Apoptosis in developmental and repair-related human tooth remodeling: a view from the inside

3D relationship between hierarchical canal network and gradient mineralization of shark tooth osteodentin

Osteodentin is a dominant mineralized collagenous tissue in the teeth of many fishes, with structural and histological characteristics resembling those of bone. Osteodentin, like bone, comprises osteons as basic structural building blocks, however, it lacks the osteocytes and the lacuno-canalicular network (LCN), which are known to play critical roles in controlling the mineralization of the collagenous matrix in bone. Although numerous vascular canals exist in osteodentin, their role in tooth maturation and the matrix mineralization process remain poorly understood. Here, high resolution micro-computed tomography (micro-CT) and focused ion beam-scanning electron microscopy (FIB-SEM) were used to obtain 3D structural information of osteodentin in shark teeth at multiple scales. We observed a complex 3D network of primary canals with a diameter ranging from ∼10 µm to ∼120 µm, where the canals are surrounded by osteon-like concentric layers of lamellae, with 'interosteonal' tissue intervening between neighboring osteons. In addition, numerous hierarchically branched secondary canals extended radially from the primary canals into the interosteonal tissue, decreasing in diameter from ∼10 µm to hundreds of nanometers. Interestingly, the mineralization degree increases from the periphery of primary canals into the interosteonal tissue, suggesting that mineralization begins in the interosteonal tissue. Correspondingly, the hardness and elastic modulus of the interosteonal tissue are higher than those of the osteonal tissue. These results demonstrate that the 3D hierarchical canal network is positioned to play a critical role in controlling the gradient mineralization of osteodentin, also providing valuable insight into the formation of mineralized collagenous tissue without osteocytes and LCN. STATEMENT OF SIGNIFICANCE: Bone is a composite material with versatile mechanical properties. Osteocytes and their lacuno-canalicular network (LCN) are known to play critical roles during formation of human bone. However, the bone and osteodentin of many fishes, although lacking osteocytes and LCN, exhibit similar osteon-like structure and mechanical functions. Here, using various high resolution 3D characterization techniques, we reveal that the 3D network of primary canals and numerous hierarchically branched secondary canals correlate with the mineralization gradient and micromechanical properties of osteonal and interosteonal tissues of shark tooth osteodentin. This work significantly improves our understanding of the construction of bone-like mineralized tissue without osteocytes and LCN, and provides inspirations for the fabrication of functional materials with hierarchical structure.

Comparison of Biological Properties and Clinical Application of Mesenchymal Stem Cells from the Mesoderm and Ectoderm

Since the discovery of mesenchymal stem cells (MSCs) in the 1970s, they have been widely used in the treatment of a variety of diseases because of their wide sources, strong differentiation potential, rapid expansion in vitro, low immunogenicity, and so on. At present, most of the related research is on mesoderm-derived MSCs (M-MSCs) such as bone marrow MSCs and adipose-derived MSCs. As a type of MSC, ectoderm-derived MSCs (E-MSCs) have a stronger potential for self-renewal, multidirectional differentiation, and immunomodulation and have more advantages than M-MSCs in some specific conditions. This paper analyzes the relevant research development of E-MSCs compared with that of M-MSCs; summarizes the extraction, discrimination and culture, biological characteristics, and clinical application of E-MSCs; and discusses the application prospects of E-MSCs. This summary provides a theoretical basis for the better application of MSCs from both ectoderm and mesoderm in the future.

Leukotriene B4 loaded in microspheres regulate the expression of genes related to odontoblastic differentiation and biomineralization by dental pulp stem cells

Background

Leukotriene B₄ (LTB₄) is a potent lipid mediator that stimulate the immune response. Because dental pulp inflammation and dentin repair are intrinsically related responses, the aim of this research was to investigate the potential of LTB₄ in inducing differentiation of dental pulp stem cells.

Methods

Microspheres (MS) loaded with LTB₄ were prepared using an oil emulsion solvent extraction evaporation process and sterility, characterization, efficiency of LTB₄ encapsulation and in vitro LTB₄ release assay were investigated. Mouse dental pulp stem cells (OD-21) were stimulated with soluble LTB₄ or MS loaded with LTB₄ (0.01 and 0.1 μM). Cytotoxicity and cell viability was determined by lactate dehydrogenase and methylthiazol tetrazolium assays. Gene expression were measured by quantitative reverse transcription polymerase chain reaction after 3, 6, 24, 48 and 72 h. Mineralized nodule formation was assessed after 28 days of OD-21 cell stimulation with LTB₄ in mineralized media or not. Groups were compared using one-way ANOVA test followed by Dunnett’s post-test (α = 0.05).

Results

Treatment with LTB₄ or MS loaded with LTB₄ (0.01 and 0.1 µm-μM) were not cytotoxic to OD-21 cells. Treatment with LTB₄ modulated the expression of the Ibsp (integrin binding sialoprotein) and Runx2 (runt-related transcription factor 2) genes differently depending on the experimental period analyzed. Interestingly LTB₄ loaded in microspheres (0.1 μM) allowed long term dental pulp cell differentiation and biomineralization.

Conclusion

LTB₄, soluble or loaded in MS, were not cytotoxic and modulated the expression of the Ibsp and Runx2 genes in cultured OD-21 cells. When LTB₄ was incorporated into MS, odontoblast differentiation and mineralization was induced in long term culture.

