Development of the mandibular condylar cartilage in human specimens of 10-15 weeks' gestation

Mechanical stress can regulate temporomandibular joint cavitation via signalling pathways

The temporomandibular joint (TMJ), composed of temporal fossa, mandibular condyle and a fibrocartilage disc with upper and lower cavities, is the biggest synovial joint and biomechanical hinge of the craniomaxillofacial musculoskeletal system. The initial events that give rise to TMJ cavities across diverse species are not fully understood. Most studies focus on the pivotal role of molecules such as Indian hedgehog (Ihh) and hyaluronic acid (HA) in TMJ cavitation. Although biologists have observed that mechanical stress plays an irreplaceable role in the development of biological tissues and organs, few studies have been concerned with how mechanical stress regulates TMJ cavitation. Based on the evidence from human or other animal embryos today, it is implicated that mechanical stress plays an essential role in TMJ cavitation. In this review, we discuss the relationship between mechanical stress and TMJ cavitation from evo-devo perspectives and review the clinical features and potential pathogenesis of TMJ dysplasia.

Development of the Primary and Secondary Jaw Joints in the Mouse

This study assesses the morphogenesis of the primary and secondary jaw joints. A collection of 11 murine heads, ranging from prenatal stage E13.5 to postnatal stage P10, were prepared as histological serial sections (thickness 8-10µm) and stained conventionally in order to examine them with light microscopy. Next, the regions of the developing temporomandibular joint and the middle ear ossicles were three dimensionally reconstructed using AnalySIS® software. This study gained new insight into the spatio-temporal development of the temporomandibular joint and the auditory ossicles. Furthermore, we newly visualized in 3D that during the developmental period from stages E16 to P4 two morphologically well-functional joints (the primary and secondary jaw joints) exist on either side and are mechanically connected via Meckel's cartilage. Potential separation mechanisms of these two joints are discussed and options for mathematical analysis are suggested.

Evolution and development of the mammalian jaw joint: Making a novel structure

A jaw joint between the squamosal and dentary is a defining feature of mammals and is referred to as the temporomandibular joint (TMJ) in humans. Driven by changes in dentition and jaw musculature, this new joint evolved early in the mammalian ancestral lineage and permitted the transference of the ancestral jaw joint into the middle ear. The fossil record demonstrates the steps in the cynodont lineage that led to the acquisition of the TMJ, including the expansion of the dentary bone, formation of the coronoid process, and initial contact between the dentary and squamosal. From a developmental perspective, the components of the TMJ form through tissue interactions of muscle and skeletal elements, as well as through interaction between the jaw and the cranial base, with the signals involved in these interactions being both biomechanical and biochemical. In this review, we discuss the development of the TMJ in an evolutionary context. We describe the evolution of the TMJ in the fossil record and the development of the TMJ in embryonic development. We address the formation of key elements of the TMJ and how knowledge from developmental biology can inform our understanding of TMJ evolution.

The human masseter muscle revisited: First description of its coronoid part

INTRODUCTION

The masseter muscle is considered to be bilayered, consisting of a superficial and a deep part. However, a few historical texts mention the possible existence of a third layer as well, but they are extremely inconsistent as to its position. Here we performed an anatomical study to clarify the presence and morphological characteristics of a distinct third layer of the masseter muscle.

MATERIALS AND METHODS

We dissected 12 formaldehyde-fixed human cadaver heads, analysed CTs of 16 fresh cadavers, evaluated MR data from one living subject and examined histological sections using methyl methacrylate embedding of one formaldehyde-preserved head.

RESULTS

An anatomically distinct, deep third layer of the masseter muscle was consistently demonstrated, running from the medial surface of the zygomatic process of the temporal bone to the root and posterior margin of the coronoid process. Ours is the first detailed description of this part of the masseter muscle.

CONCLUSIONS

To facilitate discussion of this newly described part of the masseter, we recommend the name M. masseter pars coronoidea (coronoid part of the masseter) as a further reference. The arrangement of its muscle fibers suggest it being involved in stabilising the mandible by elevating and retracting the coronoid process.

