Relations between Meckel's cartilage and the morphogenesis of the mandible in the human embryo

Making Space for Permanent Molars in Growing Baboon (Papio anubis) and Great Ape (Pan paniscus and P. troglodytes) Mandibles: Possible Ontogenetic Strategies and Solutions

While mandible proportions do not appear to constrain permanent molar initiation times, how adequate space is created in the corpus for these teeth in a timely way is not well understood. This question is important for explaining how primate tooth and jaw development and evolution are coordinated. Landmark and linear measurement data were used to characterize mandible shape, growth trajectory, and growth rate between two genera, Papio and Pan, with contrasting permanent molar initiation schedules and mandible proportions. 3D geometric morphometric and 2D bivariate analyses showed genus-level differences in mandible morphology from birth that were amplified by different postnatal growth trajectories. Different corpus proportions and regional variation in corpus growth rates helped create space in a timely way for the molars. Regional corpus growth rates may evolve alongside permanent molar morphology and developmental timing to modify space available in the corpus for these teeth.

The Meckel's cartilage in human embryonic and early fetal periods

The Meckel's cartilage itself and the mandible are derived from the first branchial arch, and their development depends upon the contribution of the cranial neural crest cells. The prenatal development of the Meckel's cartilage, along with its relationship to the developing mandible and the related structures, were studied histologically in human embryos and fetuses. The material was obtained from a collection of the Department of Anatomy, and laboratory procedures were used to prepare sections, which were stained according to standard light-microscopy methods. The formation of the Meckel's cartilage and its related structures was observed and documented. Some critical moments in the development of the Meckel's cartilage are suggested. The sequential development of the Meckel's cartilage started as early as stage 13 (32 days) with the appearance of condensation of mesenchymal cells within the mandibular prominence. During stage 17 (41 days), the primary ossification center of the mandible appeared on the inferior margin of the Meckel's cartilage. The muscular attachments to the Meckel's cartilage in embryos were observed at stage 18 (44 days). Their subsequent movement into the developing mandible during the 10th week seemed to diminish the role of the Meckel's cartilage as the supportive core; simultaneously, the process of regression within the cartilage was induced. During the embryonic period, the bilateral Meckel's cartilages were in closest contact at the posterior surface of their superior margins, preceding formation of the symphyseal cartilage at this site. The event sequence in the development of the Meckel's cartilage is finally discussed.

Radiological trace of mandibular primary growth center in postnatal human mandibles

The mandibular primary growth center (MdPGC) of human fetus was conspicuously defined in the soft X-ray view of fetal mandibles. As the peripheral adaptive growth of mandible advances during the postnatal period, the MdPGC image became overshadowed by condensed cortical bones in soft X-ray view. In this study, we traced a sclerotic sequela of MdPGC during the postnatal period. Panoramic radiograms of 200 adults and soft X-ray views of 30 dried adult mandibles were analyzed by statistical methods. The former clearly showed an MdPGC below the middle portion of apices of canine and first premolar, which was distinguishable from mental foramen, and the latter also showed the MdPGC at the same area as a radiating and condensed radiopaque image, measuring 0.5-1.0 cm in diameter. This MdPGC position was seldom changed in the elderly people, even in the edentulous mandibles. Additionally, in the radiological examination, the benign tumors including odontogenic cysts hardly involved the MdPGC, while the malignant tumors of both primary and metastatic cancer frequently destroyed the MdPGC.

The Pediatric Mandible: I. A Primer on Growth and Development

LEARNING OBJECTIVES

After studying this article, the participant should be able to: 1. Describe embryonic and fetal mandibular development. 2. Summarize the aggregate changes in mandibular form from birth to puberty. 3. Describe the eruption and maturation of the deciduous and permanent mandibular dentition.

BACKGROUND

In this, the first of two articles addressing the surgical management of pediatric mandibular fractures, the authors provide a detailed discussion of mandibular development and anatomy during the fetal period, infancy, and childhood.

METHODS

A review of the pertinent literature was performed. The changing structure of the developing mandible is discussed, with particular attention to surgically relevant anatomical structures.

RESULTS

Throughout development, key anatomical structures with relevance to surgical therapy change markedly in position. The mandible undergoes significant change in its bony structure and the composition of its surrounding soft tissues. The mandible's bony structure becomes more robust, with an increasingly acute gonial angle and enlargement of the ramus and body. Furthermore, the mandible provides the bony structure from which tooth buds erupt as the deciduous and permanent dentition--a process that generates significant growth of the alveolar process. As a consequence, the distance between the developing dentition and the inferior mandibular border increases. While the canal of the inferior alveolar nerve undergoes significant superior displacement, the mental foramen becomes positioned more posteriorly over time. In addition, the ligamentous and muscular attachments that surround the temporomandibular joint become increasingly robust. Throughout childhood and adolescence, the blood supply of the mandibular body changes little, with the buccal periosteal plexus and inferior dental artery making significant contributions.