The Versatile Roles of Nerve Growth Factor in Neuronal Attraction, Odontoblast Differentiation, and Mineral Deposition in Human Teeth

Nerve growth factor (NGF) is an important molecule for the development and differentiation of neuronal and non-neuronal cells. Here we analyze by immunohistochemistry the distribution of NGF in the dental pulp mesenchyme of embryonic and functional human teeth. In the dental pulp of both embryonic and healthy functional teeth, NGF is mainly expressed in the odontoblasts that are responsible for dentine formation, while in functional teeth NGF is also expressed in nerve fibers innervating the dental pulp. In injured teeth, NGF is expressed in the newly formed odontoblastic-like cells, which replace the dying odontoblasts. In these teeth, NGF expression is also upregulated in the intact odontoblasts, suggesting a role for this molecule in dental tissue repair. Similarly, in cultures of human dental pulp cells, NGF expression is strongly upregulated during their differentiation into odontoblasts as well as during the mineralization process. In microfluidic devices, release of NGF from cultured human dental pulp cells induced neuronal growth from trigeminal ganglia toward the NGF secreting cells. These results show that NGF is closely linked to the various functions of odontoblasts, including secretory and neuronal attraction processes.

Influence of Hydrogen Peroxide on Mineralization in Dental Pulp Cells: A Systematic Review

Background: Dental bleaching agents show the ability to permeate through dental hard tissues, which may lead to pulp tissue changes. This systematic review (PROSPERO register: CRD42020213767) is aimed at understanding the effects of bleaching agents on the process of mineralization of the pulp tissue.

Methods: Only in vitro studies evaluating the influence of hydrogen peroxide (HP) on mineralization in dental pulp cells were included. Studies without a non-bleached control group or cells after co-treatment with a bleaching agent other than HP and/or carbamide peroxide were excluded. The primary outcomes evaluated were alkaline phosphatase (ALP) activity and mineralized nodule deposition. The mineralization markers analysis in dental pulp cells and the cell viability were considered secondary outcomes. Two independent authors conducted a systematic search (PubMed/MEDLINE, Scopus, Embase, Cochrane Library, and OpenGrey until January 2021) with no language restrictions and performed data extraction. The quality assessment was appraised according to a modified Joanna Briggs Institute critical appraisal checklist.

Results: The search resulted in 473 studies, and 11 were considered eligible. Overall, a reduction in the process of mineralization was observed among pulp cells after bleaching. A reduction in the ALP activity was reported in the mostly bleached groups using different protocols and analysis periods of nine studies. Regarding mineralized nodule deposition, 6 studies reported a significant reduction from 7 to 21 days among bleached groups. Of those three studies that investigated other mineralization markers, two found a reduction in the expression of dentin matrix acidic phosphoprotein (DMP)-1, dentin sialophosphoprotein (DSPP), and matrix extracellular phosphoglycoprotein (MEPE) among some bleaching gel concentrations. In contrast, one study showed a greater expression of osteopontin (OPN) and osteocalcin (OCN) in 100 μmol/L HP after 5 or 10 min of exposure, and another study showed significant induction of DSPP in concentrations of up to 0.5 mmol/L HP.

Conclusion: Especially, high concentrations of bleaching gel reduce the potential of mineralization in pulp cells in in vitro studies; however, different HP concentrations, bleaching protocols, and analysis periods can influence this outcome.

Notch signaling in the dynamics of perivascular stem cells and their niches

The Notch signaling pathway is a fundamental regulator of cell fate determination in homeostasis and regeneration. In this work, we aimed to determine how Notch signaling mediates the interactions between perivascular stem cells and their niches in human dental mesenchymal tissues, both in homeostatic and regenerative conditions. By single cell RNA sequencing analysis, we showed that perivascular cells across the dental pulp and periodontal human tissues all express NOTCH3, and that these cells are important for the response to traumatic injuries in vivo in a transgenic mouse model. We further showed that the behavior of perivascular NOTCH3‐expressing stem cells could be modulated by cellular and molecular cues deriving from their microenvironments. Taken together, the present studies, reinforced by single‐cell analysis, reveal the pivotal importance of Notch signaling in the crosstalk between perivascular stem cells and their niches in tissue homeostasis and regeneration.

Exploiting teeth as a model to study basic features of signaling pathways

Teeth constitute a classical model for the study of signaling pathways and their roles in mediating interactions between cells and tissues in organ development, homeostasis and regeneration. Rodent teeth are mostly used as experimental models. Rodent molars have proved fundamental in the study of epithelial–mesenchymal interactions and embryonic organ morphogenesis, as well as to faithfully model human diseases affecting dental tissues. The continuously growing rodent incisor is an excellent tool for the investigation of the mechanisms regulating stem cells dynamics in homeostasis and regeneration. In this review, we discuss the use of teeth as a model to investigate signaling pathways, providing an overview of the many unique experimental approaches offered by this organ. We discuss how complex networks of signaling pathways modulate the various aspects of tooth biology, and the models used to obtain this knowledge. Finally, we introduce new experimental approaches that allow the study of more complex interactions, such as the crosstalk between dental tissues, innervation and vascularization.

