Morphological and molecular changes in denture-supporting tissues under persistent mechanical stress in rats

Effects of wearing removable dentures and aging on palatal mucosa blood flow by laser doppler

Aim

The purpose of this work is to analyze the effects of removable dental prostheses and aging on blood microcirculation in the palatal mucosa.

Settings and Design

Blood flow was measured in two groups using the Laser Doppler Flowmeter at three specific anatomical sites: Retro incisive papilla, medial raphe, and Schroeder area.

Materials and Methods

Group 1 included young, healthy dentulous individuals (mean age: 23 ± 3 years), and Group 2 contained elderly edentulous individuals (mean age: 62 ± 11.69 years). For Group 1, measurements were taken in a single session; for Group 2, the measurements were taken in two sessions: The first just before the prosthetic load (E1) and again 1 week after new dentures were provider (E2).

Statistical Analysis Used

Statistical analyses were performed using SAS software, Version 9.4 of the SAS System for Windows, Copyright © 2017 SAS Institute Inc. (Cary, NC, USA). A P < 0.05 was classified as statistically significant.

Results

Measurements of blood flow of the palatal mucosa showed that the healthy young dentulous participants had significantly lower perfusion unit values than the elderly edentulous participants at all three anatomical sites (P < 0.05). For Group 2, the comparisons between the measurements taken before (E1) and after (E2) new dentures were provided showed no significant differences.

Conclusion

Our results indicate that the process of aging significantly modifies the blood flow of the palatal mucosa while wearing removable dental prostheses does not modify the blood flow of the palatal mucosa in a 1week period. These results are not influenced by systemic pathology (e.g., diabetes, cardiovascular diseases) or smoking.

Histocompatibility of Novel Cycloaliphatic Comonomer in Heat-cured Denture Base Acrylic Resin: Histomorphometric Analysis in Rats

Background:

Prosthodontics is impossible without denture base resins. Allergic reactions to these resins are not uncommon, albeit favorable properties. Monomeric modifications are being done to improve the properties of the material. Tricyclodecane dimethanol diacrylate (TCDDMDA) monomer has been recently identified and experimented as a comonomer with methyl methacrylate (MMA).

Aim:

This study aimed to investigate the histocompatibility of TCDDMDA comonomer in polymerized resin at 10% and 20% (vol/vol) concentrations in rats by histomorphometric analysis.

Materials and Methods:

Twenty-four male Wistar rats were randomly divided into the following four groups: NP group (control; n = 6), with no palatal appliance, Groups P0, P10, and P20 were fixed with palatal appliances fabricated of 100% MMA, 10% TCDDMDA + 90% MMA, and 20% TCDDMDA + 80% MMA, respectively. Weights of the animals were recorded just before the appliance placement and after 14 days. The animals were sacrificed, and the palatal tissues were processed for histopathological analysis. Histomorphometric parameters assessed were total epithelial (ET), connective tissue (CT), and keratin layer (KT) thicknesses.

Results:

No significant difference was observed regarding body weight. Group P0 showed increased ET, CT, and KT when compared to other groups. Bonferroni multiple comparison tests showed a statistically significant difference between all the groups except between P10 and P20 for all the three morphometric parameters.

Conclusion:

Palatal appliances with TCDDMDA comonomer showed good histocompatibility in rats up to 20% (vol/vol) concentration.

Effects of mechanical stress on human oral mucosa-derived cells

OBJECTIVES

Placement of a denture results in the application of mechanical stress (MS), such as occlusal force, onto the oral mucosa beneath the denture. To better understand the molecular mechanism underlying MS-induced inflammation in the oral mucosa, we examined the impact of MS on human oral epithelial cells (HO-1-N-1) and human fibroblasts (HGFs) in this study.

MATERIALS AND METHODS

MS was applied on HO-1-N-1 and HGFs using a hydrostatic pressure apparatus. The expression and production of inflammatory cytokines and growth factors were examined by real-time RT-PCR and ELISA. MS-induced intracellular signal transduction via MAP kinase (MAPK) was also examined.

RESULTS

1 MPa MS resulted in a significant increase in inflammatory cytokines, and 3 MPa MS resulted in a significant increase in FGF-2. MS also increased p-38 phosphorylation and the addition of a p-38 inhibitor significantly suppressed the production of inflammatory cytokines.

