The role of osteocytes during experimental orthodontic tooth movement: A review

Effects of Diet Consistency on Rat Maxillary and Mandibular Growth within Three Generations-A Longitudinal CBCT Study

BACKGROUND

In this study, wistar rats were used to examine the impact of diet consistency on maxillary and mandibular growth over three generations.

METHODS

In this investigation, a breeding sample of 60 female and 8 male wistar rats was used. Measuring was only performed on female animals. The first generation's primary breeding sample consisted of 20 female wistar rats that were 30 days old and 4 male rats that were also 30 days old; two subsequent generations were created from these animals. At the age of 100 days, CBCTs were collected of all male rats. Twenty-eight craniofacial landmarks were selected for the linear measurements on stl format extracted from the DICOM files. A Bonferroni test was performed for the statistical analysis.

RESULTS

Means of measurements of all soft diet groups compared to corresponding measurements of the hard diet groups were significantly different. According to linear measurements, there was statistical difference on the maxillary measurements between the soft diet groups of the first and third generation, while the rest did not appear to have any statistical difference. There was significant difference for the mandibular dimensions only when the first generation soft diet group was compared with the third generation soft diet group.

CONCLUSIONS

Food consistency has a significant impact on the growth and development of the maxilla and mandible. Soft diet habits may result in retrognathic mandible, and narrower maxilla.

Mechanical-induced bone remodeling does not depend on Piezo1 in dentoalveolar hard tissue

Mechanosensory ion channels are proteins that are sensitive to mechanical forces. They are found in tissues throughout the body and play an important role in bone remodeling by sensing changes in mechanical stress and transmitting signals to bone-forming cells. Orthodontic tooth movement (OTM) is a prime example of mechanically induced bone remodeling. However, the cell-specific role of the ion channels Piezo1 and Piezo2 in OTM has not been investigated yet. Here we first identify the expression of PIEZO1/2 in the dentoalveolar hard tissues. Results showed that PIEZO1 was expressed in odontoblasts, osteoblasts, and osteocytes, while PIEZO2 was localized in odontoblasts and cementoblasts. We therefore used a Piezo1^{floxed/floxed} mouse model in combination with Dmp1^cre to inactivate Piezo1 in mature osteoblasts/cementoblasts, osteocytes/cementocytes, and odontoblasts. Inactivation of Piezo1 in these cells did not affect the overall morphology of the skull but caused significant bone loss in the craniofacial skeleton. Histological analysis revealed a significantly increased number of osteoclasts in Piezo1^{floxed/floxed};Dmp1^cre mice, while osteoblasts were not affected. Despite this increased number of osteoclasts, orthodontic tooth movement was not altered in these mice. Our results suggest that despite Piezo1 being crucial for osteoclast function, it may be dispensable for mechanical sensing of bone remodeling.

Long-Term Effect of Diet Consistency on Mandibular Growth within Three Generations: A Longitudinal Cephalometric Study in Rats

BACKGROUND

This study investigated the effect of diet consistency on mandibular growth of Wistar rats through three generations.

METHODS

A total breeding sample of 60 female and 8 male Wistar rats were used in this study. Measurements took place only on female animals. Twenty female Wistar rats at 30 days old and four male rats at 30 days old comprised the primary breeding sample of the first generation, and from these animals two different generations were reproduced. Lateral cephalometric X-rays were taken from all female rats at the age of 100 days. A total of 7 craniofacial landmarks were selected for the linear measurements, and 12 curves and 90 landmarks were selected for geometric morphometric analysis of the lateral X-rays. Bonferroni test and a permutation test were performed for the statistical analysis.

RESULTS

Means of measurements of all soft diet groups compared to hard diet groups were significantly smaller. According to linear measurements, there was a significant difference only between the first-generation soft diet with the third-generation soft diet group. According to geometric morphometric analysis, the statistical differences appeared on the condylar process and the angle of the mandible.

CONCLUSIONS

The soft diet could be responsible for less mandibular growth, and this information might be passing through generations.

Osteoimmunology in Periodontitis and Orthodontic Tooth Movement

PURPOSE OF REVIEW

To review the role of the immune cells and their interaction with cells found in gingiva, periodontal ligament, and bone that leads to net bone loss in periodontitis or bone remodeling in orthodontic tooth movement.