Dentin-Pulp Tissue Regeneration Approaches in Dentistry: An Overview and Current Trends

Conventional treatment approaches in irreversible pulpitis and apical periodontitis include the disinfection of the pulp space followed by filling with various materials, which is commonly known as the root canal treatment. Disadvantages including the loss of tooth vitality and defense mechanism against carious lesions, susceptibility to fractures, discoloration and microleakage led to the development of regenerative therapies for the dentin pulp-complex. The goal of dentin-pulp tissue regeneration is to reestablish the physiological pulp function such as pulp sensibility, pulp repair capability by mineralization and pulp immunity. Recent dentin-pulp tissue regeneration approaches can be divided into cell homing and cell transplantation. Cell based approaches include a suitable scaffold for the delivery of potent stem cells with or without bioactive molecules into the root canal system while cell homing is based on the recruitment of host endogenous stem cells from the resident tissue including periapical region or dental pulp. This review discusses the recent treatment modalities in dentin-pulp tissue regeneration through tissue engineering and current challenges and trends in this field of research.

Multifactorial Contribution of Notch Signaling in Head and Neck Squamous Cell Carcinoma

Head and neck squamous cell carcinoma (HNSCC) defines a group of solid tumors originating from the mucosa of the upper aerodigestive tract, pharynx, larynx, mouth, and nasal cavity. It has a metastatic evolution and poor prognosis and is the sixth most common cancer in the world, with 600,000 new cases reported every year. HNSCC heterogeneity and complexity is reflected in a multistep progression, involving crosstalk between several molecular pathways. The Notch pathway is associated with major events supporting cancerogenic evolution: cell proliferation, self-renewal, angiogenesis, and preservation of a pro-oncogenic microenvironment. Additionally, Notch is pivotal in tumor development and plays a dual role acting as both oncogene and tumor suppressor. In this review, we summarize the role of the Notch pathway in HNSCC, with a special focus on its compelling role in major events of tumor initiation and growth.

C5L2 Regulates DMP1 Expression during Odontoblastic Differentiation

The presence of stem cells within the dental-pulp tissue as well as their differentiation into a new generation of functional odontoblast-like cells constitutes an important step of the dentin-pulp regeneration. Recent investigations demonstrated that the complement system activation participates in 2 critical steps of dentin-pulp regeneration: pulp progenitor's recruitment and pulp nerve sprouting. Surprisingly, its implication in odontoblastic differentiation has not been addressed yet. Since the complement receptor C5a receptor-like 2 (C5L2) is expressed by different stem cells, the aim of this study is to investigate if the dental pulp stem cells express C5L2 and if this receptor participates in odontoblastic differentiation. Immunohistochemistry performed on human third molar pulp sections showed a perivascular co-localization of the mesenchymal stem cell markers STRO1 and C5L2. In vitro immunofluorescent staining confirmed that hDPSCs express C5L2. Furthermore, we determined by real-time polymerase chain reaction that the expression of C5L2 is highly modulated in human dental pulp stem cells (hDPSCs) undergoing odontoblastic differentiation. Moreover, we showed that this odontogenesis-regulated expression of C5L2 is specifically potentiated by the proinflammatory cytokine TNFα. Using a C5L2-siRNA silencing strategy, we provide direct evidence that C5L2 constitutes a negative regulator of the dentinogenic marker DMP1 (dentin matrix protein 1) expression by hDPSCs. Our findings suggest a direct correlation between the odontoblastic differentiation and the level of C5L2 expression in hDPSCs and identify C5L2 as a negative regulator of DMP1 expression by hDPSCs during the odontoblastic differentiation and inflammation processes. This work is the first to demonstrate the involvement of C5L2 in the biological function of stem cells, provides an important knowledge in understanding odontoblastic differentiation of dental pulp stem cells, and may be useful in future dentin-pulp engineering strategies.

Monitoring Notch Signaling-Associated Activation of Stem Cell Niches within Injured Dental Pulp

Dental pulp stem/progenitor cells guarantee tooth homeostasis, repair and regeneration throughout life. The decision between renewal and differentiation of these cells is influenced by physical and molecular interactions with stromal cells and extracellular matrix molecules forming the specialized microenvironment of dental pulp stem cell niches. Here we study the activation of putative pulp niches after tooth injury through the upregulation of Notch signaling pathway. Notch1, Notch2, and Notch3 molecules were used as markers of dental pulp stem/progenitor cells. Upon dental injury, Notch1 and Notch3 are detected in cells related to vascular structures suggesting a role of these proteins in the activation of specific pulpal perivascular niches. In contrast, a population of Notch2-positive cells that are actively proliferative is observed in the apical part of the pulp. Kinetics of these cells is followed up with a lipophilic DiI labeling, showing that apical pulp cells migrate toward the injury site where dynamic regenerative/repair events occur. The knowledge of the activation and regulation of dental pulp stem/progenitor cells within their niches in pathologic conditions may be helpful for the realization of innovative dental treatments in the near future.

Nerve growth factor signalling in pathology and regeneration of human teeth

Nerve growth factor (NGF) is a key regulator of the development and differentiation of neuronal and non-neuronal cells. In the present study we examined the distribution of NGF and its low and high-affinity receptors, p75^NTR and TrkA respectively, in permanent human teeth under normal and pathological conditions. In intact functional teeth, NGF, p75^NTR and TrkA are weakly expressed in dental pulp fibroblasts and odontoblasts that are responsible for dentine formation, while the NGF and p75^NTR molecules are strongly expressed in nerve fibres innervating the dental pulp. In carious and injured teeth NGF and TrkA expression is upregulated in a selective manner in odontoblasts surrounding the injury sites, indicating a link between NGF signalling and dental tissue repair events. Accordingly, NGF and TrkA expression is strongly upregulated in cultured primary human dental mesenchymal cells during their differentiation into odontoblasts. Targeted release of NGF in cultured human tooth slices induced extensive axonal growth and migration of Schwann cells towards the NGF administration site. These results show that NGF signalling is strongly linked to pathological and regenerative processes in human teeth and suggest a potential role for this neurotrophic molecule in pulp regeneration.

