FACEts of mechanical regulation in the morphogenesis of craniofacial structures

Three-dimensional evaluation of the cortical and cancellous bone density and thickness for miniscrew insertion: a CBCT study of interradicular area of adults with different facial growth pattern

AIM

The purpose of this study was to evaluate the effect of the density and the thickness of the cortical and the cancellous bone at selected inter-radicular areas in subjects with different facial growth patterns using cone beam computed tomography (CBCT) in order to choose the optimal area for miniscrew insertion.

MATERIALS AND METHODS

From 150 CBCT scans, 45 scans were included in the study. The subjects were categorized into three groups based on their skeletal growth pattern according to SN-GoMe angle and facial height index. Cortical and cancellous bone density and thickness were measured at the selected inter-radicular areas.

RESULTS

Compared to the other two groups, the hyperdivergent group had thinner cortical bone in the anterior region of the maxilla between the central and the lateral incisors on the buccal side at 4 mm from the alveolar crest (P-value: 0.012) and on the palatal side at 7 mm from the alveolar crest (P-value: 0.030). Cancellous bone density values in these areas were higher in subjects with hypodivergent and hyperdivergent growth pattern. Furthermore, in hyperdivergent group less dense cortical bone in the posterior region of the maxilla on the palatal side between the second premolar and the first molar (p-value: 0.020) and on the buccal side between the first molar and the second molar (p-value: 0.038 & 0.047) was observed. No significant differences were found in the mandible between the three groups. No significant differences were found between the male and the female subjects.

CONCLUSION

Hyperdivegents presented thinner cortical bone in the anterior of the maxilla between the central and the lateral incisors. Less dense cortical bone was found between maxillary second premolar and first molar on the palatal side and also between the maxillary first molar and the second molar on the buccal side in this group too. Normal showed higher density values in the posterior of the maxilla compared to the other two groups. No significant differences were found among three groups in mandible.

A three-dimensional method to calculate mechanical advantage in mandibular function : Intra- and interexaminer reliability study

PURPOSE

Masticatory muscles are physically affected by several skeletal features. The muscle performance depends on muscle size, intrinsic strength, fiber direction, moment arm, and neuromuscular control. To date, for the masticatory apparatus, only a two-dimensional cephalometric method for assessing the mechanical advantage, which is a measure for the ratio of the output force to the input force in a system, is available. This study determined the reliability and errors of a three-dimensional (3D) mechanical advantage calculation for the masticatory system.

METHODS

Using cone-beam computed tomography images from teenage patients undergoing orthodontic treatments, 36 craniofacial landmarks were identified, and the moment arms for seven muscles and their load moment arms (biomechanical variables) were determined. The 3D mechanical advantage for each muscle was calculated. This procedure was repeated by three examiners. Reliability was verified using the intraclass correlation coefficient (ICC) and the errors by calculating the absolute differences, variance estimator and coefficient of variation (CV).

RESULTS

Landmark coordinates demonstrated excellent intra- and interexaminer reliability (ICC 0.998-1.000; p < 0.0001). Intraexaminer data showed errors < 1.5 mm. Unsatisfactory interexaminer errors ranged from 1.51-5.83 mm. All biomechanical variables presented excellent intraexaminer reliability (ICC 0.919-1.000, p < 0.0001; CV < 7%). Interexaminer results were almost excellent, but with lower values (ICC 0.750-1.000, p < 0.0001; CV < 10%). However, the muscle moment arm and 3D mechanical advantage of the lateral pterygoid muscles had ICCs < 0.500 (p < 0.05) and CV < 30%. Intra- and interexaminer errors were ≤ 0.01 and ≤ 0.05, respectively.

CONCLUSIONS

Both landmarks and biomechanical variables showed high reliability and acceptable errors. The proposed method is viable for the 3D mechanical advantage measure.

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.

