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This study investigates the development of the primary and secondary jaw joints in humans, focusing on their concomitance and subsequent disconnection. Development begins with the primary temporomandibular joint as a connection between Meckel's cartilage and the incus, while the secondary temporomandibular joint develops anteriorly as an articulation between the mandibular condyle and the mandibular fossa. Previous research in mice has provided insights into the morphogenesis of these joints, but their specific development of the 3D morphogenesis in humans remains unclear. To address this gap, histological serial sections of embryos and fetuses ranging from 19 to 230 mm crown-rump length were analyzed. The 3D morphogenesis of the middle ear and the temporomandibular joint was examined, paying attention to the morphological characteristics, timing, and potential mechanisms of movement and disconnection. The primary jaw joint is initially formed at 25 mm (8th week), followed by the appearance of the secondary jaw joint arising at 87 mm (12th week). Both joints persist present simultaneously, until a separation occurs between 150 and 230 mm (18th-24th week). It is remarkable that both joints remain concomitant and function somehow for a period exceeding 6 weeks, with the mechanism of their separation still unclear. Understanding the precise timing and functional movements involved with these temporarily connected joints is crucial for comprehending the overall development of the temporomandibular joint. Further research is needed to explore the molecular and cellular processes underlying these developmental changes., Competing Interests: Declaration of Competing Interest Authors declare no competing interests., (Copyright © 2023 Elsevier GmbH. All rights reserved.)

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 8-10 µm) 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., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier GmbH. All rights reserved.)

Background: During physiological function of the temporomandibular joint, we have to rely only on elastic structures (in particular the bilaminar zone) for repositioning of the articular disc. A real muscle, however, would be functionally more reasonable. In patients, a decrease of this elasticity is possibly one of the reasons for temporomandibular joint malfunctions, which affect between 16% and 36% of the population., Method: This study assesses the morphogenesis of the murine (Mus musculus) temporomandibular joint with particular regard to the masticatory muscles, to throw light on this topic. To that end, a collection of 11 murine heads ranging from prenatal stages E13.25 to E20 was used and early postnatal stages P0 to P4, which were prepared as histological sections (thickness 8-10 µm) and stained conventionally in order to examine them with light microscopy. Next, the temporomandibular joint and selected surrounding structures, along with the masticatory muscles, were threedimensionally reconstructed using analySIS® software. Subsequently, specific morphometric analyses were performed., Results and Conclusions: The evaluation of the results led to the following conclusions., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021. Published by Elsevier GmbH.)

Craniofacial morphogenesis is an intricate developmental process in 3D, which therefore merits visualization and investigation in 3D. To better understand the process, we utilize μCT imaging, and describe a method to calibrate each cone beam μCT individually. Calibration is necessary, because during development, fetuses undergo tissue differentiation, which affects the acquisition process for radiographic images. Additionally, tissue fixation and conservation agents may influence the physical properties of the specimens and may affect image acquisition. After taking a μCT scan from each specimen, we separated a horizontal slice from each neck (which is inconsequential to our question with relation to the whole head). These neck specimens were prepared as horizontal histological serial sections and stained. With these as a reference, the μCT visualization parameters could be adjusted until they matched the selected virtual section planes, which correspond exactly to the planes of the histological sections with a precision (pixel size) of 0.69μm., (Copyright © 2021 Elsevier GmbH. All rights reserved.)

The human mandible is said to arise from desmal ossification, which, however, is not true for the entire body of the mandible: Meckel's cartilage itself is prone to ossification, at least its anterior part in the canine and incisor region. Also, within the coronoid and in the condylar processes there are cartilaginous cores, which eventually undergo ossification. Furthermore, there are a number of additional single cartilaginous islets arising in fetuses of 95mm CRL and more. They are located predominantly within the bone at the buccal sides of the brims of the dental compartments, mostly in the gussets between the dental primordia. They become wedge-shaped or elongated with a diameter of around 150-500μm and were also found in older stages up to 225mm CRL, which was the oldest specimen used in this study. This report is intended to visualize these single cartilaginous islets histologically and in 3-D reconstructions in stereoscopic images. Although some singular cartilaginous tissue within the mandible may be remains of the decaying Meckel's cartilage, our 3-D reconstructions clearly show that the aforementioned cartilaginous islets are independent thereof, as can be derived from their separate locations within the mandibular bone. The reasons that lead to these cartilaginous formations have remained unknown so far., (Copyright © 2016 Elsevier GmbH. All rights reserved.)