Odontoblastic Cell Quantification and Apoptosis within Pulp of Deciduous Teeth Versus Pulp of Permanent Teeth

OBJECTIVE

While the odontoblast ability to respond to injury in permanent teeth (PT) is well established, there is a lack of knowledge about deciduous teeth (DT). Aim of this study was to compare the odontoblasts activity within the pulp of DT versus the pulp of PT.

STUDY DESIGN

Dental pulp was obtained from forty-two DT and twenty-seven PT extracted from sixty-five patients (aged 6-16 years). Histomorphometry was carried out and the quantification of odontoblastic layer was assessed. Dental pulps of DT and PT were stained for anti-ssDNA, BCL-2, BCL-x, BAX, caspase3.

RESULTS

Pulps from DT were characterized by reduction of odontoblastic layer and greater occurrence of apoptotic odontoblasts. Pro-apoptotic BAX phenotype expression on odontoblasts correlated with the occurrence of numerous activated caspase3 odontoblasts in DT. The number of BAX positive cells was significantly higher compared to BCL-2 positive cells in the odontoblastic layer of the DT (p=0.03). Since BAX and BCL-2 proteins have an inverse role in the regulation of the apoptosis, this finding suggests that odontoblasts have a predominant pro-apoptotic phenotype in DT.

CONCLUSION

According to our results, the odontoblasts of DT can be assumed to have a lower reparative activity if compared to odontoblasts of PT.

C5L2 Receptor Represses Brain-Derived Neurotrophic Factor Secretion in Lipoteichoic Acid-Stimulated Pulp Fibroblasts

The anaphylatoxin C5a constitutes a powerful fragment generated by complement system activation. Interestingly, this complement active fragment is also an important mediator of tissue regeneration. Recent findings suggest that C5a could be an initial signal orchestrating pulp nerve sprouting beneath carious injury, a critical step in dentin-pulp regeneration. Indeed, the expression and activation of the C5a active receptor (C5aR/CD88) by injured pulp fibroblasts controls the direction of neurite outgrowth toward carious injuries by modulating the secretion of brain-derived neurotrophic factor (BDNF) by pulp fibroblasts. A second C5a receptor, C5L2, has also been cloned but has received much less attention because its interaction with the ligand induces no signaling. This work aims to investigate the role of C5L2 in pulp nerve regeneration in the secretion of BDNF by pulp fibroblasts under sites of carious injury. Using fluorescence immunostaining on human tooth sections in vivo and on primary human pulp fibroblasts in vitro, the authors reveal that C5L2 and C5aR are co-expressed by pulp fibroblasts under lipoteichoic acid (LTA) stimulation. Moreover, silencing C5L2 significantly increases BDNF secretion by LTA-stimulated pulp fibroblasts. Finally, an analysis of the subcellular distribution of C5aR and C5L2 indicates that the negative regulation of BDNF secretion by C5L2 correlates with C5aR activation and its subsequent intracellular co-localization with C5L2. Overall, the current study sheds light on the mechanism of pulp nerve regeneration by identifying C5L2 as a negative regulator of BDNF secretion by pulp fibroblasts under carious teeth. This knowledge significantly increases the understanding of the functional mechanism linking C5aR and C5L2 in pulp nerve regeneration, which may be useful in future dentin-pulp engineering strategies that target fibroblast C5L2 to induce pulp innervation.

Derivation and growth characteristics of dental pulp stem cells from patients of different ages

The dental pulp contains a relatively low number of stem cells; however, it is considered to be a promising source of stem cells for use in regenerative therapy. To date, it has remained elusive whether there are certain differences in the dental pulp stem cells (DPSCs) from donors of different ages. In the present study, DPSC lines were derived using teeth from children, adolescents, adults and aged donors. The derivation efficiency, the proliferative and apoptotic rate, cell marker expression and the differentiation capacity were investigated and compared among these DPSC lines. The derivation efficacy was decreased with increasing donor age. Although a large part of cell surface markers was expressed in all DPSC lines, the expression of CD29 was downregulated in the DPSCs from aged teeth. In addition, the doubling time of DPSCs from aged teeth was prolonged and the number of apoptotic cells was increased with the propagation. These DPSCs were able to differentiate into a neuronal linage, which positively expressed the neuron-specific class III beta-tubulin and microtubule‑associated protein 2, as well as into an osteogenic lineage, which positively expressed CD45; however, these DPSCs from aged teeth were completely or partially deprived of differentiation capacity. By contrast, DPSCs from younger teeth displayed significantly higher vitality and a higher potential for use in dental regenerative medicine.

Analysis of expression patterns of IGF-1, caspase-3 and HSP-70 in developing human tooth germs

AIMS

To analyze expression patterns of IGF-1, caspase-3 and HSP-70 in human incisor and canine tooth germs during the late bud, cap and bell stages of odontogenesis.

MATERIALS AND METHODS

Head areas or parts of jaw containing teeth from 10 human fetuses aged between 9th and 20th developmental weeks were immunohistochemically analyzed using IGF-1, active caspase-3 and HSP-70 markers. Semi-quantitative analysis of each marker's expression pattern was also performed.

RESULTS

During the analyzed period, IGF-1 and HSP-70 were mostly expressed in enamel organ. As development progressed, expression of IGF-1 and HSP-70 became more confined to differentiating tissues in the future cusp tip area, as well as in highly proliferating cervical loops. Few apoptotic bodies highly positive to active caspase-3 were observed in enamel organ and dental papilla from the cap stage onward. However, both enamel epithelia moderately expressed active caspase-3 throughout the investigated period.