Jak2 and Jaw Muscles Are Required for Buccopharyngeal Membrane Perforation during Mouth Development

The mouth is a central feature of our face, without which we could not eat, breathe, or communicate. A critical and early event in mouth formation is the creation of a "hole" which connects the digestive system and the external environment. This hole, which has also been called the primary or embryonic mouth in vertebrates, is initially covered by a 1-2 cell layer thick structure called the buccopharyngeal membrane. When the buccopharyngeal membrane does not rupture, it impairs early mouth functions and may also lead to further craniofacial malformations. Using a chemical screen in an animal model (Xenopus laevis) and genetic data from humans, we determined that Janus kinase 2 (Jak2) has a role in buccopharyngeal membrane rupture. We have determined that decreased Jak2 function, using antisense morpholinos or a pharmacological antagonist, caused a persistent buccopharyngeal membrane as well as the loss of jaw muscles. Surprisingly, we observed that the jaw muscle compartments were connected to the oral epithelium that is continuous with the buccopharyngeal membrane. Severing such connections resulted in buccopharyngeal membrane buckling and persistence. We also noted puncta accumulation of F-actin, an indicator of tension, in the buccopharyngeal membrane during perforation. Taken together, the data has led us to a hypothesis that muscles are required to exert tension across the buccopharyngeal membrane, and such tension is necessary for its perforation.

Emerging Roles of YAP/TAZ in Tooth and Surrounding: from Development to Regeneration

Yes associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) are ubiquitous transcriptional co-activators that control organ development, homeostasis, and tissue regeneration. Current in vivo evidence suggests that YAP/TAZ regulates enamel knot formation during murine tooth development, and is indispensable for dental progenitor cell renewal to support constant incisor growth. Being a critical sensor for cellular mechano-transduction, YAP/TAZ lays at the center of the complex molecular network that integrates mechanical cues from the dental pulp chamber and surrounding periodontal tissue into biochemical signals, dictating in vitro cell proliferation, differentiation, stemness maintenance, and migration of dental stem cells. Moreover, YAP/TAZ-mediated cell-microenvironment interactions also display essential regulatory roles during biomaterial-guided dental tissue repair and engineering in some animal models. Here, we review recent advances in YAP/TAZ functions in tooth development, dental pulp, and periodontal physiology, as well as dental tissue regeneration. We also highlight several promising strategies that harness YAP/TAZ activation for promoting dental tissue regeneration.

LATS1/2 control TGFB-directed epithelial-to-mesenchymal transition in the murine dorsal cranial neuroepithelium through YAP regulation

Hippo signaling, an evolutionarily conserved kinase cascade involved in organ size control, plays key roles in various tissue developmental processes, but its role in craniofacial development remains poorly understood. Using the transgenic Wnt1-Cre2 driver, we inactivated the Hippo signaling components Lats1 and Lats2 in the cranial neuroepithelium of mouse embryos and found that the double conditional knockout (DCKO) of Lats1/2 resulted in neural tube and craniofacial defects. Lats1/2 DCKO mutant embryos had microcephaly with delayed and defective neural tube closure. Furthermore, neuroepithelial cell shape and architecture were disrupted within the cranial neural tube in Lats1/2 DCKO mutants. RNA sequencing of embryonic neural tubes revealed increased TGFB signaling in Lats1/2 DCKO mutants. Moreover, markers of epithelial-to-mesenchymal transition (EMT) were upregulated in the cranial neural tube. Inactivation of Hippo signaling downstream effectors, Yap and Taz, suppressed neuroepithelial defects, aberrant EMT and TGFB upregulation in Lats1/2 DCKO embryos, indicating that LATS1/2 function via YAP and TAZ. Our findings reveal important roles for Hippo signaling in modulating TGFB signaling during neural crest EMT.

