Immunohistochemical study of hard tissue formation in the rat pulp cavity after tooth replantation

Fam20C overexpression in odontoblasts regulates dentin formation and odontoblast differentiation

FAM20C phosphorylates secretory proteins at S-x-E/pS motifs, and previous studies of Fam20C-dificient mice revealed that FAM20C played essential roles in bone and tooth formation. Inactivation of FAM20C in mice led to hypophosphatemia that masks direct effect of FAM20C in these tissues, and consequently the direct role of FAM20C remains unknown. Our previous study reported that osteoblast/odontoblast-specific Fam20C transgenic (Fam20C-Tg) mice had normal serum phosphate levels and that osteoblastic FAM20C-mediated phosphorylation regulated bone formation and resorption. Here, we investigated the direct role of FAM20C in dentin using Fam20C-Tg mice. The tooth of Fam20C-Tg mice contained numerous highly phosphorylated proteins, including SIBLINGs, compared to that of wild-type mice. In Fam20C-Tg mice, coronal dentin volume decreased and mineral density unchanged at early age, while the volume unchanged and the mineral density elevated at maturity. In these mice, radicular dentin volume and mineral density decreased at all ages, and histologically, the radicular dentin had wider predentin and abnormal apical-side dentin with embedded cells and argyrophilic canaliculi. Immunohistochemical analyses revealed that abnormal apical-side dentin had bone and dentin matrix properties accompanied with osteoblast-lineage cells. Further, in Fam20C-Tg mice, DSPP content which is important for dentin formation, was reduced in dentin, especially radicular dentin, which might lead to defects mainly in radicular dentin. Renal subcapsular transplantations of tooth germ revealed that newly formed radicular dentin replicated apical abnormal dentin of Fam20C-Tg mice, corroborating that FAM20C overexpression indeed caused the abnormal dentin. Our findings indicate that odontoblastic FAM20C-mediated phosphorylation in the tooth regulates dentin formation and odontoblast differentiation.

Histological analysis for pulp mineralisation after severe intrusive luxation of immature molars in rats

BACKGROUND/AIM

Pulp mineralisation is a survival process that may occur in the pulp of immature teeth following trauma. However, the mechanism of this process remains unclear. The aim of this study was to evaluate the histological manifestations of pulp mineralisation after intrusion in immature molars of rats.

MATERIALS AND METHODS

Three-week-old male Sprague-Dawley rats were subjected to intrusive luxation of the right maxillary second molar by an impact force from a striking instrument through a metal force transfer rod. The left maxillary second molar of each rat was used as a control. The control and injured maxillae were collected at 3, 7, 10, 14, and 30 days after trauma (n = 15 per time group) and evaluated using haematoxylin and eosin staining and immunohistochemistry. Independent two-tailed Student's t-test was used for statistical comparison of the immunoreactive area.

RESULTS

Pulp atrophy and mineralisation were observed in 30%-40% of the animals, and no pulp necrosis occurred. Ten days after trauma, pulp mineralisation, with osteoid tissue rather than reparative dentin, formed around the newly vascularised areas in the coronal pulp. CD90-immunoreactive cells were observed in the sub-odontoblastic multicellular layer in control molars, whereas the number of these cells was decreased in the traumatised teeth. CD105 localised in cells around the pulp osteoid tissue of the traumatised teeth, whereas in control teeth, it was only expressed in the vascular endothelial cells of capillaries in the odontoblastic or sub-odontoblastic layers. In specimens with pulp atrophy at 3-10 days after trauma, hypoxia inducible factor expression and CD11b-immunoreactive inflammatory cells increased.

CONCLUSIONS

Following intrusive luxation of immature teeth without crown fractures in rats, no pulp necrosis occurred. Instead, pulp atrophy and osteogenesis around neovascularisation with activated CD105-immunoreactive cells were observed in the coronal pulp microenvironment characterised by hypoxia and inflammation.