CONCLUSIONS

Expression patterns of IGF-1, active caspase-3 and HSP-70 imply importance of these factors for early human tooth development. IGF-1 and HSP-70 have versatile functions in control of proliferation, differentiation and anti-apoptotic protection of epithelial parts of human enamel organ. Active caspase-3 is partially involved in formation and apoptotic removal of primary enamel knot, although present findings might reflect its ability to perform other non-death functions such as differentiation of hard dental tissues secreting cells and guidance of ingrowth of proliferating cervical loops.

Apoptosis activation in human carious dentin. An immunohistochemical study

The exact mechanisms and enzymes involved in caries progression are largely unclear. Apoptosis plays a key role in dentin remodelling related to damage repair; however, it is unclear whether apoptosis in decayed teeth is activated through the extrinsic or the intrinsic pathway. This ex vivo immunohistochemical study explored the localization of TRAIL, DR5, Bcl-2 and Bax, the main proteins involved in apoptosis, in teeth with advanced caries. To evaluate TRAIL, DR5, Bcl-2 and Bax immunoexpressions twelve permanent carious premolars were embedded in paraffin and processed for immunohistochemistry. The results showed that TRAIL and DR5 were overexpressed in dentin and in pulp vessels and mononuclear cells; strong Bax immunostaining was detected in dilated dentinal tubules close to the lesion, and Bcl-2 staining was weak in some dentin areas under the cavity or altogether absent. These findings suggest that both apoptosis pathways are activated in dental caries. Further studies are required to gain insights into its biomolecular mechanisms.

Hydrogen peroxide induces apoptosis in human dental pulp cells via caspase-9 dependent pathway

INTRODUCTION

Reactive oxygen species are a group of metabolic intermediates produced during oxidative metabolism in eukaryotic cells. They include superoxide anion (O2(-)), hydrogen peroxide (H2O2), hydroxyl radical (·OH), and (1)O2. Of these intermediates, H2O2 is the most stable. Dental pulp cells can be invaded by tooth bleaching, laser radiation, and dental materials. This can influence the intracellular level of reactive oxygen species. Apoptosis, which is the best-known form of programmed cell death, is pivotal to tissue development and regeneration. Little information is available regarding the relationship between H2O2 and apoptosis of human dental pulp cells (hDPCs). The purpose of this study was to investigate whether H2O2 can induce apoptosis in hDPCs and its signaling way.

METHODS

HDPCs were obtained by using a modified tissue explant technique in vitro and cultured at 37°C, 20% O2 (5% CO2, 95% air) in Dulbecco modified Eagle medium. Cell viability was investigated by methyl-thiazol-tetrazolium assay. Cell apoptosis was detected by using the annexin V-fluorescein isothiocyanate/propidium iodide apoptosis assay and flow cytometry. Expression of activated caspase-3, cleaved caspase-9, and β-actin was analyzed by using Western blot.

RESULTS

Cell viability of hDPCs decreased more in treated groups than in the control group from days 1 to 7. The relative number of apoptotic cells and the expression of activated caspase-3 and cleaved caspase-9 were much higher in groups exposed to 20 and 50 μmol/L H2O2.

CONCLUSIONS

These results imply that low concentrations of H2O2 are cytotoxic to hDPCs and induce apoptosis in hDPCs in a caspase-9-dependent way.

Is hard tissue formation in the dental pulp after the death of the primary odontoblasts a regenerative or a reparative process?

OBJECTIVES

Conceptually, two types of tertiary dentine may be produced in response to caries and environmental irritations: "reactionary dentine" that is secreted by existing primary odontoblasts and "reparative dentine", formed after the death of the odontoblasts by proliferation and differentiation of progenitor cells into odontoblast-like cells. Because histologic evidence for tubular dentine generated by newly differentiated odontoblast-like cells is lacking in human teeth, the present study examined pulpal cellular changes associated with caries/restorations, in the presence or absence of pulpal exposures.

METHODS

Ninety-six extracted human teeth were histologically processed and serial sectioned for light microscopy: 65 contained untreated enamel/dentine caries; 20 were heavily restored and 11 had carious exposures managed by direct pulp-capping.

RESULTS

Sparsely distributed, irregularly arranged dentinal tubules were identified from the tertiary dentine formed in teeth with unexposed medium/deep caries and in restored teeth; those tubules were continuous with the tubules of secondary dentine; in some cases, tubules were absent. The palisade odontoblast layer was reduced to a single layer of flattened cells. In direct pulp-capping of pulp exposures, the defects were repaired by the deposition of an amorphous dystrophic calcified tissue that resembled pulp stones more than dentine, sometimes entrapping pulpal remnants. This atubular hard tissue was lined by fibroblasts and collagen fibrils.

CONCLUSIONS

Histological evidence from the present study indicates that reparative dentinogenesis cannot be considered as a regenerative process since the so-formed hard tissue lacks tubular features characteristic of genuine dentine. Rather, this process represents a repair response that produces calcified scar tissues by pulpal fibroblasts.

CLINICAL SIGNIFICANCE

Formation of hard tissue in the dental pulp after the death of the primary odontoblasts has often been regarded by clinicians as regeneration of dentine. If the objective of the clinical procedures involved is to induce healing, reduce dentine hypersensitivity, or minimise future bacteria exposure, such procedures may be regarded as clinical success. However, current clinical treatment procedures are not adept at regenerating physiological dentne because the tissues formed in the dental pulp are more likely the result of repair responses via the formation of calcified scar tissues.