CONCLUSIONS

Mandibular growth provides the basis for normal occlusal relations and the generation of increasingly large masticatory force. Although the exact mechanisms of bone remodeling during mandibular development remain unclear, the process likely receives contributions from primary growth centers and the response to local alterations in biomechanical force produced by surrounding soft-tissue structures. A working knowledge of the changing mandibular anatomy is a prerequisite for effective clinical management of traumatic injury.

Prenatal development of the human mandible

In an effort to better understand the interrelationship of the growth and development pattern of the mandible and condyle, a sequential growth pattern of human mandibles in 38 embryos and 111 fetuses were examined by serial histological sections and soft X-ray views. The basic growth pattern of the mandibular body and condyle appeared in week 7 of fertilization. Histologically, the embryonal mandible originated from primary intramembranous ossification in the fibrous mesenchymal tissue around the Meckel cartilage. From this initial ossification, the ramifying trabecular bones developed forward, backward and upward, to form the symphysis, mandibular body, and coronoid process, respectively. We named this initial ossification site of embryonal mandible as the mandibular primary growth center (MdPGC). During week 8 of fertilization, the trabecular bone of the mandibular body grew rapidly to form muscular attachments to the masseter, temporalis, and pterygoid muscles. The mandible was then rapidly separated from the Meckel cartilage and formed a condyle blastema at the posterior end of linear mandibular trabeculae. The condyle blastema, attached to the upper part of pterygoid muscle, grew backward and upward and concurrent endochondral ossification resulted in the formation of the condyle. From week 14 of fertilization, the growth of conical structure of condyle became apparent on histological and radiological examinations. The mandibular body showed a conspicuous radiating trabecular growth pattern centered at the MdPGC, located around the apical area of deciduous first molar. The condyle growth showed characteristic conical structure and abundant hematopoietic tissue in the marrow. The growth of the proximal end of condyle was also approximated to the MdPGC on radiograms. Taken together, we hypothesized that the MdPGC has an important morphogenetic affect for the development of the human mandible, providing a growth center for the trabecular bone of mandibular body and also indicating the initial growth of endochondral ossification of the condyle.

Development of Meckel's cartilage in the symphyseal region in man

BACKGROUND

The aim of this work is to clarify the aspects which are at present most controversial about the development of the anterior segments of Meckel's cartilage, such as the role of and determination of the area that is incorporated in the development of the human mandible.

METHODS

Light microscope studies were done on 25 embryos and human fetuses from the collection of the Institute of Embryology at the University Complutense of Madrid and the Department of Morphological Science from the University of Granada. Specimen length was between 18 and 125 mm crown-rump.

RESULTS

During the embryonic period, Meckel's cartilages were placed in the midline of the mandibular arch but fusion was not observed between them. Ossification of Meckel's cartilage begins at the end of the embryonic period and is completed in the fetal period and the portion that participates in mandibular formation is determined. This segment extends from the mental foramen to near the midline of the mandible. In this region, on the dorsal surface of the symphysis, cartilaginous nodules that originate from Meckel's cartilage are isolated.

CONCLUSIONS

The ventral portions of Meckel's cartilage do not fuse in the midline of the mandibular arch. These present endo- and perichondral ossification and the section from the mental foramen to near the midline (mandibular symphysis) participates in mandibular formation. The ventral ends of Meckel's cartilage, i.e., the ends nearest the midline, do not ossify and remain isolated on the dorsal surface of the fetal mandibular symphysis.

On the presence of a secondary cartilage in the mental symphyseal region of human embryos and fetuses

The presence of a secondary cartilage in the mental symphyseal region was examined in this study. A double-staining method with alcian blue and alizarin red S was performed on both whole human embryos and fetuses (developmental age between 8 and 17 weeks, crown-rump length, CRL, between 37 and 124 mm) and their disjointed mandibles. Histological and histochemical techniques were applied to transverse serial sections of whole disjointed fetal heads. The ossification process observed in the mental symphysis is quite different from that of the mandibular body, whose membranous ossification is induced by the contiguous Meckel's cartilage. No evidence of any fusion of Meckel's cartilage with the symphyseal cartilage, that lies within the symphyseal space, was detected. On the basis of these findings, we suggested that the mental secondary cartilage is able to change into bone according to an endochondral ossification process. Moreover, the role of mechanical causes in the development of the mental symphysis was hypothesized.