Desmin and nerve terminal expression during embryonic development of the lateral pterygoid muscle in mice.

OBJECTIVE: In adults, the lateral pterygoid muscle (LPM) is usually divided into the upper and lower head, between which the buccal nerve passes. Recent investigations have demonstrated foetal developmental changes in the topographical relationship between the human LPM and buccal nerve. However, as few studies have investigated this issue, we clarified the expression of desmin and nerve terminal distribution during embryonic development of the LPM in mice. METHODS: We utilized immunohistochemical staining and reverse transcription chain reaction (RT-PCR) to clarify the expression of desmin and nerve terminal distribution. RESULTS: We observed weak expression of desmin in the LPM at embryonic day (ED) 11, followed by an increase in expression from embryonic days 12-15. In addition, starting at ED 12, we observed preferential accumulation of desmin in the vicinity of the myotendinous junction, a trend that did not change up to ED 15. Nerve terminal first appeared at ED 13 and formed regularly spaced linear arrays at the centre of the muscle fibre by ED 15. The results of immunohistochemical staining agreed with those of RT-PCR analysis. CONCLUSION: We found that desmin accumulated in the vicinity of the myotendinous junction starting at ED 12, prior to the onset of jaw movement. We speculate that the accumulation of desmin is due to factors other than mechanical stress experienced during early muscle contraction. Meanwhile, the time point at which nerve terminals first appeared roughly coincided with the onset of jaw movement.

Fetal developmental change in topographical relationship between the human lateral pterygoid muscle and buccal nerve.

In adults, the lateral pterygoid muscle (LPM) is usually divided into the upper and lower heads, between which the buccal nerve passes. Using sagittal or horizontal sections of 14 fetuses and seven embryos (five specimens at approximately 20-25 weeks; five at 14-16 weeks; four at 8 weeks; seven at 6-7 weeks), we examined the topographical relationship between the LPM and the buccal nerve. In large fetuses later than 15 weeks, the upper head of the LPM was clearly discriminated from the lower head. However, the upper head was much smaller than the lower head in the smaller fetuses. Thus, in the latter, the upper head was better described as an 'anterior slip' extending from the lower head or the major muscle mass to the anterior side of the buccal nerve. The postero-anterior nerve course seemed to be determined by a branch to the temporalis muscle (i.e. the anterior deep temporal nerve). At 8 weeks, the buccal nerve passed through the roof of the small, fan-like LPM. At 6-7 weeks, the LPM anlage was embedded between the temporobuccal nerve trunk and the inferior alveolar nerve. Therefore, parts of the LPM were likely to 'leak' out of slits between the origins of the mandibular nerve branches at 7-8 weeks, and seemed to grow in size during weeks 14-20 and extend anterosuperiorly along the infratemporal surface of the prominently developing greater wing of the sphenoid bone. Consequently, the topographical relationship between the LPM and the buccal nerve appeared to 'change' during fetal development due to delayed development of the upper head.

Spry1 and spry2 are essential for development of the temporomandibular joint.

The temporomandibular joint (TMJ) is a specialized synovial joint essential for the function of the mammalian jaw. The main components of the TMJ are the mandibular condyle, the glenoid fossa of the temporal bone, and a fibrocartilagenous disc interposed between them. The genetic program for the development of the TMJ remains poorly understood. Here we show the crucial role of sprouty (Spry) genes in TMJ development. Sprouty genes encode intracellular inhibitors of receptor tyrosine kinase (RTK) signaling pathways, including those triggered by fibroblast growth factors (Fgfs). Using in situ hybridization, we show that Spry1 and Spry2 are highly expressed in muscles attached to the TMJ, including the lateral pterygoid and temporalis muscles. The combined inactivation of Spry1 and Spry2 results in overgrowth of these muscles, which disrupts normal development of the glenoid fossa. Remarkably, condyle and disc formation are not affected in these mutants, demonstrating that the glenoid fossa is not required for development of these structures. Our findings demonstrate the importance of regulated RTK signaling during TMJ development and suggest multiple skeletal origins for the fossa. Notably, our work provides the evidence that the TMJ condyle and disc develop independently of the mandibular fossa.

[Comparative morphometric analysis of structural organization of separate heads of the human lateral pterygoid muscle during prenatal period and the first 3 years of the postnatal period].

It was discovered some features in structure and differentiation of different types of the muscular fibers in each of the two heads of the lateral pterygoid muscle during ontogenesis. Taken characteristics gave possibility to think about different functional significance of data of structure in action of the temporomandibular joint of a man.