RECENT FINDINGS

Periodontal disease is one of the most common oral diseases causing inflammation in the soft and hard tissues of the periodontium and is initiated by bacteria that induce a host response. Although the innate and adaptive immune response function cooperatively to prevent bacterial dissemination, they also play a major role in gingival inflammation and destruction of the connective tissue, periodontal ligament, and alveolar bone characteristic of periodontitis. The inflammatory response is triggered by bacteria or their products that bind to pattern recognition receptors that induce transcription factor activity to stimulate cytokine and chemokine expression. Epithelial, fibroblast/stromal, and resident leukocytes play a key role in initiating the host response and contribute to periodontal disease. Single-cell RNA-seq (scRNA-seq) experiments have added new insight into the roles of various cell types in the response to bacterial challenge. This response is modified by systemic conditions such as diabetes and smoking. In contrast to periodontitis, orthodontic tooth movement (OTM) is a sterile inflammatory response induced by mechanical force. Orthodontic force application stimulates acute inflammatory responses in the periodontal ligament and alveolar bone stimulated by cytokines and chemokines that produce bone resorption on the compression side. On the tension side, orthodontic forces induce the production of osteogenic factors, stimulating new bone formation. A number of different cell types, cytokines, and signaling/pathways are involved in this complex process. Inflammatory and mechanical force-induced bone remodeling involves bone resorption and bone formation. The interaction of leukocytes with host stromal cells and osteoblastic cells plays a key role in both initiating the inflammatory events as well as inducing a cellular cascade that results in remodeling in orthodontic tooth movement or in tissue destruction in periodontitis.

Liposomal Doxorubicin Kinetic Study in an In vitro 2D and 3D Tumor Model for Osteosarcoma in a Perfusion Bioreactor

BACKGROUND

In vivo drug screening in animal models is contrary to ethical values, costly and time-consuming. Traditional static in vitro models do not reflect the basic characteristics of bone tumor microenvironments; therefore, perfusion bioreactors, in particular, would be an applicable choice due to their advantages to regenerate versatile bone tumor models for studying in vitro novel drug delivery systems.

METHODS

In this study, an optimal drug formulation of liposomal doxorubicin was prepared, and the release kinetics of the drug and its toxicity effect on MG-63 bone cancer cell line were investigated in two-dimensional, static three-dimensional media on a PLGA/β-TCP scaffold and also in a dynamic media in a perfusion bioreactor. In this assay, the efficacy of the IC50 of this formulation which had been obtained in two-dimensional cell culture (= 0.1 μg/ml), was studied in static and dynamic threedimensional media after 3 and 7 days. Liposomes with good morphology and encapsulation efficiency of 95% had release kinetics of the Korsmeyer-Peppas model.

RESULTS

The results of cell growth before treatment and cell viability after treatment in all three environments were compared. Cell growth in 2D was rapid, while it was slow in static 3D conditions. In the dynamic 3D environment, it was significant compared to the static tumor models. Cell viability after 3 and 7 days from treatment was 54.73% and 13.39% in 2D conditions, 72.27% and 26.78% in the static 3D model, while 100% and 78.92% in the dynamic culture indicating the effect of drug toxicity over time, but drug resistance of 3D models compared to 2D culture. In the bioreactor, the formulation used in the mentioned concentration showed very small cytotoxicity demonstrating the dominance of mechanical stimuli on cell growth over drug toxicity.

CONCLUSION

Increasing drug resistance in 3D models compared to 2D models indicates the superiority of liposomal Dox over free form to reduce IC50 concentration.

Bone permeability and mechanotransduction: Some current insights into the function of the lacunar-canalicular network

Lacunar-canalicular (LC) permeability involves the passage of fluids, nutrients, oxygen, ions, and signalling molecules through bone tissue, facilitating the maintenance of bone vitality and function and responses to various physiological conditions and diseases. LC permeability and fluid flow-shear stress/drag force play important roles in mechanotransduction in bone tissue by inducing mechanical stimuli in osteocytes, modulating cellular functions, and determining bone adaptation. Alterations in LC structure may therefore influence the fluid flow pattern through the LC network, thereby affecting the ability of osteocytes to sense and translate mechanical signals and possibly contributing to bone remodelling. Several bone-health conditions are associated with changes in LC structure and function and may affect mechanotransduction and responses, although the mechanisms underlying these associations are still not fully understood. In this review, recent studies of LC networks, their formation and transfer mechanical stimuli, and changes in structure, functional permeability, and mechanotransduction that result from age, pathology, and mechanical loading are discussed. Additionally, applications of vibration and low-intensity pulsed ultrasound in bone healthcare and regeneration fields are also presented.

Acceleration of orthodontic tooth movement and root resorption with near and distant surgical insults: An in-vivo study on a rat model

OBJECTIVE

The purpose of this study was to determine the effects of alveolar decortications (AD) closer and farther from the tooth on Orthodontic tooth movement (OTM) and root resorption.

MATERIALS AND METHODS

Twenty-four Wistar rats (age 17 weeks) were used in the study. Three groups were formed by randomly dividing the rats into: 1) Control Group (N=8): OTM (14 days) without any surgical insult; 2) Near Group (N=8) OTM (14 days)+Alveolar Decortications (AD) adjacent to roots of the maxillary first molar; 3) Far Group (N=8) OTM (14 days)+AD 5mm away from the roots of the maxillary first molar. OTM was performed using a closed coil Nickel-Titanium (NiTi) spring to apply a mesial force on maxillary first molars of 8-10 grams. AD was performed with high-speed quarter round bur on the palatal aspect of alveolar bone. The rats were euthanized after 2 weeks of OTM and microfocus computed tomography and histological analysis were performed.