Pulp fibroblasts synthesize functional complement proteins involved in initiating dentin-pulp regeneration

The complement system is an efficient plasma immune surveillance system that controls tissue injury and infection. Although the liver constitutes the primary circulating complement protein synthesis site, extrahepatic synthesis is known to optimize local tissue inflammatory reaction. Because dentin-pulp regeneration is known to be regulated locally, we investigated activation of the local complement system within the dental pulp and its role in initiating the regeneration process. Membrane attack complex (C5b-9) formation and Gram's staining revealed that complement activation is correlated with the presence of Gram-positive bacteria in carious human teeth. RT-PCR analysis demonstrated that cultured human pulp fibroblasts stimulated with lipoteichoic acid produce all the proteins required for efficient complement activation. This was demonstrated in vitro by C5b-9 formation and C5a active fragment production in the absence of plasma proteins. Finally, the dynamic migration assays performed in μ-Slide chemotaxis chambers and use of a C5aR-specific antagonist (W54011) demonstrated that the activation of complement proteins synthesized by pulp fibroblasts and the subsequent release of C5a specifically induced pulp progenitor cell recruitment. Our study reveals human pulp fibroblasts as the first nonimmune cell type capable of synthesizing all complement proteins. These fibroblasts cells contribute significantly to tissue regeneration by recruiting pulp progenitors via complement activation, which suggests to a potential therapeutic strategy of targeting pulp fibroblasts in dentin-pulp regeneration.

Pulp progenitor cell recruitment is selectively guided by a C5a gradient

It recently became evident that activation of the complement system also contributes to tissue regeneration after infection/injury. The complement-derived fragment C5a induces vascular modifications and attracts cells expressing its receptor (C5aR/CD88) to the site of infection and tissue injury. Besides inflammatory cells, various tissue cells express this receptor. We hypothesized that pulp progenitor cells, being exposed to local complement activation in caries lesions, may respond to C5a via the C5aR. Our work aimed at evaluating the ability of C5a to induce pulp progenitor cell migration that may link complement activation to dentin regeneration. Immunofluorescence analysis of third molar pulp sections showed perivascular localization of the mesenchymal stem cell markers STRO-1 and C5aR. RT-PCR on STRO-1-sorted pulp progenitor cells, co-expressing both STRO-1 and C5aR, revealed high C5aR mRNA levels. Experiments with the C5aR antagonist W54011 revealed that C5a specifically bound to progenitor cells via C5aR, inducing their selective migration toward the C5a gradient. Since we could also demonstrate C5b-9 formation by immunohistochemistry in carious teeth, our findings suggest that, upon local complement activation, C5a induces pulp progenitor cell migration, which may be critical in initiating the regenerative process after dentin/pulp injury.

Sodium fluoride induces apoptosis in odontoblasts via a JNK-dependent mechanism

Sodium fluoride (NaF) is widely used for the treatment of dental caries and dentin hypersensitivity. However, its pro-apoptotic effect on odontoblasts may lead to harmful side-effects. The purpose of this study was to evaluate the pro-apoptotic effects of NaF in odontoblasts and elucidate the possible underlying molecular mechanisms. NaF generated cytotoxic effects in odontoblast-lineage cell (OLC) in a dose- and time-dependent manner. Exposure of cells to 4mM NaF for 24h induced caspase-3 activation, ultrastructural alterations, and resulted in the translocation of Bax to the mitochondria and the release of cytochrome c from the mitochondrial inter-membrane space into the cytosol, indicating that fluoride-mediated apoptosis is mitochondria-dependent. Fluoride treatment also increased phosphorylation of JNK and ERK, but not p38, and apoptosis induced by fluoride was notably or partly suppressed by treatment with JNK or ERK inhibitors, respectively. Taken together, these findings suggest that NaF induces apoptosis in OLC odontoblasts through a JNK-dependent mitochondrial pathway.

Nanodentistry: combining nanostructured materials and stem cells for dental tissue regeneration

Regenerative dentistry represents an attractive multidisciplinary therapeutic approach that complements traditional restorative/surgery techniques and benefits from recent advances in stem cell biology, molecular biology, genomics and proteomics. Materials science is important in such advances to move regenerative dentistry from the laboratory to the clinic. The design of novel nanostructured materials, such as biomimetic matrices and scaffolds for controlling cell fate and differentiation, and nanoparticles for diagnostics, imaging and targeted treatment, is needed. The combination of nanotechnology, which allows the creation of sophisticated materials with exquisite fine structural detail, and stem cell biology turns out to be increasingly useful in regenerative medicine. The administration to patients of dynamic biological agents comprising stem cells, bioactive scaffolds and/or nanoparticles will certainly increase the regenerative impact of dental pathological tissues. This overview briefly describes some of the actual benefits and future possibilities of nanomaterials in the emerging field of stem cell-based regenerative dentistry.