The underlying mechanisms of human bone morphogenesis leading to a topologically specific shape remain unknown, despite increasing knowledge of the basic molecular aspects of bone formation and its regulation. The formation of the alveolar bone, which houses the dental primordia, and later the dental roots, may serve as a model to approach general questions of bone formation. Twenty-five heads of human embryos and fetuses (Radlanski-Collection, Berlin) ranging from 19mm to 270mm (crown-rump-length) CRL were prepared as histological serial sections. For each stage, virtual 3D-reconstructions were made in order to study the morphogenesis of the mandibular molar primordia with their surrounding bone. Special focus was given to recording the bone-remodeling pattern, as diagnosed from the histological sections. In early stages (19-31mm CRL) developing bone was characterized by appositional only. At 41, in the canine region, mm CRL bony extensions were found forming on the bottom of the trough. Besides general apposition, regions with resting surfaces were also found. At a fetal size of 53mm CRL, septa have developed and led to a compartment for canine development. Furthermore, one shared compartment for the incisor primordia and another shared compartment for the molars also developed. Moreover, the inner surfaces of the dental crypts showed resorption of bone. From this stage on, a general pattern became established such that the compartmentalizing ridges and septa between all of the dental primordia and the brims of the crypts were noted, and were due to appositional growth of bone, while the crypts enlarged on their inner surfaces by resorption. By 160mm CRL, the dental primordia were larger, and all of the bony septa had become reduced in size. The primordia for the permanent teeth became visible at 225mm CRL and shared the crypts of their corresponding deciduous primordia., (Copyright © 2016 Elsevier GmbH. All rights reserved.)

Despite increasing knowledge of the basic molecular aspects of bone formation and its regulation, the mechanisms of bone morphogenesis leading to a topologically specific shape remain unknown. The formation of the alveolar bone, which houses the dental primordia and later, the dental roots, may serve as a model to understand the formation of bone form in general. Thirty-eight heads of mice (C57 Bl/6J) ranging from stages E13-P20 were used to prepare histological serial sections. For each stage, virtual 3D-reconstructions were made in order to study the morphogenesis of the mandibular molar primordia concomitantly with their surrounding bone. Special focus was given to recording the remodeling pattern. It has been shown that, in early stages (E13, E14), bone formation is characterized by apposition only. In stage E15, the bony crypt around the dental primordia is remodeled mostly by resorption of bone. In stage E18, the bone remodeling pattern shows resorption all along the bony gutter, which houses the molar primordia. The medial and lateral margins are characterized by apposition. At birth (stage P0), a bony septum has begun to form between the primordium m1 and of m2, arising from both sides and characterized by pure apposition of bone. In stage P4, the crypts of m1 and m2, and also that of m3, show bone resorption inside, while the medial and lateral bony margins show apposition of bone throughout. Generally, during development, the bone gradually encapsulates the dental primordia, in such a way that the bone reaches over the dental primordia and leaves only a continuous longish opening of about 200μm width. The opening at the occlusal surface of m1, at the time of eruption, starting at stage P14, appears to have increased in size again. The distance between bone and dental primordium undergoes change during development. In erupted molars, it is around 100μm, during early developmental stages, it may be as less as 20μm. These data show the inevitability of bone remodeling., (Copyright © 2015 Elsevier GmbH. All rights reserved.)

This study is a first report on the modality of early dentin formation in respect to the scalloped pattern of the dentino-enamel junction (DEJ). We applied scanning electron microscopy (SEM), transmission electron microscopy (TEM), histological serial sections, and three-dimensional (3D) reconstructions. TEM and SEM showed scallops and secondary scallops on the DEJ of deciduous dental primordia and on deciduous teeth with the enamel cap removed. This peculiar outline of the DEJ requires a specific dentin formation pattern; histological sections showed that dentin formation began at the brims of the scallops, seen as triangular spikes in serial sections. The dentin formation front was not uniform; instead, it was characterized by multiple, insular forming centers, as revealed by our 3D reconstructions. As thicker dentin layers formed, the islands became confluent. Factors are discussed, which may lead to crimpling of the inner enamel epithelium, and maintained as the scalloped pattern of the DEJ develops. Signaling patterns in accordance with the insular dentin formation are unknown so far.