DISCUSSION

Our study suggested that MS applied through a denture increases the production of inflammatory cytokines from oral mucosal epithelial cells and fibroblasts via the p38 MAPK cascade. These responses to MS likely lead to inflammation of the mucosal tissue beneath dentures. On other hand, up-regulation of growth factors is likely a manifestation of the biological defense mechanism against excessive MS.

Immunohistochemical study for relationship between vessel and lymphatic properties and tooth marks in human oral mucosa

Angiogenesis is an important issue related to normal growth and differentiation, and it is a critical issue in the progression of human disease in oral mucosa. Tooth marks occur after clenching the teeth for a long period under muscle tension in the human oral cavity. However, the sites of angiogenesis, cell differentiation and microvessel density are not known for human mucosa with tooth marks. Therefore, we investigated the relationship between the markers of differentiation (Ki-67), angiogenesis (CD31, D2-40, VEGF-A), and marks from teeth in the second molar region using immunohistochemical methods. In addition, we compared these areas with the mucous membrane. Our results revealed blood and lymphoid vessels in irregular mucosa structures, and the vessels in the oral mucosa were observed in three types of samples: dentulous, denture attachment (containing partial teeth), and edentulous samples. The localization of the angiogenesis was related to the structure of the oral mucosa of connective tissue in humans, such as the mucosal fold-like of the buccal region. Using principal component analysis (PCA), we found that tooth occlusal condition, gender, anti-VEGF-A reaction levels in oral mucosa of the epithelium were positive factors in all groups, which is in contrast to the negative association of Ki-67 reaction in the epithelium and CD31 expression. In addition, Ki-67 reaction in oral mucosa had negative impacts, in contrast to the positive association of D2-40. These PCA properties provide useful information for future study of tumour progression or mechanical stress in remodelling of oral mucosa and angiogenesis. Moreover, mechanical stress of the occlusal condition may be correlated with tumour angiogenic activity and cell differentiation in human oral mucosa.

Biomechanics of oral mucosa

The prevalence of prosthodontic treatment has been well recognized, and the need is continuously increasing with the ageing population. While the oral mucosa plays a critical role in the treatment outcome, the associated biomechanics is not yet fully understood. Using the literature available, this paper provides a critical review on four aspects of mucosal biomechanics, including static, dynamic, volumetric and interactive responses, which are interpreted by its elasticity, viscosity/permeability, apparent Poisson's ratio and friction coefficient, respectively. Both empirical studies and numerical models are analysed and compared to gain anatomical and physiological insights. Furthermore, the clinical applications of such biomechanical knowledge on the mucosa are explored to address some critical concerns, including stimuli for tissue remodelling (interstitial hydrostatic pressure), pressure–pain thresholds, tissue displaceability and residual bone resorption. Through this review, the state of the art in mucosal biomechanics and their clinical implications are discussed for future research interests, including clinical applications, computational modelling, design optimization and prosthetic fabrication.

HSP70 mRNA expression by cells of the epithelial rest of Malassez due to mechanical forces in vitro

Background

The purpose of the present study was to examine the in vitro responses of ERM cells under the combination of centrifugal and compression forces, in terms of their expression of HSP70 mRNA.

Methods

The ERM cells were positive for CK19 indicating that they were derived from the odontogenic epithelium. Cultured ERM cells were applied centrifugal force and compressing force at one to three times as mechanical forces. After addition of forces, cells were observed using scanning electron microscope (SEM) and were measured expression of HSP70 mRNA by RT-PCR.

Results

SEM observations showed the cells were flattened immediately after the application of mechanical force, but nuclear protrusions recovered the same as the control 3 h later. A significantly higher expression of HSP70 mRNA was observed in ERM cells under mechanical force compared with the control, but it gradually decreased with time. No accumulation of HSP70 mRNA expression occurred with intermittent force. However, the expression of HSP70 mRNA with intermittent force repeated 3 times was significantly higher compared with intermittent force applied only once or twice.

Conclusions

These findings suggest that ERM cells express HSP70 mRNA in response to mechanical force, and that intermittent force maintains the level of HSP70 mRNA expression.