Intermittent hypoxia inhibits mandibular cartilage growth with reduced TGF-β and SOX9 expressions in neonatal rats

Intermittent hypoxia (IH) has been associated with skeletal growth. However, the influence of IH on cartilage growth and metabolism is unknown. We compared the effects of IH on chondrocyte proliferation and maturation in the mandibular condyle fibrocartilage and tibial hyaline cartilage of 1-week-old male Sprague-Dawley rats. The rats were exposed to normoxic air (n = 9) or IH at 20 cycles/h (nadir, 4% O₂; peak, 21% O₂; 0% CO₂) (n = 9) for 8 h each day. IH impeded body weight gain, but not tibial elongation. IH also increased cancellous bone mineral and volumetric bone mineral densities in the mandibular condylar head. The mandibular condylar became thinner, but the tibial cartilage did not. IH reduced maturative and increased hypertrophic chondrocytic layers of the middle and posterior mandibular cartilage. PCR showed that IH shifted proliferation and maturation in mandibular condyle fibrocartilage toward hypertrophic differentiation and ossification by downregulating TGF-β and SOX9, and upregulating collagen X. These effects were absent in the tibial growth plate hyaline cartilage. Our results showed that neonatal rats exposed to IH displayed underdeveloped mandibular ramus/condyles, while suppression of chondrogenesis marker expression was detected in the growth-restricted condylar cartilage.

Pointing on the early stages of maxillary bone and tooth development - histological findings

Although the morphological stages of tooth development, in parallel with maxillary bone construction, are known for decades, the intimate mechanisms of early development of the oral cavity structures and tooth's proper and associated tissues are still incompletely elucidated. Nowadays, the research in embryology was shifted from the morphological to the molecular and genetic approach. This new approach is accomplished by using in vivo and in vitro experimental studies performed on animal models and cell lines. The interest in the knowledge of these events at gene and molecular level is still current, aiming to sustain the progress in the endorsement of novel regenerative and restorative therapies. However, the morphological standpoint maintains its interest, because the extrapolation of the results of experimental studies in humans requires a strong confirmation. Within this context, our work aims to analyze the histological characteristics of the maxillary bone and integrated tooth germs during the early stages of embryonic development. The study group consisted in mandible fragments obtained by dissection of the cephalic extremities collected from fetuses aged from 10 to 24 weeks, after medical or spontaneous abortions. The tissue specimens were processed for the histological exam. The histoarchitectonic traits of the initial stages of mandibular bone tissue and tooth development were assessed. The results revealed the dynamics of the ossification stages, from stages of early-dispersed intramembranous ossification to the organization of the dental alveoli, incorporated step-by-step in the maxillary body, and the simultaneous presence of tooth germs with different sizes and shapes, in accordance with the development stage. Our study complements the existing data regarding the embryonic period, bringing an important contribution for the enlargement of existing morphological, visual information for maxillary bone and tooth development.

Molecular and cellular mechanisms underlying the evolution of form and function in the amniote jaw

The amniote jaw complex is a remarkable amalgamation of derivatives from distinct embryonic cell lineages. During development, the cells in these lineages experience concerted movements, migrations, and signaling interactions that take them from their initial origins to their final destinations and imbue their derivatives with aspects of form including their axial orientation, anatomical identity, size, and shape. Perturbations along the way can produce defects and disease, but also generate the variation necessary for jaw evolution and adaptation. We focus on molecular and cellular mechanisms that regulate form in the amniote jaw complex, and that enable structural and functional integration. Special emphasis is placed on the role of cranial neural crest mesenchyme (NCM) during the species-specific patterning of bone, cartilage, tendon, muscle, and other jaw tissues. We also address the effects of biomechanical forces during jaw development and discuss ways in which certain molecular and cellular responses add adaptive and evolutionary plasticity to jaw morphology. Overall, we highlight how variation in molecular and cellular programs can promote the phenomenal diversity and functional morphology achieved during amniote jaw evolution or lead to the range of jaw defects and disease that affect the human condition.