The development of the muscles in the floor of the mouth is described in 10 human embryos and fetuses ranging from 6.9 to 76 mm CRL by means of computer-aided graphical 3D-reconstructions. All primordia of the muscles in the floor of the mouth could be identified from the 15.6 mm CRL stage on. The proportions and insertion lines of the early muscles were found to be different from adult anatomy. Each muscle first inserted in the medial surface of Meckels cartilage, but during the developmental period between 19 and 68 mm CRL the insertion lines were gradually transposed to the bony ridges of the mandible which progrediently embraced Meckels cartilage. The fibers of the mylohyoid muscles left the anterior region near the symphysis mentalis free during all stages of this study. The digastric muscle revealed only one belly with a constriction of its continuous fibers where it passed the hyoid bone primordium. There was no attachment of digastric muscle fibers to the hyoid; only geniohyoid and mylohyoid fibers. Geniohyoid and genioglossus muscles basically correspond to their definite arrangement, but they underwent proportional changes. Individual specimens embodied irregularities such as accessory geniohyoid and hyoid portions and muscle fibers separate from the mylohyoide muscle.

Aim of the study was to assess the value of in vitro three-dimensional computed tomography (3D-CT) as a new method for the depiction of the fetal skull. 3D-CT images of a fetal head of 24 weeks gestational age were generated with the implemented software on the basis of contiguous CT slices of 1 mm thickness. 3D-CT gave real three-dimensional information about morphology and spatial arrangement of the cranial bones. Using cutting and rotating procedures fast evaluation of all imaged ossified structures in arbitrary views without disturbing superpositions was possible, thus allowing easy identification and accurate description of the cranial bones and making isolated examinations of particular macroscopic sections of the specimens unnecessary. Advantages of 3D-CT over other evaluation techniques for the fetal cranium and its limitations are discussed. In conclusion, 3D-CT promises to be an effective tool for the investigation of fetal cranial development.

The understanding of growth and developmental changes can be improved when shapes and changes in size, proportion, and relationships are visualized in 3 dimensions and at different stages. This applies particularly to craniofacial skeletal growth and the development of the dentition. For that purpose 3D-data were collected from prenatal human heads ranging from 18 up to 275 mm CRL and from a collection of macerated fetal and postnatal skulls. Computer-aided graphical reconstructions were obtained from histological serial sections of embryonic and early fetal specimens. Proportional changes in the growing skull were recorded by means of radiological and cephalometric evaluation. In addition, computed tomography was applied to fetal and postnatal skulls. Furthermore, the prenatal and postnatal development of the dentition was digitized. To that end 3D-polygone sets of these data were read into a workstation computer and animated by means of the software Soft Image (Microsoft). This comprehensive 4D insight into growth facilitates the understanding and teaching of normal and abnormal development.

The fetal development of the human parotid gland was studied by means of serial sections of human embryos and fetuses ranging from 19 mm to 67 mm CRL. Analysis of computer assisted 3-dimensional reconstructions and anatomical drawings leads to the following observations: 1. The parotid gland anlage is found at the most lateral and cranial point of the sulcus buccalis. 2. The location of the orifice of the parotid duct is dependent upon the developmental processes of the fetal skeleton. 3. The anlagen of anatomical structures dominating the parotid bed in the adult are found prior to the enlargement of the parotid gland. 4. The surface of the parotid gland shows impressions of the surrounding structures. 5. There is no evidence that the parotid gland is subdivided into two lobes by the facial nerve.

The fetal development and arrangement of the human submandibular gland was studied by means of serial sections of human embryos and fetuses ranging from 19 mm to 67 mm CRL. Computer assisted 3-dimensional reconstruction leads to the following observations: 1. The orifice of the submandibular duct is located in the medial paralingual sulcus. 2. There is evidence that the extension and location of WHARTON's duct is influenced by the surrounding structures. 3. The surface of the submandibular gland primordium shows impressions of the neighboring structures. 4. The glandular tissue is encapsulated in condensed mesenchyme.