Description of a Rat Palatal Acrylic Plate That Can Be Relined

PURPOSE

The objective of this article is to describe a method to construct an intraoral acrylic device that permits a reline material to be added to the inner surface of the palatal plate.

MATERIALS AND METHODS

Fifteen 60-day-old adult female rats (Rattus Norvegicus Albinus Wistar), weighing 150 to 250 g were used for this study and allocated to three groups (n = 5): G1, animals wearing a heat-polymerized acrylic resin palatal plate (Lucitone 550) for 14 days; G2, animals wearing a heat-polymerized acrylic resin palatal plate (Lucitone 550) relined with Tokuyama Rebase II for 14 days; and G3, animals maintained under the same conditions as the experimental groups, without wearing palatal plates for 14 days. The manipulation of the animals followed the guidelines of the Brazilian College of Animal Experimentation, under the approval of the animal ethics committee of the State University of Ponta Grossa. The palatal plates covered the whole palate, were fixed in the molar region with light-cured resin, and were kept there for 14 days. The animals received a paste diet and water ad libitum. Before and after the trial period, the rats were weighed individually on a precision scale. Statistical analysis was performed using a two-way analysis of variance (α = 0.05) test for comparison of the animals' weight (g) at time 0 and after 14 days of using the palatal plate.

RESULTS

No statistical differences were observed regarding the weight of the animals among the experimental groups in the study.

CONCLUSIONS

The individual master impressions, the molar teeth coverage, and the method of cementation with nonadhesive composite resin provided good stability for the palatal plate showed in this study, not disturbing the eating habits and nutrition of the animals. This model seems reproducible, offering adequate histopathological evaluation. Differences in tissue morphology exist between the animals that used the palatal plate and the animals that did not use this device. Use of these palatal plates could clarify how prostheses bring changes in the palatal mucosa of users.

Histopathological changes by the use of soft reline materials: a rat model study

Aim

To assess the histopathological changes of rat palatal mucosa exposed to soft reline materials.

Methods

Forty-five adult female Wistar rats with controlled living conditions and fed ad libitum were employed. Palatal appliances of heat-polymerized acrylic resin Lucitone 550 were manufactured and worn by forty animals during 14 days. Five animals did not use the appliances (G1) and were used to control the appliance influence. One experimental group (n = 10) used the appliances without any relining material (G2) to control the material effect. Three experimental groups (n = 10) received the following soft reline materials below appliances: Dentusoft (G3), Dentuflex (G4), and Trusoft (G5). Appliances from half of each experimental group(n = 5) was immersed in water bath at 55°C for 10 min before use. Animals were slaughtered and the palates were fixed in 10% buffered formalin. Hematoxylin and eosin stained sections of 5 µm were analyzed by computerized planimetry. Cellular compartment, keratin and total epithelial thickness, and basement membrane length were measured to histopathological description.Analysis of variance and Tukey post-hoc test were used to data examination(α = 0.05).

Results

For heat-treatment groups, G4 showed less elongated ridges at the basal layer interface (p = 0.037) than G2. When comparing the conditions with and without heat-treatment, only G2 showed a significant decrease in the cellular compartment, keratin layer and total epithelium thickness (p<0.05).

Conclusion

The post-polymerization for Lucitone 550 was an effective method to reduce the changes in the rat palatal mucosa. The soft reline materials tested did not cause significant histopathological changes in the rat palatal mucosa.

Cell proliferation and apoptosis in the fusion of human primary and secondary palates

The markers of cell proliferation (Ki-67) and apoptosis [caspase-3, TdT-mediated biotin-dUTP nick-end labelling (TUNEL)] and the expression of syndecan-1 and heat shock protein 70 (Hsp70) were analyzed immunohistochemically in 11 developing human palates, from developmental weeks 6 to 10. During fusion of the primary palate, the proportion of proliferating cells decreased from 42 to 32% and the proportion of apoptotic cells decreased from 11 to 7% in the medial-edge epithelium. At later stages, the proportions of both types of cells decreased in the ectomesenchyme, except for proliferating cells in its non-condensing part. At developmental weeks 9-10, the epithelial seam in the secondary palate comprised 28% proliferative cells and 5% apoptotic cells. While condensing ectomesenchyme contained more apoptotic cells than proliferating cells, the opposite was observed for the non-condensing ectomesenchyme. Co-expression of syndecan-1 and Hsp70 was detected in cells budding from the epithelial seam. Our study indicates similar principles for human primary palate and secondary palate fusion, and parallel persistence of proliferation and apoptotic activity. While proliferation enables growth and fusion of different palatal primordia, apoptosis results in the removal of of large numbers epithelial cells at the fusion point. The disintegration of seam remnants seems to be executed through the processes of change in protein content and cell migration, probably leading to cell death as their final outcome.

