Diverse Fate of an Enigmatic Structure: 200 Years of Meckel's Cartilage

The ciliary protein C2cd3 is required for mandibular musculoskeletal tissue patterning

The mandible is composed of several musculoskeletal tissues including bone, cartilage, and tendon that require precise patterning to ensure structural and functional integrity. Interestingly, most of these tissues are derived from one multipotent cell population called cranial neural crest cells (CNCCs). How CNCCs are properly instructed to differentiate into various tissue types remains nebulous. To better understand the mechanisms necessary for the patterning of mandibular musculoskeletal tissues we utilized the avian mutant talpid2 (ta2) which presents with several malformations of the facial skeleton including dysplastic tendons, mispatterned musculature, and bilateral ectopic cartilaginous processes extending off Meckel's cartilage. We found an ectopic epithelial BMP signaling domain in the ta2 mandibular prominence (MNP) that correlated with the subsequent expansion of SOX9+ cartilage precursors. These findings were validated with conditional murine models suggesting an evolutionarily conserved mechanism for CNCC-derived musculoskeletal patterning. Collectively, these data support a model in which cilia are required to define epithelial signal centers essential for proper musculoskeletal patterning of CNCC-derived mesenchyme.

The Contribution of Meckel's Cartilage-Derived Type II Collagen-Positive Cells to the Jawbone Development and Repair

Jawbones and long bones, despite their shared skeletal lineage, frequently exhibit distinct origins and developmental pathways. Identifying specific progenitor subsets for mandibular osteogenesis remains challenging. Type II collagen is conventionally associated with cartilaginous structures, yet our investigation has identified the presence of type II collagen positive (Col2+) cells within the jawbone development and regeneration. The role of Col2+ cells in jawbone morphogenesis and repair has remained enigmatic. In this study, we analyze single-cell RNA sequencing data from mice jawbone at embryonic day 10.5. Through fate-mapping experiments, we have elucidated that Col2+ cells and their progeny are instrumental in mandibular osteogenesis across both fetal and postnatal stages. Furthermore, lineage tracing with a tamoxifen-inducible CreER system has established the pivotal role of Col2+ cells, marked by Col2-CreER and originating from the primordial Meckel's cartilage, in jawbone formation. Moreover, our research explored models simulating jawbone defects and tooth extraction, which underscored the osteogenic differentiation capabilities of postnatal Col2+ cells during repair. This finding not only highlights the regenerative potential of Col2+ cells but also suggests their versatility in contributing to skeletal healing and regeneration. In conclusion, our findings position Col2+ cells as essential in orchestrating osteogenesis throughout the continuum of mandibular development and repair.

A common cis-regulatory variant impacts normal-range and disease-associated human facial shape through regulation of PKDCC during chondrogenesis

Genome-wide association studies (GWAS) identified thousands of genetic variants linked to phenotypic traits and disease risk. However, mechanistic understanding of how GWAS variants influence complex morphological traits and can, in certain cases, simultaneously confer normal-range phenotypic variation and disease predisposition, is still largely lacking. Here, we focus on rs6740960, a single nucleotide polymorphism (SNP) at the 2p21 locus, which in GWAS studies has been associated both with normal-range variation in jaw shape and with an increased risk of non-syndromic orofacial clefting. Using in vitro derived embryonic cell types relevant for human facial morphogenesis, we show that this SNP resides in an enhancer that regulates chondrocytic expression of PKDCC - a gene encoding a tyrosine kinase involved in chondrogenesis and skeletal development. In agreement, we demonstrate that the rs6740960 SNP is sufficient to confer chondrocyte-specific differences in PKDCC expression. By deploying dense landmark morphometric analysis of skull elements in mice, we show that changes in Pkdcc dosage are associated with quantitative changes in the maxilla, mandible, and palatine bone shape that are concordant with the facial phenotypes and disease predisposition seen in humans. We further demonstrate that the frequency of the rs6740960 variant strongly deviated among different human populations, and that the activity of its cognate enhancer diverged in hominids. Our study provides a mechanistic explanation of how a common SNP can mediate normal-range and disease-associated morphological variation, with implications for the evolution of human facial features.