Mitochondrial Autophagy and Lipofuscin Accumulation in Aging Odontoblasts

Aging of long-lived post-mitotic cells is characterized as a progressive and irreversible reduction of functional activity. In such cells, mitochondria are organelles critical for bioenergetic supply, whose turnover is mediated by an autophagic-lysosomal pathway. In human teeth, odontoblasts are post-mitotic cells responsible for sensory function and dentin preservation. Here, human odontoblasts were processed for immunohistochemistry with antibodies against mitochondrial (MTCO2) and lysosomal (LAMP2) markers, and comparatively analyzed in two age groups (young-adult and adult) with light and electron microscopy. Selective engulfment of mitochondrial profiles into autophagic vacuoles is common in young-adult odontoblasts, suggesting a microautophagic pathway. With age, the odontoblast layer is reduced in width, and mitochondrial elements converge around large clusters of autofluorescent lipofuscin deposits. Age-related changes in odontoblasts are observed as a long-term process in which the progressive accumulation of intralysosomal debris limits the autophagic turnover of mitochondrial components, causing an eventual decline in physiological cell functions, which leads to increased vulnerability under stress conditions.

Apoptotic signaling in mouse odontogenesis

Apoptosis is an important morphogenetic event in embryogenesis as well as during postnatal life. In the last 2 decades, apoptosis in tooth development (odontogenesis) has been investigated with gradually increasing focus on the mechanisms and signaling pathways involved. The molecular machinery responsible for apoptosis exhibits a high degree of conservation but also organ and tissue specific patterns. This review aims to discuss recent knowledge about apoptotic signaling networks during odontogenesis, concentrating on the mouse, which is often used as a model organism for human dentistry. Apoptosis accompanies the entire development of the tooth and corresponding remodeling of the surrounding bony tissue. It is most evident in its role in the elimination of signaling centers within developing teeth, removal of vestigal tooth germs, and in odontoblast and ameloblast organization during tooth mineralization. Dental apoptosis is caspase dependent and proceeds via mitochondrial mediated cell death with possible amplification by Fas-FasL signaling modulated by Bcl-2 family members.

Future dentistry: cell therapy meets tooth and periodontal repair and regeneration

Cell-based tissue repair of the tooth and – tooth-supporting – periodontal ligament (PDL) is a new attractive approach that complements traditional restorative or surgical techniques for replacement of injured or pathologically damaged tissues. In such therapeutic approaches, stem cells and/or progenitor cells are manipulated in vitro and administered to patients as living and dynamic biological agents. In this review, we discuss the clonogenic potential of human dental and periodontal tissues such as the dental pulp and the PDL and their potential for tooth and periodontal repair and/or regeneration. We propose novel therapeutic approaches using stem cells or progenitor cells, which are targeted to regenerate the lost dental or periodontal tissue.

Dental pulp stem cells, niches, and notch signaling in tooth injury

Stem cells guarantee tissue repair and regeneration throughout life. The decision between cell self-renewal and differentiation is influenced by a specialized microenvironment called the 'stem cell niche'. In the tooth, stem cell niches are formed at specific anatomic locations of the dental pulp. The microenvironment of these niches regulates how dental pulp stem cell populations participate in tissue maintenance, repair, and regeneration. Signaling molecules such as Notch proteins are important regulators of stem cell function, with various capacities to induce proliferation or differentiation. Dental injuries often lead to odontoblast apoptosis, which triggers activation of dental pulp stem cells followed by their proliferation, migration, and differentiation into odontoblast-like cells, which elaborate a reparative dentin. Better knowledge of the regulation of dental pulp stem cells within their niches in pathological conditions will aid in the development of novel treatments for dental tissue repair and regeneration.

Odontoblast-targeted Bcl-2 overexpression promotes dentine damage repair

OBJECTIVE

Bcl-2 is widely expressed in a developing tooth organ and regulates tooth morphogenesis. However, whether Bcl-2 is related to tooth damage repair is unknown yet. Using an odontoblast-targeted Bcl-2 overexpression transgenic mouse (Col2.3Bcl-2) and artificial cavity preparation as a model system, the relationship between Bcl-2 and reparative dentinogenesis is investigated in this study.

METHODS

The odontoblastic-like cell cultures derived from mouse molar pulps were established. The expression of transgenic human Bcl-2 (hBcl-2) and endogenous mouse Bcl-2 (mBcl-2) and mouse Bax (mBax, a Bcl-2 antagonist) was detected in vivo and in vitro by Western blot and immunocytochemistry, respectively. Basal level and artificial cavity-induced odontoblast apoptosis was detected by the Deoxynucleotidyl Transferase (TdT) dUTP Nick End labelling (TUNEL) technique. Reparative dentine formation induced by artificial cavity drilled to a half dentine thickness on mesial cervical region of mandibular first molars 2, 4, and 6 weeks post-op was evaluated histologically and via micro-CT.

RESULTS

The transgenic hBcl-2 was stably expressed in odontoblasts of the transgenic animals without interference with the expression of mBcl-2 and mBax. Basal level as well as artificial cavity- induced odontoblast apoptosis was prevented by the transgene. Compared to the wild type, the transgenic animals produced reparative dentine with significantly higher mineral density 6 weeks after the operation.

CONCLUSIONS

Bcl-2 overexpression prevents odontoblast apoptosis and promotes dentine damage repair, indicating that genetic manipulation of Bcl-2 may be a novel strategy to maintain the vitality and function of dentine-pulp complex under detrimental mechanical stimuli.

Inorganic phosphate stimulates apoptosis in murine MO6-G3 odontoblast-like cells

OBJECTIVE

Dental pathologies such as caries are the most prevalent disease worldwide with infectious and social complications. During the process of caries formation, the tooth is degraded and demineralization of enamel and dentine leads to the release of large amounts of inorganic phosphate (Pi) within dental tubuli. As Pi has been shown to induce apoptosis in skeletal cells, including osteoblasts and chondrocytes, we questioned whether high concentrations of Pi could affect odontoblast viability, proliferation and apoptosis.