Insertion of the Superior Head of the Lateral Pterigoid Muscle in the Human Fetuses / Inserción de la Cabeza Superior del Músculo Pterigoideo Lateral en Fetos Humanos

The lateral pterygoid muscle, more specifically its superior head, as we know, is closely related to the temporomandibular joint (TMJ). Particularly in children, in contrast with what was observed in adults, these joints have been rarely studied, by the anatomic functional aspect, little knowing about its functions in the embryonic and fetal periods. We used, in this work, 12 fetuses ranging in age from 16 to 39 weeks of intrauterine life, where we observed that the superior head of the lateral pterygoid muscle is inserted in the disc and in the articular capsule, in all age groups studied, and also, that the fibers and the thickness of the articular disc is, as well as the articular capsule suffer modifications in accordance with the period of development.

El músculo petrigoideo lateral, más específicamente su cabeza superior, como es conocida, está estrechamente relacionada con la articulación témporomandibular. Particularmente en niños, en contraste con lo observado en adultos, estas articulaciones han sido raramente estudiadas, por aspectos anatomofuncionales, escasos conocimientos de sus funciones en los períodos embrionario y fetal. Fueron utilizados 12 fetos, de 16 a 19 semanas de vida intrauterina, en los cuales fue observada que la cabeza superior del músculo petrigoideo lateral estaba insertada en el disco y en la cápsula articular, en todos los grupos estudiados. Además, fue posible observar que, tanto las fibras y el espesor del disco articular, como la cápsula articular, sufren modificaciones de acuerdo con el período de desarrollo.

Morphology of the lateral pterygoid muscle associated to the mandibular condyle in the human prenatal stage.

The lateral pterygoid muscle (LPM) inserts at the condyle and the articular disc and plays a central role in mandibular movement via the Temporomandibular Articular Complex. The aim of this study was to examine the association between the morphology of LPM muscular fascicles and the degree of mineralization of the mandibular condyle in the prenatal stage employing structural, ultrastructural and microanalytical evaluation. Sixteen human fetuses at 11-37 weeks of gestation, with no apparent pathology and resulting from spontaneous abortions, were included in the study. Samples from lateral pterygoid muscle and the mandibular condyle were processed for light microscopy and electron microscopy and microanalysis. Desmin immunolabeling (dilution 1: 25 Dako) and alpha sarcomeric actin immunolabeling (dilution 1:50 Dako) employing the avidin-biotin system were used in paraffin embedded samples. Contralateral samples were examine by transmission electron microscopy. Four condyles (at 17-21 weeks of gestation) were used to measure the relative content of calcium and phosphorous employing the X-ray diffraction microanalytical technique. At 11-16 weeks of gestation, the LPM was composed of secondary myotubes associated to satellite cells and nerve fibers. At 18 weeks, the muscle exhibited multiple compact fascicles and the condyle showed a thin, external, subperiostal mineralized layer with few central bone spicules. At 20 weeks, at the site of insertion of the LPM, the bone trabeculae of the condyle contained an electrondense matrix with abundant mineralization nuclei. At 17-21 weeks of gestation no significant variations in the contents of phosphorous and calcium were observed. At 24 weeks, transmission electron calcium and microscopy studies revealed a marked increase in the functional units of the muscle fascicles. Also, at this age muscle fibers exhibited differences in the expression of desmin and alpha sarcomeric actin. At 37 weeks the muscle became multipennate in appearance, exhibiting a more complex organization than younger fetuses. Alpha sarcomeric actin labeling became light with age. This results suggest that between 16 and 22 weeks of gestation the differentiation and maturation process of the muscle fibers precedes and prevails over the development and mineralization process from mandibular condyle. The rudimentary performance of the prenatal LPM would be one of the factors that regulate the process of ossification at the level of the mandibular condyle. The rate of ossification would increase starting from 22 of gestation week.

Computerized 3-dimensional study of the embryologic development of the human masticatory muscles and temporomandibular joint.