Hypoxia-inducible factor 1α induces osteo/odontoblast differentiation of human dental pulp stem cells via Wnt/β-catenin transcriptional cofactor BCL9

Accelerated dental pulp mineralization is a common complication in avulsed/luxated teeth, although the mechanisms underlying this remain unclear. We hypothesized that hypoxia due to vascular severance may induce osteo/odontoblast differentiation of dental pulp stem cells (DPSCs). This study examined the role of B-cell CLL/lymphoma 9 (BCL9), which is downstream of hypoxia-inducible factor 1α (HIF1α) and a Wnt/β-catenin transcriptional cofactor, in the osteo/odontoblastic differentiation of human DPSCs (hDPSCs) under hypoxic conditions. hDPSCs were isolated from extracted healthy wisdom teeth. Hypoxic conditions and HIF1α overexpression induced significant upregulation of mRNAs for osteo/odontoblast markers (RUNX2, ALP, OC), BCL9, and Wnt/β-catenin signaling target genes (AXIN2, TCF1) in hDPSCs. Overexpression and suppression of BCL9 in hDPSCs up- and downregulated, respectively, the mRNAs for AXIN2, TCF1, and the osteo/odontoblast markers. Hypoxic-cultured mouse pulp tissue explants showed the promotion of HIF1α, BCL9, and β-catenin expression and BCL9-β-catenin co-localization. In addition, BCL9 formed a complex with β-catenin in hDPSCs in vitro. This study demonstrated that hypoxia/HIF1α-induced osteo/odontoblast differentiation of hDPSCs was partially dependent on Wnt/β-catenin signaling, where BCL9 acted as a key mediator between HIF1α and Wnt/β-catenin signaling. These findings may reveal part of the mechanisms of dental pulp mineralization after traumatic dental injury.

Investigation of in vitro odonto/osteogenic capacity of cannabidiol on human dental pulp cell

INTRODUCTION

Vital pulp treatment (VPT) maintains tooth vitality with certain dental materials by protecting pulp from noxious stimulation and promoting repair through enhancing cell proliferation/differentiation, migration, and inducing odontogenesis. As a non-psychotropic cannabis constituent, cannabidiol (CBD) possesses the properties of analgesic, anti-inflammation, and osteogenesis. Therefore, we hypothesize that CBD may induce the odonto/osteogenesis of human dental pulp cells (HDPCs), a critical feature using as effective pulp capping agent for VPT.

MATERIALS AND METHODS

In this in vitro study, the cytotoxicity of CBD on HDPCs was determined by MTT assay. Scratch assay was performed to analyze HDPC migration. The biomineralization was examined by collagen synthesis and calcium nodule formation and related odonto/osteogenic and angiogenic genes. Cannabinoid receptor (CB) specificity was evaluated by Western blotting and Von Kossa staining using specific antagonists AM251 for cannabinoid receptor 1 (CB1) and AM 630 targeted at cannabinoid receptor 2 (CB2). In addition, the underlying molecular mechanism of CBD-induced biomineralization were investigated by examining CB-dependent MAPK signaling pathways.

RESULTS

CBD demonstrated bi-phasic effects on HDPC viability in tested concentrations. We found CBD significantly promoted cell migration, enhanced collagen synthesis and mineralized deposits in HDPCs when treated by 1 μM CBD supplemented in the differentiation media. RT-PCR revealed CBD increased the expression of angiogenic and odontogenic genes, such as DSPP, DMP-1, OPN, ALP, Runx2, VEGFR1 and ICAM-1. These effects were via MAPK activation in a manner mainly mediated by CB2.

CONCLUSION

The results from this study suggested that CBD can induce odonto/osteogenesis from HDPCs and has the potential to develop new therapeutics in VPT in dentistry.

Histological analysis of dental pulp response in immature or mature teeth after extra-oral subcutaneous transplantation into mice dorsum

PURPOSE

The aim of this study was to assess the response of dental pulp associated with donor or host cells in the pulp chamber and root canal after extra-oral transplantation.

METHODS

Wild type or green fluorescent protein (GFP) transgenic first molars from 3-week, 6-week, and 12-week mice were transplanted into the subcutaneous layer of GFP mice or wild type mice. The teeth were histologically and immunohistochemically examined at 5 weeks after transplantation.