ESCRT-dependent control of craniofacial morphogenesis with concomitant perturbation of NOTCH signaling

Craniofacial development is orchestrated by transcription factor-driven regulatory networks, epigenetic modifications, and signaling pathways. Signaling molecules and their receptors rely on endo-lysosomal trafficking to prevent accumulation on the plasma membrane. ESCRT (Endosomal Sorting Complexes Required for Transport) machinery is recruited to endosomal membranes enabling degradation of such endosomal cargoes. Studies in vitro and in invertebrate models established the requirements of the ESCRT machinery in membrane remodeling, endosomal trafficking, and lysosomal degradation of activated membrane receptors. However, investigations during vertebrate development have been scarce. By ENU-induced mutagenesis, we isolated a mouse line, Vps25ENU/ENU, carrying a hypomorphic allele of the ESCRT-II component Vps25, with craniofacial anomalies resembling features of human congenital syndromes. Here, we assessed the spatiotemporal dynamics of Vps25 and additional ESCRT-encoding genes during murine development. We show that these genes are ubiquitously expressed although enriched in discrete domains of the craniofacial complex, heart, and limbs. ESCRT-encoding genes, including Vps25, are expressed in both cranial neural crest-derived mesenchyme and epithelium. Unlike constitutive ESCRT mutants, Vps25ENU/ENU embryos display late lethality. They exhibit hypoplastic lower jaw, stunted snout, dysmorphic ear pinnae, and secondary palate clefting. Thus, we provide the first evidence for critical roles of ESCRT-II in craniofacial morphogenesis and report perturbation of NOTCH signaling in craniofacial domains of Vps25ENU/ENU embryos. Given the known roles of NOTCH signaling in the developing cranium, and notably the lower jaw, we propose that the NOTCH pathway partly mediates the craniofacial defects of Vps25ENU/ENU mouse embryos.

Middle ear innovation in Early Cretaceous eutherian mammals

The middle ear ossicles in modern mammals are repurposed from postdentary bones in non-mammalian cynodonts. Recent discoveries by palaeontological and embryonic studies have developed different models for the middle ear evolution in mammaliaforms. However, little is known about the evolutionary scenario of the middle ear in early therians. Here we report a detached middle ear preserved in a new eutherian mammal from the Early Cretaceous Jehol Biota. The well-preserved articulation of the malleus and incus suggest that the saddle-shaped incudomallear joint is a major apomorphy of Early Cretaceous eutherians. By contrast to the distinct saddle-like incudomallear articulation in therians, differences between the overlapping versus the half-overlapping incudomallear joints in monotremes and stem mammals would be relatively minor. The middle ear belongs to the microtype by definition, indicating its adaptation to high-frequency hearing. Current evidence indicates that significant evolutionary innovations of the middle ear in modern therians evolved in Early Cretaceous.

Embryonic cranial cartilage defects in the Fgfr3Y367C /+ mouse model of achondroplasia

Achondroplasia, the most common chondrodysplasia in humans, is caused by one of two gain of function mutations localized in the transmembrane domain of fibroblast growth factor receptor 3 (FGFR3) leading to constitutive activation of FGFR3 and subsequent growth plate cartilage and bone defects. Phenotypic features of achondroplasia include macrocephaly with frontal bossing, midface hypoplasia, disproportionate shortening of the extremities, brachydactyly with trident configuration of the hand, and bowed legs. The condition is defined primarily on postnatal effects on bone and cartilage, and embryonic development of tissues in affected individuals is not well studied. Using the Fgfr3Y367C/+ mouse model of achondroplasia, we investigated the developing chondrocranium and Meckel's cartilage (MC) at embryonic days (E)14.5 and E16.5. Sparse hand annotations of chondrocranial and MC cartilages visualized in phosphotungstic acid enhanced three-dimensional (3D) micro-computed tomography (microCT) images were used to train our automatic deep learning-based 3D segmentation model and produce 3D isosurfaces of the chondrocranium and MC. Using 3D coordinates of landmarks measured on the 3D isosurfaces, we quantified differences in the chondrocranium and MC of Fgfr3Y367C/+ mice relative to those of their unaffected littermates. Statistically significant differences in morphology and growth of the chondrocranium and MC were found, indicating direct effects of this Fgfr3 mutation on embryonic cranial and pharyngeal cartilages, which in turn can secondarily affect cranial dermal bone development. Our results support the suggestion that early therapeutic intervention during cartilage formation may lessen the effects of this condition.