PURPOSE: In this study, the development of human embryonic temporomandibular joint (TMJ) and masticatory muscles were investigated by using computed 3-dimensional reconstructions. MATERIALS AND METHODS: Sixteen human embryos and fetuses, ranging from 6.5 to 107 mm crown-rump length, were examined. RESULTS: At 10 weeks, a band of mesenchyme extending from the attachment of the lateral pterygoid muscle to the condylar process was observed to pass through the medial side of the condylar process to attach to the malleus. The temporal, masseter, and pterygoid muscles develop from the so called "temporal muscle" primordium, and the temporal muscle was in continuity with the masseter muscle until 14 weeks of fetal life. CONCLUSIONS: The study shows that the muscles of mastication arise from a single primordium. It also confirms the presence of a ligamentous attachment between the lateral pterygoid muscle and the malleus.

Development of the human temporomandibular joint. Computer-aided 3D-reconstructions.

Computer-aided graphical three-dimensional reconstructions of histological serial sections of 12 human embryos and fetuses (25-250 mm Crown-rump length (CRL)) were used to trace the prenatal development of the elements of the human temporomandibular joint. The primordia of the condylar and coronoid processes could be identified as two bony peaks at the dorsal ends of the mandible at the stage of 25 mm CRL. The primordium of the temporal bone already existed at the stage of 37 mm CRL. The bone was apparent with a convex contour towards the condyle. The glenoid fossa was not yet visible. At 65 mm CRL, the osseous glenoid fossa could be distinguished at the enlarged temporal bone formation. The glenoid fossa developed posteriorly and medially from the condyle and extended in cranial and anterior direction. The glenoid fossa had various contours, changing from flat and slightly convex (65 mm CRL) to concave (250 mm CRL) with an articular tubercle. The distance between fossa and condyle increased proportionally. The lower joint cavity appeared at an earlier stage (65 mm CRL) than the upper joint cavity (70 mm CRL). Both cavities started development as isolated compartments fusing later on. The upper joint cavity followed the contour of the fossa, whereas the lower joint cavity followed the form of the condyle. The biconcave shape of the articular disc as well as the attachment of the lateral pterygoid muscle could be observed very early (70 mm CRL).

Apoptosis in the development of the temporomandibular joint.

Apoptosis has been shown to be involved in remodeling of organs during development, and derangement of the apoptotic process may result in temporomandibular joint (TMJ) dysfunction or congenital malformation. To investigate the relationship between the development of the TMJ and apoptosis, rat fetuses at 17.5-20.5 days of gestation (E17.5-20.5, vaginal plug=E0) and rats at postnatal days 1, 2, 3, 5, and 10 (P1, 2, 3, 5, and 10) were examined by light (LM) and transmission electron microscopy (TEM) and electrophoretic analysis of DNA fragmentation. At E17.5 and 18.5, a few layers of slender mesenchymal cells which eventually develop into the TMJ disk were observed, although TEM or electrophoresis did not reveal apoptotic cells at these stages. At E19.5 and 20.5, all structures of the TMJ except the lower joint cavity could be distinguished, but at these stages apoptotic cells were not observed. In P1 condyles, apoptotic cells were observed by TEM both at the subsurface of the condyle and in the region at which the lateral pterygoid muscle attaches to the condyle. These apoptotic cells showed irregular chromatin condensation, convolution of the cell membrane, and fragmentation and disintegration of the cytoplasm. Electrophoretic analysis of the P1 condyle further confirmed DNA fragmentation. Apoptosis was not observed in all specimens at the P1 stage. It was confirmed in 8 out of 20 animals (10 out of 27 joints) by TEM and/or electrophoretic analysis. The shape of the upper portion of the condyle flattened progressively from E20.5 to P2. At this stage, the lower joint cavity was developing, as observed by LM. These findings suggest that the morphological changes of the mandibular condyle effected by apoptosis, together with development of the lower joint cavity, play important roles in the postnatal functional adaptation to external stimuli such as mechanical strain.

[The development and morphofunctional characteristics of the human lateral pterygoid muscle]. / Razvitie i morfofunktsional'naia kharakteristika lateral'noi krylovidnoi myshtsy cheloveka.

Peculiarities of the development of superior and inferior heads of lateral pterygoid muscle (LPM) as well as their micro- and ultramicroscopic structure were studied as related to the temporomandibular joint elements in human fetuses and newborns. As a result, heterogeneity and asynchronism of the development of myogenic elements of certain LPM heads were demonstrated. The process of muscle fibres differentiation in LPM superior head leads to more mature structural organization and runs faster than in the inferior head. By the moment of birth average section area of superior head muscle fibres is significantly higher than the inferior head similar parameter.

A morphometric investigation of myotube formation in rabbit embryo medial pterygoid muscle.