DESIGN

Using the odontoblast-like MO6-G3 cell line as a model, we used cell counting and MTS-based colorimetric assays to measure cell viability and proliferation. Apoptosis was assessed using Hoechst nuclei staining and detection of the early apoptotic markers annexin V and Apo2.7.

RESULTS

We show for the first time that a high Pi concentration (7 mM) induced a decrease in odontoblast viability and proliferation together with a large increase in apoptosis. These effects were blunted in calcium-free medium, possibly due to the formation of calcium-phosphate crystals in the presence of high Pi concentrations.

CONCLUSION

This study contributes to clarifying the effect of Pi on odontoblast viability and apoptosis, which may improve our understanding of the role of Pi during caries formation.

Lipopolysaccharide-induced dental pulp cell apoptosis and the expression of Bax and Bcl-2 in vitro

Lipopolysaccharide exerts many effects on many cell lines, including cytokine secretion, and cell apoptosis and necrosis. We investigated the in vitro effects of lipopolysaccharide on apoptosis of cultured human dental pulp cells and the expression of Bcl-2 and Bax. Dental pulp cells showed morphologies typical of apoptosis after exposure to lipopolysaccharide. Flow cytometry showed that the rate of apoptosis of human dental pulp cells increased with increasing lipopolysaccharide concentration. Compared with controls, lipopolysaccharide promoted pulp cell apoptosis (P < 0.05) from 0.1 to 100 μg/mL but not at 0.01 μg/mL. Cell apoptosis was statistically higher after exposure to lipopolysaccharide for 3 days compared with 1 day, but no difference was observed between 3 and 5 days. Immunohistochemistry showed that expression of Bax and Bcl-2 was enhanced by lipopolysaccharide at high concentrations, but no evident expression was observed at low concentrations (0.01 and 0.1 μg/mL) or in the control groups. In conclusion, lipopolysaccharide induced dental pulp cell apoptosis in a dose-dependent manner, but apoptosis did not increase with treatment duration. The expression of the apoptosis regulatory proteins Bax and Bcl-2 was also up-regulated in pulp cells after exposure to a high concentration of lipopolysaccharide.

BMPs and FGFs target Notch signalling via jagged 2 to regulate tooth morphogenesis and cytodifferentiation

The Notch signalling pathway is an evolutionarily conserved intercellular signalling mechanism that is essential for cell fate specification and proper embryonic development. We have analysed the expression, regulation and function of the jagged 2 (Jag2) gene, which encodes a ligand for the Notch family of receptors, in developing mouse teeth. Jag2 is expressed in epithelial cells that give rise to the enamel-producing ameloblasts from the earliest stages of tooth development. Tissue recombination experiments showed that its expression in epithelium is regulated by mesenchyme-derived signals. In dental explants cultured in vitro, the local application of fibroblast growth factors upregulated Jag2 expression, whereas bone morphogenetic proteins provoked the opposite effect. Mice homozygous for a deletion in the Notch-interaction domain of Jag2 presented a variety of severe dental abnormalities. In molars, the crown morphology was misshapen, with additional cusps being formed. This was due to alterations in the enamel knot, an epithelial signalling structure involved in molar crown morphogenesis, in which Bmp4 expression and apoptosis were altered. In incisors, cytodifferentiation and enamel matrix deposition were inhibited. The expression of Tbx1 in ameloblast progenitors, which is a hallmark for ameloblast differentiation and enamel formation, was dramatically reduced in Jag2(-/-) teeth. Together, these results demonstrate that Notch signalling mediated by Jag2 is indispensable for normal tooth development.

Disruptions of the pulp-dentin complex in an investigated case of pink secondary retained molar

A pink retained left mandibular first molar without carious lesions was diagnosed in a healthy 12-year-old girl presenting normal clinical tests. An orthopantomogram failed to detect other retained teeth. Both periapical radiography and computed tomography showed the absence of a periodontal ligament space in the bifurcation area and the presence of radiolucency or calcifications in the pulp cavity. The coronal part of the removed tooth was subjected to histological and immunohistochemical analysis using anti-PCNA (proliferation marker) and anti-p53 (apoptosis marker) antibodies. Root surfaces were observed by scanning electron microscopy. The pink color of the molar reflected the extension of resorptive tissue into the clinical crown and the underlining proliferation of pulp vessels. Ankylosis observed in the bifurcation area was also detected in the coronal part of the pulp. Whereas odontoblasts secreted tertiary dentin despite no evidence for a carious lesion, only osteocytes in the newly formed bone were apoptotic and the root surfaces were free of resorption lacunae. The etiopathology of the lesion in this case indicated a pulp origin, suggesting that new therapies targeting this tissue should be developed.

Cell fate determination during tooth development and regeneration

Teeth arise from sequential and reciprocal interactions between the oral epithelium and the underlying cranial neural crest-derived mesenchyme. Their formation involves a precisely orchestrated series of molecular and morphogenetic events, and gives us the opportunity to discover and understand the nature of the signals that direct cell fates and patterning. For that reason, it is important to elucidate how signaling factors work together in a defined number of cells to generate the diverse and precise patterned structures of the mature functional teeth. Over the last decade, substantial research efforts have been directed toward elucidating the molecular mechanisms that control cell fate decisions during tooth development. These efforts have contributed toward the increased knowledge on dental stem cells, and observation of the molecular similarities that exist between tooth development and regeneration.