Cranial structure and condylar asymmetry of patients with juvenile idiopathic arthritis: a risky growth pattern

The aim of the study was to evaluate the cephalometric differences and condylar asymmetry between patients with juvenile idiopathic arthritis (JIA) and normal control group. Sixty-two JIA patients with a latero-lateral cephalogram and orthopantomography, seeking for orthodontic therapy, and 62 normal matched subjects were comprised in the study. Cephalometric analysis was used for the evaluation of facial morphology while the method of Habets et al. (J Oral Rehabil 15(5): 465-471, 1988) was used to compare the condyles in orthopantomography. The significance of between-group differences was assessed using the Mann-Whitney test, as appropriate. The results showed a prevalence of the upper maxilla with hypomandibulia (class II), hyperdivergency with short vertical ramus posterior and posterior rotation of the mandible in JIA children (SNB, ANB, NSL/ML, Fh/ML, NL/ML, ArGo, ML P < 0.0001, ML/Oc P < 0.004, ArGo/GoGn P = 0.02, no difference for SNA). The condyles of the JIA group resulted highly asymmetric (P < 0.0001). The growth pattern of JIA patients resulted clearly different from normal subjects. This serious impairment of the cranial growth may be considered as an indicator of the need for early and continuous orthognatodonthic therapy during the entire period of development for all JIA patients, independently from temporomandibular joint signs or symptoms. To this end, it is important that rheumatologists and orthognathodontists set up a multidisciplinary treatment planned to control the side effects of a deranged growing pattern, to strictly avoid any orthodontic therapies that may worsen function and growth, and to promote treatments improving the physiology and biology of the cranial development.

Observing the development of the temporomandibular joint in embryonic and post-natal mice using various staining methods

The temporomandibular joint (TMJ) is a specialized synovial joint that is essential for the movement and function of the mammalian jaw. The TMJ develops from two mesenchymal condensations, and is composed of the glenoid fossa that originates from the otic capsule by intramembranous ossification, the mandibular condyle of the temporal bone and a fibrocartilagenous articular disc derived from a secondary cartilaginous joint by endochondral ossification. However, the development of the TMJ remains unclear. In the present study, the formation and development of the mouse TMJ was investigated between embryonic day 13.5 and post-natal day 180 in order to elucidate the morphological and molecular alterations that occur during this period. TMJ formation appeared to proceed in three stages: Initiation or blastema stage; growth and cavitation stage; and the maturation or completion stage. In order to investigate the activity of certain transcription factors on TMJ formation and development, the expression of extracellular matrix (ECM), sex determining region Y-box 9, runt-related transcription factor 2, Indian hedgehog homolog, Osterix, collagen I, collagen II, aggrecan, total matrix metalloproteinase (MMP), MMP-9 and MMP-13 were detected in the TMJ using in situ and/or immunohistochemistry. The results indicate that the transcription factors, ECM and MMP serve critical functions in the formation and development of the mouse TMJ. In summary, the development of the mouse TMJ was investigated, and the molecular regulation of mouse TMJ formation was partially characterized. The results of the present study may aid the systematic understanding of the physiological processes underlying TMJ formation and development in mice.

Development of synovial membrane in the temporomandibular joint of the human fetus

The development of the synovial membrane was analyzed in serial sections of 21 temporomandibular joints of human fetuses at 9 to 13 weeks of gestation. Sections of two fetuses at 12 weeks of development were used to perform immunohistochemical expression of the markers CD68 and Hsp27 on the synovial lining. Macrophage-like type A and fibroblast-like type B cells, which express CD68 and Hsp27, respectively, were observed at the twelfth week of development. Our results suggest that the development of the synovial membrane is related to the vascularization of the joint and the formation of the articular cavities.

