pterygoid of both young and elderly contained predominantly slow MyHC, and fast A MyHC was the major fast isoform. In contrast, the digastric was composed of slow, fast A and fast X MyHCs in about equal proportions in both age groups. About half of the lateral pterygoid fibres contained mixtures of slow and fast MyHCs, often together with alpha-cardiac MyHC. In the digastric, co-existence of slow and fast MyHCs was rare, and alpha-cardiac MyHC was lacking. On the other hand, co-expression of fast A and fast X MyHCs was found more often in the digastric than in the lateral pterygoid. In both age groups about half of the digastric IIB fibres contained solely fast X MyHC. In the lateral pterygoid, type IIB fibres with pure fast X MyHC was found in only one subject. The lateral pterygoid in elderly showed a significant amount of fibres with solely fast A MyHC, which were occasionally found in young adults. In the digastric, no significant differences were found between young and elderly, although the muscles of elderly contained lower mean value of slow MyHC, as compared to that of young muscles. It is concluded that the lateral pterygoid and the digastric muscles differ not only in the MyHC composition but also in modifications of the MyHC phenotypes during aging, suggesting that they have separate roles in jaw opening function.

Regional differences of metabolism in human masseter muscle by two-dimensional 31P-chemical shift imaging

Many reports have demonstrated significant region-dependent differences in the fiber-type composition of the human masseter muscle. Therefore, it is considered that there is intramuscular heterogeneity of metabolic activity in the muscle. The present study was carried out, with two-dimensional Chemical Shift Imaging, to detect differences between the deep and superficial parts of the human masseter muscle at rest. Masseter muscle from 11 volunteers, from 20 to 27 years old, was examined, and characteristic spectra of the inorganic phosphate (Pi), phosphocreatine (PCr), and alpha-, beta-, and gamma-adenosine triphosphate (ATP) from each part of the muscles were obtained. In this study, the deep and superficial parts of the masseter muscle were distinguished by the existence of aponeurosis. The Pi/PCr, PCr/beta-ATP, and Pi/beta-ATP ratios as well as the pH in the deep and superficial parts were calculated from the peak spectra. Compared with the deep part, the Pi/PCr of the superficial part was lower (p < 0.05) and the PCr/beta-ATP was higher (p < 0.01). The Pi/beta-ATP and pH showed no significant differences between the two parts. The results indicate that the superficial part of the masseter muscle contains more PCr than the deep part, and this may be related to functional differences between these two parts. In future examinations of the metabolic activity of the human masseter muscle, the deep and superficial parts must be measured separately.

Oro-facial muscles: internal structure, function and ageing

Structure and function are reviewed in the masticatory muscles and in the muscles of the lower face and tongue. The enormous strength of jaw closure is in large part due to the pinnated arrangement of the muscle fibres in the masseter. This muscle, like other masticatory muscles, is unusual in that the cell bodies of the muscle spindle afferents lie in the brain stem rather than in an external ganglion; spindles are absent in the lower facial muscles. Although few data are available, the numbers of motor units in the masticatory muscles, and probably in the lower facial muscles also, appear to be much greater than in limb muscles. The motor units in the facial and tongue muscles are largely composed of histochemical type II ('fast-twitch') fibres, but in the masticatory muscles there are substantial numbers of fibres intermediate between type I ('slow twitch') and type II, and fibre type grouping is present. In comparison with limb muscles, there is little information on ageing changes in oro-facial muscles. The masticatory muscles do, however, show some atrophy and loss of X-ray density, while motor unit twitches are prolonged. Strength is reduced in the tongue and masticatory muscles. It is known that limb muscle properties are largely governed by their innervation, both through the pattern and amount of impulse activity, and the delivery of trophic messengers; the situation for oro-facial muscles is unclear. The structural and functional differences between the two types of muscle indicate the need for conducting ageing studies on the oro-facial muscles, rather than relying on extrapolations from limb muscles.