RESULTS

Near-AD and Far-AD groups exhibited significantly increased OTM and reduction of Bone volume fraction compared to control. The root volume was significantly decreased (increased root resorption) in the Near-AD group. Histological analysis showed no statistically significant difference between the groups.

CONCLUSIONS

Alveolar decortications lead to a significant increase in the rate of OTM. Increased root resorption was observed when the alveolar decortications were performed close to the maxillary molar.

The role of inhibition of osteocyte apoptosis in mediating orthodontic tooth movement and periodontal remodeling: a pilot study

BACKGROUND

Orthodontic tooth movement (OTM) has been shown to induce osteocyte apoptosis in alveolar bone shortly after force application. However, how osteocyte apoptosis affects orthodontic tooth movement is unknown. The goal of this study was to assess the effect of inhibition of osteocyte apoptosis on osteoclastogenesis, changes in the alveolar bone density, and the magnitude of OTM using a bisphosphonate analog (IG9402), a drug that affects osteocyte and osteoblast apoptosis but does not affect osteoclasts.

MATERIAL AND METHODS

Two sets of experiments were performed. Experiment 1 was used to specifically evaluate the effect of IG9402 on osteocyte apoptosis in the alveolar bone during 24 h of OTM. For this experiment, twelve mice were divided into two groups: group 1, saline administration + OTM_24-h (n=6), and group 2, IG9402 administration + OTM_24-h (n=6). The contralateral unloaded sides served as the control. The goal of experiment 2 was to evaluate the role of osteocyte apoptosis on OTM magnitude and osteoclastogenesis 10 days after OTM. Twenty mice were divided into 4 groups: group 1, saline administration without OTM (n=5); group 2, IG9402 administration without OTM (n=5); group 3, saline + OTM_10-day (n=6); and group 4, IG9402 + OTM_10-day (n=4). For both experiments, tooth movement was achieved using Ultra Light (25g) Sentalloy Closed Coil Springs attached between the first maxillary molar and the central incisor. Linear measurements of tooth movement and alveolar bone density (BVF) were assessed by MicroCT analysis. Cell death (or apoptosis) was assessed by terminal dUTP nick-end labeling (TUNEL) assay, while osteoclast and macrophage formation were assessed by tartrate-resistant acid phosphatase (TRAP) staining and F4/80+ immunostaining.

RESULTS

We found that IG9402 significantly blocked osteocyte apoptosis in alveolar bone (AB) at 24 h of OTM. At 10 days, IG9402 prevented OTM-induced loss of alveolar bone density and changed the morphology and quality of osteoclasts and macrophages, but did not significantly affect the amount of tooth movement.

CONCLUSION

Our study demonstrates that osteocyte apoptosis may play a significant role in osteoclast and macrophage formation during OTM, but does not seem to play a role in the magnitude of orthodontic tooth movement.

Biomechanical and biological responses of periodontium in orthodontic tooth movement: up-date in a new decade

Nowadays, orthodontic treatment has become increasingly popular. However, the biological mechanisms of orthodontic tooth movement (OTM) have not been fully elucidated. We were aiming to summarize the evidences regarding the mechanisms of OTM. Firstly, we introduced the research models as a basis for further discussion of mechanisms. Secondly, we proposed a new hypothesis regarding the primary roles of periodontal ligament cells (PDLCs) and osteocytes involved in OTM mechanisms and summarized the biomechanical and biological responses of the periodontium in OTM through four steps, basically in OTM temporal sequences, as follows: (1) Extracellular mechanobiology of periodontium: biological, mechanical, and material changes of acellular components in periodontium under orthodontic forces were introduced. (2) Cell strain: the sensing, transduction, and regulation of mechanical stimuli in PDLCs and osteocytes. (3) Cell activation and differentiation: the activation and differentiation mechanisms of osteoblast and osteoclast, the force-induced sterile inflammation, and the communication networks consisting of sensors and effectors. (4) Tissue remodeling: the remodeling of bone and periodontal ligament (PDL) in the compression side and tension side responding to mechanical stimuli and root resorption. Lastly, we talked about the clinical implications of the updated OTM mechanisms, regarding optimal orthodontic force (OOF), acceleration of OTM, and prevention of root resorption.