To determine the times of the appearance of myoblasts, early myotubes, late myotubes, and myofibers, we studied a region between two aponeuroses of the medial pterygoid masticatory muscle in embryos of two strains of rabbits, without disturbing the normal innervation. The objectives of this study were to define the quantitative relations among these cells and to determine their kinetics statistically. We used Fauve de Bourgogne and New Zealand rabbit embryos on day 17, day 17 plus 12 hours, day 18, day 18 plus 12 hours, and days 20, 22, and 28 of gestation. Cell proliferation was studied with a light microscope, by means of counting methods. Similar development was observed in the two strains of rabbits. The numbers of myoblasts decreased as follows: (i) a marked decrease; (ii) a sudden cessation of the decrease, marked by a rebound at 18 days, and lasting less than 24 hours; and (iii) a plateau between embryonic days 22 and 28. The onset of reduction in the number of early myotubes coincided with the rebound of myoblasts. The number of late myotubes increased at the time of maximal early myotube density and during rebound of the myoblasts. Myofiber densities were similar to late myotube densities on day 22. We suggest that early myotubes are formed very gradually by fusion of myoblasts, and that the significant increase in the numbers of myoblasts corresponds to the second generation of myoblasts necessary for differentiation of late myotubes.

The morphogenesis of the human discomalleolar and sphenomandibular ligaments.

The morphogenesis of the discomalleolar ligament and its relationship with the sphenomandibular ligament were studied in human embryos and fetuses, on histological grounds. Total number of 18 embryos and fetuses, ranging from 6.5 to 230 mm (5-25 weeks of fertilization age) were examined. The discomalleolar ligament emerged from the posterior part of the temporomandibular joint capsule and disc, passing through the squamotympanic fissure, joined the malleus. The superior fibres of the ligament inserted on the anterior process of the malleus and on the bony wall of the squamotympanic fissure. The inferior fibres of the discomalleolar ligament encircles the anterior malleolar ligament, the remnant of Meckel's cartilage, and chorda tympani, and inserted on the tympanic wall of the temporal bone. Within the tympanic cavity, the discomalleolar ligament and the anterior malleolar ligament, a continuation of the sphenomandibular ligament, formed a horizontal 'V' shape at the attachment site on the ventral surface of the malleus. The study shows that the discomalleolar ligament is an embryological continuation of the sheath of the lateral pterygoid muscle. The sphenomandibular ligament derives from Meckel's cartilage.

The development of the human lateral pterygoid muscle and the temporomandibular joint and related structures: a three-dimensional approach.

The development of the human fetal temporomandibular joint and surrounding structures was investigated in 11 specimens on the basis of histologic examination and three-dimensional reconstructions. Until the 10th week, there were no signs of the disc, joint spaces and capsule formation of the temporomandibular joint (TMJ). Apart from Meckel's cartilage, all the temporomandibular joint and related structures attained their adult shape at 14 weeks. Throughout the embryologic and fetal development, the relative positions of the branches of the mandibular nerve remained unchanged. From 11-12 weeks onwards, the lateral pterygoid muscle became a complex structure which was segmented by aponeuroses dividing the muscle into three main parts: superior, infero-medial and infero-anterior parts. The superior segment was attached to the TMJ disc superiorly and medially. The infero-medial segment was inserted onto the antero-medial aspect of the TMJ condyle and disc. The infero-anterior portion was attached to the anterior aspect of the condyle.

The embryologic development of the human lateral pterygoid muscle and its relationships with the temporomandibular joint disc and Meckel's cartilage.

The morphology of the developing lateral pterygoid muscle and its relationships with the temporomandibular joint disc and Meckel's cartilage were studied in 16 human embryos and fetuses ranging in age from 5 weeks to 14 weeks. All the temporomandibular joint structures and the lateral pterygoid muscle assumed their adult shapes by the 14th week of fetal life. At this stage, the lateral pterygoid muscle is a complex structure with several aponeuroses dividing the muscle into three main parts: superior, inferomedial, and inferoanterior. The superior part is attached to the disc superiorly and medially. The inferomedial part inserts into the anteromedial aspect of the condyle and disc. The inferoanterior portion is attached to the anterior aspect of the condyle. Anteriorly, the buccal nerve and associating blood vessels traverse the muscle in a mediolateral direction, dividing it into superior (small) and inferior (large) compartments. Posteriorly, the muscle remains intact with no separation. The lateral pterygoid muscle fibers show no direct attachment to Meckel's cartilage at any stage of development.