The relationship between Meckel's cartilage resorption and incisor tooth germ in mice

Our understanding of the initiation and cellular mechanisms underlying endochondral resorption of Meckel's cartilage (MC) remains limited. Several studies have shown that the resorption site of MC and the mandibular incisor tooth germ are located close to each other. However, whether incisor tooth germ development is involved in MC resorption remains unclear. In this study, we aimed to elucidate the spatio-temporal interaction between the initiation site of MC resorption and the development of incisor tooth germs in an embryonic mouse model. To this effect, we developed a histology-based three-dimensional (3D) reconstruction technique using paraffin-embedded serial sections of various tissues in the jaw. The serial sections were cut in the frontal section and the tissue constituents (e.g., MC, incisor, and mineralized mandible) were studied using conventional and enzyme-based histochemistry. The outline of each component was marked on the frontal sectional images and 3D structures were constructed. To assess the vascular architecture at the site of MC resorption, immunohistochemical staining using anti-laminin, anti-factor VIII, and anti-VEGF antibodies was performed. MC resorption was first observed on the lateral incisor-facing side of the cartilage rods at sites anterior to the mental foramen on E16.0. The 3D analysis suggested that: (a) the posterior region of the clastic cartilage resorption corresponds to the cervical loop of the incisor; (b) the cervical portion of the tooth germ inflates probably due to temporal cellular congestion prior to differentiation into matrix-producing cells; (c) the incisor tooth germ tissue is present in close proximity to MC even in mouse with continuously growing tooth and determines the disappearance of MC as the tooth development.

Zebrafish crocc2 mutants exhibit divergent craniofacial shape, misregulated variability, and aberrant cartilage morphogenesis

BACKGROUND

Phenotypic variation is of paramount importance in development, evolution, and human health; however, the molecular mechanisms that influence organ shape and shape variability are not well understood. During craniofacial development, the behavior of skeletal precursors is regulated by both biochemical and environmental inputs, and the primary cilia play critical roles in transducing both types of signals. Here, we examine a gene that encodes a key constituent of the ciliary rootlets, crocc2, and its role in cartilage morphogenesis in larval zebrafish.

RESULTS

Geometric morphometric analysis of crocc2 mutants revealed altered craniofacial shapes and expanded variation. At the cellular level, we observed altered chondrocyte shapes and planar cell polarity across multiple stages in crocc2 mutants. Notably, cellular defects were specific to areas that experience direct mechanical input. Cartilage cell number, apoptosis, and bone patterning were not affected in crocc2 mutants.

CONCLUSIONS

Whereas "regulatory" genes are widely implicated in patterning the craniofacial skeleton, genes that encode "structural" aspects of the cell are increasingly implicated in shaping the face. Our results add crocc2 to this list, and demonstrate that it affects craniofacial geometry and canalizes phenotypic variation. We propose that it does so via mechanosensing, possibly through the ciliary rootlet. If true, this would implicate a new organelle in skeletal development and evolution.

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.

Altered developmental programs and oriented cell divisions lead to bulky bones during salamander limb regeneration

There are major differences in duration and scale at which limb development and regeneration proceed, raising the question to what extent regeneration is a recapitulation of development. We address this by analyzing skeletal elements using a combination of micro-CT imaging, molecular profiling and clonal cell tracing. We find that, in contrast to development, regenerative skeletal growth is accomplished based entirely on cartilage expansion prior to ossification, not limiting the transversal cartilage expansion and resulting in bulkier skeletal parts. The oriented extension of salamander cartilage and bone appear similar to the development of basicranial synchondroses in mammals, as we found no evidence for cartilage stem cell niches or growth plate-like structures during neither development nor regeneration. Both regenerative and developmental ossification in salamanders start from the cortical bone and proceeds inwards, showing the diversity of schemes for the synchrony of cortical and endochondral ossification among vertebrates.