RESULTS

Blood vessels present in the original coronal pulp had anastomosed with those from the recipient tissue that had invaded the root canal. Two distinct eosin-stained extracellular matrices were observed in the pulp chamber and root canal. Acellular matrix composed of nestin-positive, odontoblast-like cells invaded from the outside and was seen in the root canal of 3-week teeth. Cellular matrix comprising alkaline phosphatase (ALP)-positive fibroblast-like cells appeared in the original coronal pulp. In the root canal of the 6-week and 12-week teeth, cellular extracellular matrix consisting of ALP-positive fibroblast-like cells had invaded the recipient tissue.

CONCLUSION

Dental pulp from immature teeth might be able to regenerate dentin-like tissue. This model could be useful in the development of an optimized vitalization treatment.

Boric acid inhibits alveolar bone loss in rat experimental periodontitis through diminished bone resorption and enhanced osteoblast formation

Background/purpose

Inhibition of bone resorption is essential for periodontal treatment. Recently, it has been suggested that boric acid suppresses periodontitis, but the mechanism of this inhibition is still not well understood. Therefore, to analyze the cellular response to boric acid administration, we histologically evaluated alveolar bone in experimental periodontitis of rats administered boric acid.

Materials and methods

5-0 silk ligatures were placed around the cervix of the second maxillary molars of 4 week-old rats treated with or without boric acid. Five and 14 days after ligature placement, the periodontal tissues between first and second molars were investigated histologically and immunohistochemically using antibodies to CD68, cathepsin K, and α-smooth muscle actin (SMA).

Results

Five days after the beginning of the experiment, many CD68-positive cells appeared in the periodontal tissues with ligature placement without boric acid administration. Also, the number of cathepsin K-positive osteoclasts had increased on the surface of alveolar bone. However, boric acid administration prevented severe bone resorption and reduced the number of cells positive for CD68 and cathepsin K. At day 14 post treatment, cells positive for α-SMA were seen in the periodontal tissues after boric acid administration, whereas no such cells were found around the alveolar bone without the administration of boric acid.

Conclusion

Boric acid inhibited the inflammation of ligature-induced periodontitis. This agent might reduce bone resorption by inhibiting osteoclastogenesis and also could accelerate osteoblastogenesis.

Pulp healing in immature replanted permanent teeth: A competing risk analysis

BACKGROUND/AIM

Tooth displacement during avulsion causes total rupture of the pulp's neurovascular supply. Revascularization and pulp healing may occur in immature teeth, which gives rise to the recommendation that root canal treatment may not be required. The aim of this study was to evaluate the prognostic factors for the pulp's response after replantation of young permanent teeth.

METHODS

Records from 117 patients with 133 replanted permanent immature teeth were reviewed, and pulp outcomes were classified as healing (hard tissue deposition on the dentinal walls followed by narrowing of the pulp lumen or ingrowth of bone-like tissue inside the pulp canal) or non-healing (pulp necrosis with infection). The effect of clinical and demographic co-variates on the hazards of both outcomes was assessed performing a competing risk model.

RESULTS

Pulp necrosis with infection was diagnosed in 78.2% of the teeth, and healing was observed in 12.8% of the teeth. A total of 12 teeth (9.0%) were censored due to prophylactic removal of the pulp or severe external root resorption caused by eruption of adjacent canines. The cs-Cox model demonstrated that the hazards of pulp healing increased in teeth with extra-alveolar periods <15 min (csHR: 7.83, 95% CI 1.76-34.80, p = .01), while the hazards of pulp necrosis with infection decreased (csHR: 0.31, 95% CI: 0.10-0.92, p = .04). Teeth replanted with Moorrees' stages 4 and 5 of root development had higher hazards of pulp necrosis with infection than teeth with stage 2 of root development (csHR: 2.23, 95% CI 1.11-4.50, p = .03; csHR: 2.89, 95% CI: 1.40-5.95; p = .01).