An Immunohistochemical Study of Matrix Components in Early-Stage Vascular Canals Within Mandibular Condylar Cartilage in Midterm Human Fetuses

Matrix components of vascular canals (VCs) in human fetal mandibular condylar cartilage (15-16 weeks of gestation) were analyzed by immunohistochemistry. Prevascular canals (PVCs), consisting of spindle-shaped cells without capillary invasion, were observed within the cartilage. Intense immunoreactivity for collagen type I, weak immunoreactivity for aggrecan and tenascin-C, weak hyaluronan (HA) staining, and abundant argyrophilic fibers in PVCs indicated that they contain noncartilaginous fibrous connective tissues that was different from those in the perichondrium/periosteum. These structural and immunohistochemical features of PVCs are different from those of previously reported cartilage canals of the long bone. Capillaries entered the VCs from the periosteum and ascended through VCs. Following capillary invasion, loose connective tissue had formed in the lower part of VCs, and immunoreactivity for collagen types I and III, tenascin-C, and HA staining was evident in the matrix of loose connective tissue. No chondroclasts or osteogenic cells were seen at the front of capillary invasion, although small, mononuclear tartrate-resistant acid phosphatase (TRAP)-positive cells were present. Meanwhile, TRAP-positive, multinucleated chondroclasts and flattened, osteoblast-like cells were observed in the loose connective tissue at the lower part of VCs. These results may indicate slow progress of endochondral ossification in human fetal mandibular condyle. Further, unique matrix components in PVCs/VCs, which were different from those in cartilage canals in long bone, may reflect the difference of speed of endochondral ossification in cartilage canals and human fetal mandibular condyles.

An immunohistochemical study of matrix proteins in the craniofacial cartilage in midterm human fetuses

Immunohistochemical localization of collagen types I, II, and X, aggrecan, versican, dentin matrix protein (DMP)-1, martix extracellular phosphoprotein (MEPE) were performed for Meckel's cartilage, cranial base cartilage, and mandibular condylar cartilage in human midterm fetuses; staining patterns within the condylar cartilage were compared to those within other cartilaginous structures. Mandibular condylar cartilage contained aggrecan; it also had more type I collagen and a thicker hypertrophic cell layer than the other two types of cartilage; these three characteristics are similar to those of the secondary cartilage of rodents. MEPE immunoreactivity was first evident in the cartilage matrix of all types of cartilage in the human fetuses and in Meckel's cartilage of mice and rats. MEPE immunoreactivity was enhanced in the deep layer of the hypertrophic cell layer and in the cartilaginous core of the bone trabeculae in the primary spongiosa. These results indicated that MEPE is a component of cartilage matrix and may be involved in cartilage mineralization. DMP-1 immunoreactivity first became evident in human bone lacunae walls and canaliculi; this pattern of expression was comparable to the pattern seen in rodents. In addition, chondroid bone was evident in the mandibular (glenoid) fossa of the temporal bone, and it had aggrecan, collagen types I and X, MEPE, and DMP-1 immunoreactivity; these findings indicated that chondroid bone in this region has phenotypic expression indicative of both hypertrophic chondrocytes and osteocytes.

Morphological and morphometric study on sphenoid and basioccipital ossification in normal human fetuses

Congenital anomalies of the brain frequently correspond to cranial base anomalies, and a detailed description of morphology and individual variations in the developing cranial base is of clinical importance for diagnosing anomalies. Development of the human cranial base has been studied using dissection, computed tomography, and magnetic resonance imaging, each of which has advantages and disadvantages. We here examined development of the normal human fetal cranial base using bone staining, which allows for direct observation of the ossification centers and precise three-dimensional measurements. We observed alizarin red S-stained sphenoids and basiocciputs of 22 normal formalin-fixed human fetuses with crown-rump lengths (CRL) of 115-175 mm. We defined landmarks and measured sphenoids and basiocciputs using a fine caliper. Growth patterns of these ossifying bones were obtained, and we found similarities and differences among the growth patterns. We also observed individual variations in the ossification patterns, in particular, single- or double-ossification center patterns for the basisphenoid. The orbitosphenoid and basisphenoid widths and ratios of the widths to the total cranial base width were significantly different between the two pattern groups, whereas the other measurements and their ratios to the total cranial base did not differ between the groups. We measured the cerebrum and pons in different sets of 22 human fetuses with CRLs of 105-186 mm and found close relationships with the development of corresponding parts of the cranial base. The results contribute to the quantitative and qualitative information about the growth patterns and variations during human fetal cranial base development.