Endothelial cells during craniofacial development: Populating and patterning the head

Major organs and tissues require close association with the vasculature during development and for later function. Blood vessels are essential for efficient gas exchange and for providing metabolic sustenance to individual cells, with endothelial cells forming the basic unit of this complex vascular framework. Recent research has revealed novel roles for endothelial cells in mediating tissue morphogenesis and differentiation during development, providing an instructive role to shape the tissues as they form. This highlights the importance of providing a vasculature when constructing tissues and organs for tissue engineering. Studies in various organ systems have identified important signalling pathways crucial for regulating the cross talk between endothelial cells and their environment. This review will focus on the origin and migration of craniofacial endothelial cells and how these cells influence the development of craniofacial tissues. For this we will look at research on the interaction with the cranial neural crest, and individual organs such as the salivary glands, teeth, and jaw. Additionally, we will investigate the methods used to understand and manipulate endothelial networks during the development of craniofacial tissues, highlighting recent advances in this area.

Species-specific sensitivity to TGFβ signaling and changes to the Mmp13 promoter underlie avian jaw development and evolution

Precise developmental control of jaw length is critical for survival, but underlying molecular mechanisms remain poorly understood. The jaw skeleton arises from neural crest mesenchyme (NCM), and we previously demonstrated that these progenitor cells express more bone-resorbing enzymes including Matrix metalloproteinase 13 (Mmp13) when they generate shorter jaws in quail embryos versus longer jaws in duck. Moreover, if we inhibit bone resorption or Mmp13, we can increase jaw length. In the current study, we uncover mechanisms establishing species-specific levels of Mmp13 and bone resorption. Quail show greater activation of and sensitivity to transforming growth factor beta (TGFβ) signaling than duck; where intracellular mediators like SMADs and targets like Runt-related transcription factor 2 (Runx2), which bind Mmp13, become elevated. Inhibiting TGFβ signaling decreases bone resorption, and overexpressing Mmp13 in NCM shortens the duck lower jaw. To elucidate the basis for this differential regulation, we examine the Mmp13 promoter. We discover a SMAD-binding element and single nucleotide polymorphisms (SNPs) near a RUNX2-binding element that distinguish quail from duck. Altering the SMAD site and switching the SNPs abolish TGFβ sensitivity in the quail Mmp13 promoter but make the duck promoter responsive. Thus, differential regulation of TGFβ signaling and Mmp13 promoter structure underlie avian jaw development and evolution.

Meckel's Cartilage in Mandibular Development and Dysmorphogenesis

The Fgfr2c^C342Y/+ Crouzon syndrome mouse model carries a cysteine to tyrosine substitution at amino acid position 342 (Cys342Tyr; C342Y) in the fibroblast growth factor receptor 2 (Fgfr2) gene equivalent to a FGFR2 mutation commonly associated with Crouzon and Pfeiffer syndromes in humans. The Fgfr2c C342Y mutation results in constitutive activation of the receptor and is associated with upregulation of osteogenic differentiation. Fgfr2c^C342Y/+ Crouzon syndrome mice show premature closure of the coronal suture and other craniofacial anomalies including malocclusion of teeth, most likely due to abnormal craniofacial form. Malformation of the mandible can precipitate a plethora of complications including disrupting development of the upper jaw and palate, impediment of the airway, and alteration of occlusion necessary for proper mastication. The current paradigm of mandibular development assumes that Meckel’s cartilage (MC) serves as a support or model for mandibular bone formation and as a template for the later forming mandible. If valid, this implies a functional relationship between MC and the forming mandible, so mandibular dysmorphogenesis might be discerned in MC affecting the relationship between MC and mandibular bone. Here we investigate the relationship of MC to mandible development from the early mineralization of the mandible (E13.5) through the initiation of MC degradation at E17.7 using Fgfr2c^C342Y/+ Crouzon syndrome embryos and their unaffected littermates (Fgfr2c^+/+). Differences between genotypes in both MC and mandibular bone are subtle, however MC of Fgfr2c^C342Y/+ embryos is generally longer relative to unaffected littermates at E15.5 with specific aspects remaining relatively large at E17.5. In contrast, mandibular bone is smaller overall in Fgfr2c^C342Y/+ embryos relative to their unaffected littermates at E15.5 with the posterior aspect remaining relatively small at E17.5. At a cellular level, differences are identified between genotypes early (E13.5) followed by reduced proliferation in MC (E15.5) and in the forming mandible (E17.5) in Fgfr2c^C342Y/+ embryos. Activation of the ERK pathways is reduced in the perichondrium of MC in Fgfr2c^C342Y/+ embryos and increased in bone related cells at E15.5. These data reveal that the Fgfr2c C342Y mutation differentially affects cells by type, location, and developmental age indicating a complex set of changes in the cells that make up the lower jaw.