CONCLUSIONS

Pulp healing rarely occurred after replantation of young permanent teeth being associated with short extra-alveolar periods <15 min. Early stages of root development decreased the hazards of pulp necrosis with infection.

Expression of BSP-GFPtpz Transgene during Osteogenesis and Reparative Dentinogenesis

Bone sialoprotein (BSP) is a member of the SIBLING family with essential roles in skeletogenesis. In the developing teeth, although the expression and function of BSP in the formation of acellular cementum and periodontal attachment are well documented, there are uncertainties regarding the expression and function of BSP by odontoblasts and dentin. Reporter mice are valuable animal models for biological research, providing a gene expression readout that can contribute to cellular characterization within the context of a developmental process. In the present study, we examined the expression of a BSP-GFPtpz reporter mouse line during odontoblast differentiation, reparative dentinogenesis, and bone. In the developing teeth, BSP-GFPtpz was expressed at high levels in cementoblasts but not in odontoblasts or dentin. In bones, the transgene was highly expressed in osteoblasts at an early stage of differentiation. Interestingly, despite its lack of expression in odontoblasts and dental pulp during tooth development, the BSP-GFPtpz transgene was detected during in vitro mineralization of primary pulp cultures and during reparative dentinogenesis following pulp exposures. Importantly, under these experimental contexts, the expression of BSP-GFPtpz was still exclusive to DSPP-Cerulean, an odontoblast-specific reporter gene. This suggests that the combinatorial use of BSP-GFPtpz and DSPP-Cerulean can be a valuable experimental tool to distinguish osteogenic from dentinogenic cells, thereby providing an avenue to investigate mechanisms that distinctly regulate the lineage progression of progenitors into odontoblasts versus osteoblasts.

Immunohistochemistry and gene expression of GLUT1, RUNX2 and MTOR in reparative dentinogenesis

OBJECTIVES

To determine glucose transporter 1 (GLUT1) and runt-related transcription factor 2 (RUNX2) expression during reparative dentinogenesis after pulpotomy with mineral trioxide aggregate (MTA) capping.

SUBJECTS AND METHODS

Eight-week-old male Wistar rats were used. Pulp of the upper left first molar was exposed and capped with MTA. The upper right first molar of the same animal was used as a control. After collecting molars at various time points, GLUT1, RUNX2 and mammalian target of rapamycin (MTOR) were examined by immunohistochemistry. mRNA levels of Slc2a1 (encoding GLUT1), Runx2, Nestin and Mtor were determined by real-time PCR.

RESULTS

Pulp exhibited progressive formation of reparative dentine lined with GLUT1- and MTOR-immunoreactive odontoblast-like cells at 5 days after pulpotomy. RUNX2 was detected in nuclei of most pulp tissue cells at day 5 after pulpotomy. Double immunofluorescence staining revealed GLUT1 immunoreactivity on odontoblast-like cells positive for Nestin or RUNX2, 5 days after pulpotomy. Slc2a1, Runx2, Nestin and Mtor mRNA levels were significantly upregulated on days 3-5 after pulpotomy.

CONCLUSIONS

After rat molar pulpotomy, dental pulp induced formation of reparative dentine with colocalization of GLUT1 and Nestin or RUNX2. Moreover, mRNA levels of Slc2a1, Runx2, Nestin and Mtor were significantly upregulated in pulpotomized dental pulp.

Calcific metamorphosis of pulp after extrusive luxation

BACKGROUND/AIM

The literature on the pathogenesis of extrusive dental luxation has been focused on periodontal tissue responses, with little attention given to the pulp. The aim of this study was to evaluate the response of dental pulp of teeth following extrusive luxation in a rat model.

MATERIAL AND METHODS

The maxillary right central incisors of 30 rats were extrusively luxated and repositioned after 5 minutes. The animals were euthanized after 7, 15, and 30 days to provide three groups: I, II, and III, respectively (n = 10). Histological sections were stained with H and E for histomorphometric analysis of the odontoblast layer, reparative dentin deposition, Hertwig's epithelial root sheath, pulp necrosis, and periapical inflammatory infiltrate.