Opposite changes in myosin heavy chain composition of human masseter and biceps brachii muscles during aging

The myosin heavy chain (MyHC) content in functionally different parts of the human masseter muscle of six elderly and five young adult subjects (mean age 74 and 22 years, respectively) was determined, using gel electrophoresis. The MyHC composition of the old masseter was also studied by enzyme- and immunohistochemical methods and compared with previous data for young adults. For comparison, the biceps brachii muscle of the same subjects was also analysed. The old masseter contained smaller amounts of slow and larger amounts of fast and fetal MyHCs. These differences were region-dependent and were more pronounced in the superficial portion. There was also a larger proportion of "hybrid" fibres, containing two to four MyHC isoforms (42%), compared with the young adult masseter (23%). No such differences were observed between old and young biceps. In contrast to the masseter, the old biceps contained more slow MyHC and less fast MyHC. This investigation demonstrates that the aging process in human skeletal muscle is accompanied by a modification in the muscle phenotype which is both muscle and region specific; a transformation towards a fast and fetal phenotype concomitant with an increased number of fibres with a mixture of different MyHC isoforms in the masseter; and an opposite shift towards a slower phenotype in the biceps brachii. The results might reflect differences between jaw and limb muscles in genetic programs and adaptive responses to changed functional demands following aging.

Regional differences in fibre type composition in the human temporalis muscle

Anatomical and electromyographic studies point to regional differences in function in the human temporalis muscle. During chewing and biting the anterior portions of the muscle are in general more intensively activated and they are capable of producing larger forces than the posterior portions. It was hypothetised that this heterogeneity in function is reflected in the fibre type composition of the muscle. The composition and surface area of different fibre types in various anteroposterior portions of the temporalis muscle were investigated in 7 cadavers employing immunohistochemistry with a panel of monoclonal antibodies against different isoforms of myosin heavy chain. Pure slow muscle fibres, type I, differed strongly in number across the muscle. In the most posterior portion of the muscle there were 24% type I fibres, in the intermediate portion 57%, and in the most anterior portion 46%. The mean fibre cross-sectional area (m-fcsa) of type I fibres was 1849 microm2, which did not differ significantly across the muscle. The proportion of pure fast muscle fibres, type IIA and IIX, remained more or less constant throughout the muscle at 13% and 11% respectively; their m-fcsa was 1309 microm2 and 1206 microm2, respectively, which did not differ significantly throughout the muscle. Pure type IIB fibres were not found. The relative proportion of hybrid fibres was 31% and did not differ significantly among the muscle portions. Fibre types I + IIA and cardiac alpha + I + IIA were the most abundant hybrid fibre types. In addition, 5% of the type I fibres had an additional myosin isoform which has only recently been described by means of electrophoresis and was named Ia. In the present study they were denoted as hybrid type I + Ia muscle fibres. It is concluded that intramuscular differences in type I fibre distribution are in accordance with regional differences in muscle function.

Expression of an embryonic fibronectin splicing variant in human masseter muscle

Fibronectin is an extracellular matrix protein that is a key constituent of all skeletal muscles. Its deposition increases in a number of pathological conditions, including some muscular dystrophies in which a progressive increase in lower-face height is often noted. It has been shown in other organ systems that increased deposition of fibronectin is associated with changes in the expression of differentially spliced isoforms of the molecule. This investigation documents the expression of mRNA coding for fibronectin and its splicing variants, EIIIA and EIIIB, in biopsies of masseter muscle from normal and long-face patients, using reverse transcriptase-polymerase chain reaction. Expression was compared with that occurring in anatomically and embryologically differing somatic skeletal muscle. Masseter expressed fibronectin mRNA containing the EIIIA but not the EIIIB splicing variant. Conversely, somatic skeletal muscle expressed neither the EIIIA nor EIIIB variants. There were no differences between expression of fibronectin containing the EIIIA splicing variants in masseter from normal and long faces. These results suggest that variations in fibronectin expression reflect the differing functional demands of muscles from different anatomical locations and indicate that jaw and somatic muscle belong to different allotypes.