Mechanotransductive Mechanisms of Biomimetic Hydrogel Cues Modulating Meckel's Cartilage Degeneration

Meckel's cartilage, a cartilage rod present in the mandible during developmental stages, shows a unique developmental fate: while the anterior and posterior portions undergo ossification, the middle part degenerates. Previously, it was shown that a stiff environment promoted cartilage degeneration in the middle region, while a soft environment enhanced the mineralization in the anterior region of Meckel's cartilage. This study aims to elucidate the spatio-temporal changes in the mechanosensing properties of Meckel's cartilage during its early developmental stages and clarify the mechanotransduction-related mechanisms involved in its degeneration. The results show that the expression of Hippo pathway effector yes-associated protein (YAP) is only detectable in the Meckel's cartilage onward embryonic day (E)14.5, indicating that mechanosensing is dependent on the tissue developmental stage. Consistently, microenvironmental stiffness-induced cartilage degeneration can only be induced in cartilages onward E14.5, but not in those at earlier developmental stages. Expressions of integrin-β1 and cartilage matrix-degrading enzymes, matrix metalloproteinase 1 (MMP-1) and MMP-13, are significantly enhanced in the degeneration area. Moreover, verteporfin (YAP inhibitor) and integrin-β1 antibody block the substrate stiffness-induced degeneration by suppressing the expressions of MMP-1 and MMP-13. These data provide new insights into the interplay between biochemical and mechanical cues determining the fate of Meckel's cartilage.

Morphometric study of the human malleus during prenatal development

OBJECTIVES

Establish normal reference values for the human malleus during the second and third trimesters of pregnancy, and describe its growth dynamics and ossification process.

METHODS

Thirty spontaneously aborted human fetuses, 12-36 weeks of age, were examined. Temporal bones were obtained en bloc and processed using the skeletal clearing and staining technique. Each malleus was obtained by microdissection. Measurements of anatomical points of the malleus were performed. Ossification was recorded. Statistics were applied.

RESULTS

Sixty mallei were obtained. At the end of development, the following measurements were obtained in millimeters: head length, 2.87 (2.30-3.44); neck length, 1.15 (0.92-1.38); manubrium length, 4.45 (3.75-5.16); total length, 8.51 (8.02-8.99); neck width, 1.33 (0.70-1.97); manubrium width, 0.84 (0.82-0.86); and Meckel cartilage width, 0.43 (0.06-0.95). The angle of the longitudinal axis of the malleus was 160° (156-164°). The ossification process of the malleus followed a stereotyped pattern that began at week 16, at the level of the neck, with a single center that expanded to the head and then to the manubrium. Toward the end of development, only the umbo remained cartilaginous. There were no sex-based differences in human malleus size during prenatal development. From the beginning of the second trimester of pregnancy, the longitudinal axis of the malleus acquires dimensions comparable to adults.

CONCLUSION

The study of the malleus development provides clues that would explain some anomalies in its development, and can influence decision-making during the placement and design of prostheses in the malleus.

A new mammal from the Lower Cretaceous Jehol Biota and implications for eutherian evolution