RESULTS

In most cases, new vascular formation occured in association with reparative dentin deposition on the root walls and within the pulp. In some cases, dentin deposition occupied the entire pulp space over time, with no other types of non-odontogenic hard tissues being observed. Pulp necrosis with the presence of periapical inflammatory infiltrate was also observed in a few cases. No statistical differences were observed among the studied groups.

CONCLUSIONS

Following extrusive luxation, calcific metamorphosis of the pulp is very likely to occur.

Required Time for Migration of Bone Marrow-derived Cells to Dental Pulp after Bone Marrow Transplantation

INTRODUCTION

This study aimed to evaluate the time required for bone marrow-derived cells (BMDCs) from transgenic green fluorescent protein (GFP)+ donor mice (GFP+ mice) to migrate into the dental pulp of wild-type GFP- recipient mice (GFP- mice) by using bone marrow transplantation (BMT) as an in vivo model for tracking BMDCs from GFP+ mice (GFP+ BMDCs).

METHODS

GFP+ BMDCs were injected into irradiated GFP- mice. Maxillary arches, tibiae, and femora from GFP- mice were isolated and processed at 24 hours, 48 hours, 4, 7, and 14 days, and 7 weeks after BMT. Confocal laser microscopy analyses were performed to assess the presence of GFP+ BMDCs in the dental pulp, and flow cytometry of BM was performed to confirm the efficiency of engraftment of GFP+ BMDCs.

RESULTS

Confocal laser microscopy analyses evidenced the presence of GFP+ BMDCs in the dental pulp of GFP- mice from 14 days to 7 weeks after BMT. There was no presence of GFP+ BMDCs at 24 hours, 48 hours, 4 days, and 7 days. Flow cytometry of the BM of GFP- mice demonstrated a constant increase in the presence of GFP+ BMDCs at 24 hours, 48 hours, and 4 days after BMT, which stabilized from 7 days to 7 weeks.

CONCLUSIONS

The study demonstrated the presence of GFP+ BMDCs in the dental pulp from 14 days to 7 weeks after BMT and the feasibility of using GFP+ animals and BMT as an in vivo model for tracking GFP+ BMDCs.

Healing response of rat pulp treated with an injectable keratin hydrogel

Background

Keratin has shown promising outcomes as a biomaterial due to its inherent bioactivity, biocompatibility and regenerative effects. The effect of keratin on repair and regeneration of dental tissues has never been studied before. Current therapies to treat pulp tissues involve its replacement with inert, synthetic materials that do not have a proper biological function, leading to failure and tooth loss. This study aimed to develop a biocompatible keratin hydrogel (KH) suitable for pulp therapies.

Methods

Keratins extracted from sheep wool were isolated, quantified and reconstituted to form KH. Different concentrations of keratin gel suitable for dental application were characterized by rheological analysis. The optimized gel based on flow characteristics was studied further for microstructure including porosity, percentage swelling ratio and contact angle measurements, using analytical tools such as scanning electron microscopy (SEM), micro-computed tomography and goniometer. To assess both biocompatibility and pulpal response, KH was implanted into rat upper molar teeth following partial pulpotomy. After 28 days, the tissue sections were analyzed by histological and immunohistochemical methods to identify dentin matrix protein 1 (DMP-1) formation and compared with control (Ca(OH)₂-treated) teeth.

Results

The results of the study demonstrated a viscous and injectable, porous, dimensionally stable, hydrophilic and biocompatible gel that allowed pulp healing to occur by a reparative response, with widespread DMP-1 expression.

Conclusions

The findings of this study indicate that keratins can be developed as a biomaterial source for alternate biological treatment options for pulp therapies.