Response of a preosteoblastic cell line to cyclic tensile stress conditioning and growth factors for bone tissue engineering

Bone regeneration can be accelerated by utilizing mechanical stress and growth factors (GFs). However, a limited understanding exists regarding the response of preosteoblasts to tensile stress alone or with GFs. We measured cell proliferation and expression of heat-shock proteins (HSPs) and other bone-related proteins by preosteoblasts following cyclic tensile stress (1%-10% magnitude) alone or in combination with bone morphogenetic protein-2 (BMP-2) and transforming growth factor-β1 (TGF-β1). Tensile stress (3%) with GFs induced greater gene upregulation of osteoprotegerin (3.3 relative fold induction [RFI] compared to sham-treated samples), prostaglandin E synthase 2 (2.1 RFI), and vascular endothelial growth factor (VEGF) (11.5 RFI), compared with samples treated with stimuli alone or sham-treated samples. The most significant increases in messenger RNA expression occurred with GF addition to either static-cultured or tensile-loaded (1% elongation) cells for the following genes: HSP47 (RFI=2.53), cyclooxygenase-2 (RFI=72.52), bone sialoprotein (RFI=11.56), and TGF-β1 (RFI=8.05). Following 5% strain with GFs, VEGF secretion increased 64% (days 3-6) compared with GF alone and cell proliferation increased 23% compared with the sham-treated group. GF addition increased osteocalcin secretion but decreased matrix metalloproteinase-9 significantly (days 3-6). Tensile stress and GFs in combination may enhance bone regeneration by initiating angiogenic and anti-osteoclastic effects and promote cell growth.

Fabrication of a multi-applicable removable intraoral denture system for rodent research

The objective was to engineer an inexpensive intraoral removable denture system for rodents that can be utilised in numerous oral health research applications. At the forefront is biofilm research related to Candida-associated denture stomatitis. Previously described intraoral devices are primitive and inadequate. The denture system was engineered consisting of a fixed part that is anchored to the posterior palate by orthodontic wires and acrylic resin and a removable part fitted to the anterior palate that is retained by magnets embedded in the fixed part. Both parts are custom fitted to the rodent palate by impression making and cast fabrication. Rats fitted with the intraoral denture system maintained body weight and normal activity with the device maintaining integrity and durability for upwards of 8 weeks. The denture system was used successfully to establish a working model of denture stomatitis. This newly engineered inexpensive intraoral removable denture system for rodents can be utilised in numerous oral health research applications, including denture-associated infections, biofilms and a variety of biomaterial applications. The removable portion is advantageous for longitudinal analyses and charging/discharging of biomaterials.

Effect of stretching force on the cells of epithelial rests of malassez in vitro

Background and Objective. The aim of this study was to investigate the behavior of cells from epithelial rest of Malassez (ERM) against stretching force. Material and Methods. ERM-cultured cells were stretched for 1 hour, at the cycle of 18% elongation for 1 second followed by 1-second relaxation. The cells without addition of stretching force were used as controls. The cells were observed by immunohistochmical staining using actin 0, 12, 24, 36, 48, and 72 hours. Furthermore, expressions of HSP70-, VEGF-, and OPN-mRNAs of cells were also evaluated using quantitative RT-PCR. Results. Actin filaments were randomly orientated in the cytoplasm in the control group, whereas in the stretching group, actin filaments were orientated comparatively parallel to the stretching direction. Expression of HSP70-mRNA in the stretching group was significantly higher than that of control group at 12, 24, 36 hours (P < .05). Expression of VEGF-mRNA in the stretching group was significantly higher than that of control group at 24, 36, 48, and 72 hours (P < .05). Expression of OPN-mRNA in the stretching group was significantly higher than that of control group at 12 and 24 hours (P < .05). Conclusion. ERM cells response against the stretching force by expressing HSP70, VEGF, and OPN.