Here we report on a new Early Cretaceous eutherian represented by a partial skeleton from the Jiufotang Formation at Sihedang site, Lingyuan City, Liaoning Province that fills a crucial gap between the earliest eutherians from the Yixian Formation and later Cretaceous eutherians. The new specimen reveals, to our knowledge for the first time in eutherians, that the Meckelian cartilage was ossified but reduced in size, confirming a complete detachment of the middle ear from the lower jaw. Seven hyoid elements, including paired stylohyals, epihyals and thyrohyals and the single basihyal are preserved. For the inner ear the ossified primary lamina, base of the secondary lamina, ossified cochlear ganglion and secondary crus commune are present and the cochlear canal is coiled through 360°. In addition, plesiomorphic features of the dentition include weak conules, lack of pre- and post-cingula and less expanded protocones on the upper molars and height differential between the trigonid and talonid, a large protoconid and a small paraconid on the lower molars. The new taxon displays an alternating pattern of tooth replacement with P3 being the last upper premolar to erupt similar to the basal eutherian Juramaia. Parsimony analysis places the new taxon with Montanalestes, Sinodelphys and Ambolestes as a sister group to other eutherians. This article is part of the theme issue 'The impact of Chinese palaeontology on evolutionary research'.

PRDM paralogs antagonistically balance Wnt/β-catenin activity during craniofacial chondrocyte differentiation

Cranial neural crest cell (NCC)-derived chondrocyte precursors undergo a dynamic differentiation and maturation process to establish a scaffold for subsequent bone formation, alterations in which contribute to congenital birth defects. Here, we demonstrate that transcription factor and histone methyltransferase proteins Prdm3 and Prdm16 control the differentiation switch of cranial NCCs to craniofacial cartilage. Loss of either paralog results in hypoplastic and disorganized chondrocytes due to impaired cellular orientation and polarity. We show that these proteins regulate cartilage differentiation by controlling the timing of Wnt/β-catenin activity in strikingly different ways: Prdm3 represses whereas Prdm16 activates global gene expression, although both act by regulating Wnt enhanceosome activity and chromatin accessibility. Finally, we show that manipulating Wnt/β-catenin signaling pharmacologically or generating prdm3-/-;prdm16-/- double mutants rescues craniofacial cartilage defects. Our findings reveal upstream regulatory roles for Prdm3 and Prdm16 in cranial NCCs to control Wnt/β-catenin transcriptional activity during chondrocyte differentiation to ensure proper development of the craniofacial skeleton.

Spatial and chronological localization of septoclasts in the mouse Meckel's cartilage

Meckel's cartilage (MC) in the first branchial arch of mammals is a transient structure that disappears before birth, except for the most anterior and posterior portions. Recent studies reported that some congenital abnormalities in craniofacial regions are linked with the persistence or dysplasia of MC. However, the mechanisms underlying the resorption of MC have not been elucidated. Cartilage resorption in endochondral ossification is performed by multinuclear osteoclasts/chondroclasts as well as mononuclear septoclasts, which were newly added to the list of cartilage phagocytes. Septoclasts located exclusively at the chondro-osseous junction of the growth plate resorb the uncalcified cartilage matrix. We hypothesized that septoclasts participate in the resorption of MC and attempted to clarify the localization and roles of septoclasts in MC of mouse using a specific immunohistochemistry marker, epidermal type-fatty acid-binding protein (E-FABP/FABP5). E-FABP-immunopositive septoclasts were detected for the first time at the beginning of MC resorption and localized along the resorption surface. Septoclasts of MC in embryonic mice possessed several processes that elongated toward the uncalcified cartilage matrix, expressed cathepsin B, and exhibited characteristic pericapillary localization. Additionally, they localized between hypertrophied cartilage and osteoclasts/chondroclasts in the resorption surface. Confocal laser-scanning microscopy revealed a decrease in the numbers of septoclasts and their processes with the progression of MC disappearance before birth. The present study showed that E-FABP-immunopositive septoclasts participated in the disappearance of MC through the resorption of the uncalcified cartilage matrix and that they have different roles from osteoclasts/chondroclasts.

Osteogenic and angiogenic profiles of the palatal process of the maxilla and the palatal process of the palatine bone