Revisiting the embryogenesis of lip and palate development

Clefts of the lip and palate (CLP), the major causes of congenital facial malformation globally, result from failure of fusion of the facial processes during embryogenesis. With a prevalence of 1 in 500-2500 live births, CLP causes major morbidity throughout life as a result of problems with facial appearance, feeding, speaking, obstructive apnoea, hearing and social adjustment and requires complex, multi-disciplinary care at considerable cost to healthcare systems worldwide. Long-term outcomes for affected individuals include increased mortality compared with their unaffected siblings. The frequent occurrence and major healthcare burden imposed by CLP highlight the importance of dissecting the molecular mechanisms driving facial development. Identification of the genetic mutations underlying syndromic forms of CLP, where CLP occurs in association with non-cleft clinical features, allied to developmental studies using appropriate animal models is central to our understanding of the molecular events underlying development of the lip and palate and, ultimately, how these are disturbed in CLP.

Spatiotemporal Gene Expression Regions along the Anterior-Posterior Axis in Mouse Embryos before and after Palatal Elevation

The mammalian secondary palate is formed through complex developmental processes: growth, elevation, and fusion. Although it is known that the palatal elevation pattern changes along the anterior-posterior axis, it is unclear what molecules are expressed and whether their locations change before and after elevation. We examined the expression regions of molecules associated with palatal shelf elevation (Pax9, Osr2, and Tgfβ3) and tissue deformation (F-actin, E-cadherin, and Ki67) using immunohistochemistry and RT-PCR in mouse embryos at E13.5 (before elevation) and E14.5 (after elevation). Pax9 was expressed at significantly higher levels in the lingual/nasal region in the anterior and middle parts, as well as in the buccal/oral region in the posterior part at E13.5. At E14.5, Pax9 was expressed at significantly higher levels in both the lingual/nasal and buccal/oral regions in the anterior and middle parts and the buccal/oral regions in the posterior part. Osr2 was expressed at significantly higher levels in the buccal/oral region in all parts at E13.5 and was more strongly expressed at E13.5 than at E14.5 in all regions. No spatiotemporal changes were found in the other molecules. These results suggested that Pax9 and Osr2 are critical molecules leading to differences in the elevation pattern in palatogenesis.

Three-Dimensional Analysis of Posterior Mandibular Displacement in Rats

Mandibular protrusion and its treatment is challenging for the orthodontist. The aim of the present research was to identify macroscopic changes in the mandible, based on three-dimensional Cone Beam Computed Tomography analysis. Seventy-two male Wistar rats were divided into two equal groups, experimental (group A) and control (group B). Each consisted of three equal subgroups of 12 rats (A1, A2, A3, B1, B2, B3). Full-cast orthodontic intraoral devices were attached to the maxillary incisors of the experimental animals, and effected functional posterior mandibular displacement. Throughout the experimental period, all animals were fed with mashed food. Animals were sacrificed at 30 days (A1, B1), 60 days (A2, B2) and 90 days (A3, B3). At the 60th day of the experiment, the orthodontic devices were removed from the remaining experimental subgroup A3. Measurements revealed significant differences in the anteroposterior dimensions between experimental and control subgroups. However, the observed changes in the vertical dimensions, Condylion/Go’–Menton and the Intercondylar distance proved insignificant. Posterior mandibular displacement of the mandible in growing rats affects the morphology of the mandible and culminates in the development of a smaller mandible at a grown age.

Three-dimensional reconstruction of systematic histological sections: application to observations on palatal shelf elevation

Normal mammalian secondary palate development undergoes a series of processes, including palatal shelf (PS) growth, elevation, adhesion and fusion, and palatal bone formation. It has been estimated that more than 90% of isolated cleft palate is caused by defects associated with the elevation process. However, because of the rapidly completed elevation process, the entire process of elevation will never be easy to clarify. In this article, we present a novel method for three-dimensional (3D) reconstruction of thick tissue blocks from two-dimensional (2D) histological sections. We established multiplanar sections of the palate and tongue in coronal and sagittal directions, and further performed 3D reconstruction to observe the morphological interaction and connection between the two components prior to and during elevation. The method completes an imaging system for simultaneous morphological analysis of thick tissue samples using both synthetic and real data. The new method will provide a comprehensive picture of reorientation morphology and gene expression pattern during the palatal elevation process.