Establishment of in vitro three-dimensional cementocyte differentiation scaffolds to study orthodontic root resorption

Orthodontic-induced root resorption is a severe side effect that can lead to tooth root shortening and loss. Compressive force induces tissue stress in the cementum that covers the tooth root, which is associated with activation of bone metabolism and cementum resorption. To investigate the role of cementocytes in mechanotransduction and osteoclast differentiation, the present study established an in vitro three-dimensional (3D) model replicating cellular cementum and observed the effects of static compression on the cellular behavior of the cementocytes. Cell Counting Kit-8 assay, alkaline phosphatase staining and dentin matrix protein 1 quantification were used to evaluate the cementocyte differentiation in the 3D scaffolds. Cellular viability under static compression was evaluated using live/dead staining, and expression of mineral metabolism-related genes were analyzed via reverse transcription-quantitative PCR. The results suggested that the cementocytes maintained their phenotype and increased the expression of osteoprotegerin (OPG), receptor activator of NF-κB ligand (RANKL) and sclerostin (SOST) in the 3D model compared with cells cultured in two dimensions. Compression force increased cell death and induced osteoclastic differentiation via the upregulation of SOST and RANKL/OPG ratio, and the downregulation of osteocalcin. The effect of compression showed a force magnitude-dependent pattern. The present study established an in vitro model of cellular cementum to study the biology of cementocytes. The results indicated that cementocytes are sensitive to mechanical loading and may serve potential roles in the metabolic regulation of minerals during orthodontic root resorption. These findings provide a novel tool to study biological processes in the field of orthodontics and expand knowledge of the biological function of cementocytes.

Tackling the Concept of Symbiotic Implantable Medical Devices with Nanobiotechnologies

This review takes an approach to implanted medical devices that considers whether the intention of the implanted device is to have any communication of energy or materials with the body. The first part describes some specific examples of three different classes of implants, analyzed with regards to the type of signal sent to cells. Through several examples, the authors describe that a one way signaling to the body leads to encapsulation or degradation. In most cases, those phenomena do not lead to major problems. However, encapsulation or degradation are critical for new kinds of medical devices capable of duplex communication, which are defined in this review as symbiotic devices. The concept the authors propose is that implanted medical devices that need to be symbiotic with the body also need to be designed with an intended duplex communication of energy and materials with the body. This extends the definition of a biocompatible system to one that requires stable exchange of materials between the implanted device and the body. Having this novel concept in mind will guide research in a new field between medical implant and regenerative medicine to create actual symbiotic devices.

RANKL deletion in periodontal ligament and bone lining cells blocks orthodontic tooth movement

The bone remodeling process in response to orthodontic forces requires the activity of osteoclasts to allow teeth to move in the direction of the force applied. Receptor activator of nuclear factor-κB ligand (RANKL) is essential for this process although its cellular source in response to orthodontic forces has not been determined. Orthodontic tooth movement is considered to be an aseptic inflammatory process that is stimulated by leukocytes including T and B lymphocytes which are presumed to stimulate bone resorption. We determined whether periodontal ligament and bone lining cells were an essential source of RANKL by tamoxifen induced deletion of RANKL in which Cre recombinase was driven by a 3.2 kb reporter element of the Col1α1 gene in experimental mice (Col1α1.CreERTM+.RANKLf/f) and compared results with littermate controls (Col1α1.CreERTM-.RANKLf/f). By examination of Col1α1.CreERTM+.ROSA26 reporter mice we showed tissue specificity of tamoxifen induced Cre recombinase predominantly in the periodontal ligament and bone lining cells. Surprisingly we found that most of the orthodontic tooth movement and formation of osteoclasts was blocked in the experimental mice, which also had a reduced periodontal ligament space. Thus, we demonstrate for the first time that RANKL produced by periodontal ligament and bone lining cells provide the major driving force for tooth movement and osteoclastogenesis in response to orthodontic forces.

Osteocyte regulation of orthodontic force-mediated tooth movement via RANKL expression

Orthodontic tooth movement is achieved by the remodeling of the alveolar bone surrounding roots of teeth. Upon the application of orthodontic force, osteoclastic bone resorption occurs on the compression side of alveolar bone, towards which the teeth are driven. However, the molecular basis for the regulatory mechanisms underlying alveolar bone remodeling has not been sufficiently elucidated. Osteoclastogenesis is regulated by receptor activator of nuclear factor-κB ligand (RANKL), which is postulated to be expressed by the cells surrounding the tooth roots. Here, we show that osteocytes are the critical source of RANKL in alveolar bone remodeling during orthodontic tooth movement. Using a newly established method for the isolation of periodontal tissue component cells from alveolar bone, we found that osteocytes expressed a much higher amount of RANKL than other cells did in periodontal tissue. The critical role of osteocyte-derived RANKL was confirmed by the reduction of orthodontic tooth movement in mice specifically lacking RANKL in osteocytes. Thus, we provide in vivo evidence for the key role of osteocyte-derived RANKL in alveolar bone remodeling, establishing a molecular basis for orthodontic force-mediated bone resorption.