Hard palate consists anteriorly of the palatal process of the maxilla (ppmx) and posteriorly of the palatal process of the palatine (ppp). Currently, palatal osteogenesis is receiving increasing attention. This is the first study to provide an overview of the osteogenesis process of the mouse hard palate. We found that the period in which avascular mesenchymal condensation becomes a vascularized bone structure corresponds to embryonic day (E) 14.5 to E16.5 in the hard palate. The ppmx and ppp differ remarkably in morphology and molecular respects during osteogenesis. Osteoclasts in the ppmx and ppp are heterogeneous. There was a multinucleated giant osteoclast on the bone surface at the lateral-nasal side of the ppmx, while osteoclasts in the ppp were more abundant and adjacent to blood vessels but were smaller and had fewer nuclei. In addition, bone remodeling in the hard palate was asymmetric and exclusively occurred on the nasal side of the hard palate at E18.5. During angiogenesis, CD31-positive endothelial cells were initially localized in the surrounding of palatal mesenchymal condensation and then invaded the condensation in a sprouting fashion. At the transcriptome level, we found 78 differentially expressed genes related to osteogenesis and angiogenesis between the ppmx and ppp. Fifty-five related genes were up/downregulated from E14.5 to E16.5. Here, we described the morphogenesis and the heterogeneity in the osteogenic and angiogenic genes profiles of the ppmx and ppp, which are significant for subsequent studies of normal and abnormal subjects.

Mandible Explant Assay for the Analysis of Meckel's Cartilage Development

Ex vivo explant models are a valuable tool for analyzing organ and tissue morphogenesis, providing the opportunity to manipulate and interrogate specific cellular and/or molecular pathways that may not be possible using conventional methods in vivo. The mandible primordia is a remarkably self-organizing structure that has the ability to develop cartilage, bone, teeth, epithelial tissue, and the tongue when grown in culture ex vivo and closely mimics the development of these structures in vivo. Here we describe a robust protocol for the culture of mandibular explants using serum-free, chemically defined culture media. We also describe methods for manipulating mandible and/or Meckel's cartilage development by implantation of agarose beads soaked in various molecular factors to augment mandible development, as well as methods for Alcian blue staining of Meckel's cartilage and immunohistochemistry. This culture method can also be adapted for other molecular analyses, including addition of small-molecule inhibitors and/or growth factors to the culture media, as well as culturing explants from genetically modified mice.

Calcification and resorption of mouse Meckel's cartilage analyzed by von Kossa and tartrate-resistant acid phosphatase histochemistry and scanning electron microscopy/energy-dispersive X-ray spectrometry

Meckel's cartilage is essential for the normal development of the mandible. The fate of the intermediate portion of Meckel's cartilage is unique as most of it disappears soon after birth except for the part that forms the sphenomandibular ligament. The mechanism of the disappearance of Meckel's cartilage is unknown; therefore, this study was designed to investigate the process of Meckel's cartilage degradation, focusing on cartilage matrix calcification and the appearance of chondroclasts. Developing mouse mandibles at embryonic days 15, 16, 17, and 18, and postnatal day 2 were processed for whole-mount staining with alcian blue and alizarin red. The mandibles on embryonic days 15, 16, 17, and 18 were fixed and embedded in paraffin. Adjacent sections were processed for von Kossa and tartrate-resistant acid phosphatase (TRAP) histochemistry and scanning electron microscopy/energy-dispersive X-ray spectrometry (SEM/EDS). Calcification and the element concentrations of calcium, phosphorus, and carbon were examined with von Kossa histochemistry and SEM/EDS. The involvement of chondroclasts was investigated using TRAP histochemistry. The results demonstrated that the intermediate portion of Meckel's cartilage is resorbed by chondroclasts after chondrocyte hypertrophy and cartilage matrix calcification and that the mineral concentration of calcified Meckel's cartilage is comparable to that of the surrounding bone. This study contributes to the understanding of the mechanism of Meckel's cartilage resorption and provides useful insights into the development of the mandible.

Histone methyltransferase SET domain bifurcated 1 negatively regulates parathyroid hormone/parathyroid hormone-related peptide receptor to control chondrocyte proliferation in Meckel's cartilage

OBJECTIVE

The aim of this study is to show that the proliferation of chondrocytes is regulated by SET domain bifurcated 1 (SETDB1) along with the downregulation of parathyroid hormone (PTH)/parathyroid hormone-related peptide (PTHrP) receptor in Meckel's cartilage.