Biomimetic microenvironments for regenerative endodontics

Regenerative endodontics has been proposed to replace damaged and underdeveloped tooth structures with normal pulp-dentin tissue by providing a natural extracellular matrix (ECM) mimicking environment; stem cells, signaling molecules, and scaffolds. In addition, clinical success of the regenerative endodontic treatments can be evidenced by absence of signs and symptoms; no bony pathology, a disinfected pulp, and the maturation of root dentin in length and thickness. In spite of the various approaches of regenerative endodontics, there are several major challenges that remain to be improved: a) the endodontic root canal is a strong harbor of the endodontic bacterial biofilm and the fundamental etiologic factors of recurrent endodontic diseases, (b) tooth discolorations are caused by antibiotics and filling materials, (c) cervical root fractures are caused by endodontic medicaments, (d) pulp tissue is not vascularized nor innervated, and (e) the dentin matrix is not developed with adequate root thickness and length. Generally, current clinical protocols and recent studies have shown a limited success of the pulp-dentin tissue regeneration. Throughout the various approaches, the construction of biomimetic microenvironments of pulp-dentin tissue is a key concept of the tissue engineering based regenerative endodontics. The biomimetic microenvironments are composed of a synthetic nano-scaled polymeric fiber structure that mimics native pulp ECM and functions as a scaffold of the pulp-dentin tissue complex. They will provide a framework of the pulp ECM, can deliver selective bioactive molecules, and may recruit pluripotent stem cells from the vicinity of the pulp apex. The polymeric nanofibers are produced by methods of self-assembly, electrospinning, and phase separation. In order to be applied to biomedical use, the polymeric nanofibers require biocompatibility, stability, and biodegradability. Therefore, this review focuses on the development and application of the biomimetic microenvironments of pulp-dentin tissue among the current regenerative endodontics.

The prolyl hydroxylase inhibitor dimethyloxalylglycine enhances dentin sialophoshoprotein expression through VEGF-induced Runx2 stabilization

Prolyl hydroxylase (PHD) inhibitors are suggested as therapeutic agents for tissue regeneration based on their ability to induce pro-angiogenic responses. In this study, we examined the effect of the PHD inhibitor dimethyloxalylglycine (DMOG) on odontoblast maturation and sought to determine the underlying mechanism using MDPC-23 odontoblast-like cells. DMOG significantly enhanced matrix mineralization, confirmed by alizarin red staining and by measurement of the calcium content. DMOG dose-dependently increased alkaline phosphatase activity and the expressions of dentin sialophosphoprotein (Dspp) and osteocalcin. To determine the underlying events leading to DMOG-induced Dspp expression, we analyzed the effect of DMOG on Runx2. Knockdown of Runx2 using siRNAs decreased Dspp expression and prevented DMOG-induced Dspp expression. DMOG enhanced the transcriptional activity and level of Runx2 protein but not Runx2 transcript, and this enhancement was linked to the inhibitory effects of DMOG on the degradation of Runx2 protein. The vascular endothelial growth factor (VEGF) siRNAs profoundly decreased the Runx2 protein levels and inhibited the DMOG-increased Runx2 protein. Recombinant VEGF protein treatment significantly and dose-dependently increased the transcriptional activity and level of the Runx2 protein but not Runx2 transcript. Dspp expression was also enhanced by VEGF. Last, we examined the involvement of the Erk mitogen-activated protein kinase and Pin1 pathway in VEGF-enhanced Runx2 because this pathway can regulate the stability and activity of the Runx2 protein. VEGF stimulated Erk activation, and the inhibitors of Erk and Pin1 hampered VEGF-enhanced Runx2 protein. Taken together, the results of this study provide evidence that DMOG can enhance Dspp expression through VEGF-induced stabilization of Runx2 protein, and thus, suggest that DMOG can be used as a therapeutic tool for enhancing odontoblast maturation in dental procedures.