The development of the temporomandibular joint: a review with regard to the lateral pterygoid muscle.

This review sets out to explore the relationship between the lateral pterygoid muscle and the meniscus. It examines the embryological evidence to explain the controversy surrounding the apparent morphological discrepancy in the attachment of the lateral pterygoid muscle to the meniscus. It shows that the contribution of the lateral pterygoid muscle to the formation of the meniscus remains in dispute. Morphogenetic studies have not been able to support either one or other viewpoint and no direct conclusions can therefore be made on the embryology of the temporomandibular joint.

[A histological study of the meniscal insertion of the external pterygoid muscle]. / Studio istologico dell'inserzione meniscale del muscolo pterigoideo esterno.

The paper reports a study of the relationship between the upper head of the pterygoid muscle and the temporomandibular joint disc. Using optical microscopy, TMJ serial sections of 10 human embryos and fetuses aged from 6 to 22 weeks were examined. From the observation of serial sections, it appears that from the start of histogenesis the lateral pterygoid muscle is in contact with the condylar blastema and later with the disc and the condyle. A fibrous attachment was detected between the muscle tendon and the meniscus, and oxytalan and elastic fibres were observed near this attachment.

Longitudinal growth of the lateral pterygoid muscle in human fetuses.

The length of the upper and lower heads of the lateral pterygoid muscle was estimated in 44 fetuses aged 9 to 35 weeks. The lower head of the muscle is always longer than the upper one. Intensive increase in the length of the muscle is observed between 11th and 20th week.

The attachments of the temporomandibular joint meniscus in the human fetus.

The temporomandibular joints of human fetuses aged between 13 and 21 weeks have been examined by gross dissection and serial sections. An uninterrupted continuation of fibres has been observed from the superior head of the lateral pterygoid muscle to the malleus.

Morphology of the pterygomandibular raphe in human fetuses and adults.

The pterygomandibular raphe as described in current anatomy textbooks is not supported by actual observations in cadavers. A study was made on 60 adult Caucasian and Negro cadavers (52 right and 58 left sides, giving a total of 110 sides) providing comparison with an earlier study on Japanese specimens. In addition, 50 fetuses (25 mm crown-rump length to term) were examined to determine the arrangement of the raphe prenatally. Variations in the morphology of the raphe region were classified into three types: Type A--only the upper portion of the raphe could be identified and had a broad, triangular shape. Type B--the buccinator and superior pharyngeal constrictor muscles were widely separated by a broad, fascial region. Type C--the raphe was absent with complete continuity of the buccinator and superior pharyngeal constrictor muscles. A prominent, narrow, tendinous band with attachments as described in current textbooks was never found in adults. There was a complete absence of the raphe in 36% of the specimens resulting in continuity of the buccinator and superior pharyngeal constrictor muscles (type C). However, the remaining adult specimens (64%) exhibited some form of a broad, fascial region that either completely (type B, 36%) or partially (type A, 28%) separated the two muscles. All of the fetuses exhibited the type B arrangement exclusively, indicating that changes in the shape of the raphe occur postnatally. The frequency of appearance of the raphe types in adults differs significantly according to race.

Critical periods in the prenatal morphogenesis of the human lateral pterygoid muscle, the mandibular condyle, the articular disk, and medial articular capsule.

This study was prompted by the renewed clinical interests in understanding the natural history or early morphogenesis of the human temporomandibular joint. Using histologic preparations of 52 representative human embryos and fetuses, each of the major components of the joint was systematically assessed for its changing structure and related to an approximate time scale. The emergence and continued morphogenesis of the joint articular fossa, mandibular condyle, disk, capsule, lateral pterygoid muscle fibers, and both joint cavities occur in regular temporal and spatial pattern. A key observation from this study of embryos and fetuses ranging in age from 32 days to 22 weeks is that each of the component parts of the TMJ progressively emerge with some kind of continuity from a common mass of embryonic mesenchyme interposed between the future temporal bone and mandibular regions. The observations of this study lead to the suggestion that significant developmental disturbances to this common tissue mass or "developmental field" can lead to anomalous morphogenesis of those structures expected to emerge over time from the "developmental field" of the temporomandibular joint. Timing and the identification of a critical time period for the joint are important variables. This study identifies the critical period in the early morphogenesis of TMJ structures as generally falling between the early 7th and 11th prenatal weeks.