DESIGN

Setdb1 was knocked down or overexpressed in a mouse chondrogenic ATDC5 cells, by transfecting the cells with short interfering RNA against Setdb1 or wild-type Setdb1 expression vector, respectively. Cell proliferation was detected by bromodeoxyuridine incorporation. Setdb1 was conditionally deleted in neural crest cells with Wnt1-Cre (Setdb1 conditional knockout mice). Immunofluorescence staining of paraffin sections of embryonic days 13.5 and 14.5 Setdb1 conditional knockout mice or transfected ATDC5 cells was performed to detect PTH/PTHrP receptor. Protein kinase B (AKT) phosphorylation inhibitor was added to both siRNA-transfected ATDC5 cultures to determine whether AKT activation induces PTH/PTHrP receptor expression after Setdb1 knockdown or vice versa.

RESULTS

Setdb1 knockdown in ATDC5 cells showed increased cell proliferation and parathyroid hormone receptor 1 expression. Contrasting results were observed in the Setdb1-overexpressed wild-type cells. Immunofluorescence staining showed the highly expressed PTH/PTHrP receptor in Setdb1-knocked down ATDC5 cells and in the chondrocytes of Setdb1 conditional knockout embryonic Meckel's cartilage, indicating the negative regulation of SETDB1 on PTH/PTHrP receptor. Strong staining of phosphorylated AKT was observed in Setdb1-knocked down ATDC5 cells. However, the inhibition of AKT phosphorylation significantly reduced both the PTH/PTHrP receptor staining and the Setdb1-knockdown-induced increase in ATDC5 cell proliferation.

CONCLUSIONS

Our findings contribute new insights on SETDB1 function in relation with AKT and PTH/PTHrP receptor during chondrocyte proliferation.

The Mandibular and Hyoid Arches-From Molecular Patterning to Shaping Bone and Cartilage

The mandibular and hyoid arches collectively make up the facial skeleton, also known as the viscerocranium. Although all three germ layers come together to assemble the pharyngeal arches, the majority of tissue within viscerocranial skeletal components differentiates from the neural crest. Since nearly one third of all birth defects in humans affect the craniofacial region, it is important to understand how signalling pathways and transcription factors govern the embryogenesis and skeletogenesis of the viscerocranium. This review focuses on mouse and zebrafish models of craniofacial development. We highlight gene regulatory networks directing the patterning and osteochondrogenesis of the mandibular and hyoid arches that are actually conserved among all gnathostomes. The first part of this review describes the anatomy and development of mandibular and hyoid arches in both species. The second part analyses cell signalling and transcription factors that ensure the specificity of individual structures along the anatomical axes. The third part discusses the genes and molecules that control the formation of bone and cartilage within mandibular and hyoid arches and how dysregulation of molecular signalling influences the development of skeletal components of the viscerocranium. In conclusion, we notice that mandibular malformations in humans and mice often co-occur with hyoid malformations and pinpoint the similar molecular machinery controlling the development of mandibular and hyoid arches.

Craniofacial Development: Neural Crest in Molecular Embryology

Craniofacial development, one of the most complex sequences of developmental events in embryology, features a uniquely transient, pluripotent stem cell-like population known as the neural crest (NC). Neural crest cells (NCCs) originate from the dorsal aspect of the neural tube and migrate along pre-determined routes into the developing branchial arches and frontonasal plate. The exceptional rates of proliferation and migration of NCCs enable their diverse contribution to a wide variety of craniofacial structures. Subsequent differentiation of these cells gives rise to cartilage, bones, and a number of mesenchymally-derived tissues. Deficiencies in any stage of differentiation can result in facial clefts and abnormalities associated with craniofacial syndromes. A small number of conserved signaling pathways are involved in controlling NC differentiation and craniofacial development. They are used in a reiterated fashion to help define precise temporospatial cell and tissue formation. Although many aspects of their cellular and molecular control have yet to be described, it is clear that together they form intricately integrated signaling networks required for spatial orientation and developmental stability and plasticity, which are hallmarks of craniofacial development. Mutations that affect the functions of these signaling pathways are often directly or indirectly identified in congenital syndromes. Clinical applications of NC-derived mesenchymal stem/progenitor cells, persistent into adulthood, hold great promise for tissue repair and regeneration. Realization of NCC potential for regenerative therapies motivates understanding of the intricacies of cell communication and differentiation that underlie the complexities of NC-derived tissues.