Sp7 and Runx2 molecular complex synergistically regulate expression of target genes

Runx2 and Sp7 transcription factors are essential for skeletogenesis. Targeted deletion of either gene results in failure of osteoblast differentiation and bone formation. Loss of bone-matrix gene expression is surprisingly similar in Sp7 and Runx2 null mice. The molecular mechanisms responsible for similar transcriptional regulation of target genes remain largely unknown. Here, we demonstrate that Runx2 and Sp7 interact physically and functionally. Both proteins are co-expressed in osteoblastic cells. We first characterized a panel of Sp7 antibodies and demonstrate that majority of the published antibodies do not recognize Sp7 protein. Co-immunoprecipitation studies revealed that endogenous Runx2 protein physically interacts with Sp7 protein. We identified that runt homology domain (RHD) of Runx2 protein is involved in physical association with Sp7. Functional consequences of Runx2-Sp7 physical interaction was then assessed by promoter-reporter assays. We selected promoters of osteocalcin (OC), a marker of mature osteoblast and fibroblast growth factor 3 (FGF3), a signaling molecule that determine the fate of embryonic ecto-mesenchyme. Runx2 and Sp7 stimulate OC-promoter activity by 3-folds in epithelial cells. However, when both proteins were co-expressed, a dose-dependent synergistic activation of 22-folds was noted. Similar pattern of synergistic activation of OC-promoter was noted in mesenchymal cell. FGF3 promoter was activated by 25 - and 30-folds with Runx2 and Sp7 respectively. Again a dose-dependent synergistic activation of 130-folds was evident when Runx2 and Sp7 were co-expressed in epithelial cells. Synergistic activation of FGF3 promoter was also noted in mesenchymal cells. Together, our data demonstrated that Runx2-Sp7 molecular complex functionally cooperate for maximal induction of cell-phenotype-restricted genes.

Enhanced Bone-Forming Activity of Side Population Cells in the Periodontal Ligament

Regeneration of alveolar bone is critical for the successful treatment of periodontal diseases. The periodontal ligament (PDL) has been widely investigated as a source of cells for the regeneration of periodontal tissues. In the present study where we attempted to develop an effective strategy for alveolar bone regeneration, we examined the osteogenic potential of side population (SP) cells, a stem cell-containing population that has been shown to be highly abundant in several kinds of tissues, in PDL cells. Isolated SP cells from the rat PDL exhibited a superior ability to differentiate into osteoblastic cells compared with non-SP (NSP) and unsorted PDL cells in vitro. The mRNA expressions of osteoblast markers and bone morphogenetic protein (BMP) 2 were significantly upregulated in SP cells and were further increased by osteogenic induction. To examine the bone-forming activity of SP cells in vivo, PDL SP cells isolated from green fluorescent protein (GFP)-transgenic rats were transplanted with hydroxyapatite (HA) disks into wild-type animals. SP cells exhibited a high ability to induce the mineralized matrix compared with NSP and unsorted PDL cells. At 12 weeks after the implantation, some of the pores in the HA disks with SP cells were filled with mineralized matrices, which were positive for bone matrix proteins, such as osteopontin, bone sialoprotein, and osteocalcin. Furthermore, osteoblast- and osteocyte-like cells on and in the bone-like mineralized matrices were GFP positive, suggesting that the matrices were directly formed by the transplanted cells. These results suggest that PDL SP cells possess enhanced osteogenic potential and could be a potential source for cell-based regenerative therapy for alveolar bone.

Genetic and molecular control of osterix in skeletal formation

Osteoblast differentiation is a multi-step process where mesenchymal cells differentiate into osteoblast lineage cells including osteocytes. Osterix (Osx) is an osteoblast-specific transcription factor which activates a repertoire of genes during differentiation of preosteoblasts into mature osteoblasts and osteocytes. The essential role of Osx in the genetic program of bone formation and in bone homeostasis is well established. Osx mutant embryos do not form bone and fail to express osteoblast-specific marker genes. Inactivation of Osx in mice after birth causes multiple skeletal phenotypes including lack of new bone formation, absence of resorption of mineralized cartilage, and defects in osteocyte maturation and function. Since Osx is a major effector in skeletal formation, studies on Osx gained momentum over the last 5-7 years and implicated its important function in tooth formation as well as in healing of bone fractures. This review outlines mouse genetic studies that establish the essential role of Osx in bone and tooth formation as well as in healing of bone fractures. We also discuss the recent advances in regulation of Osx expression, which is under control of a transcriptional network, signaling pathways, and epigenetic regulation. Finally, we summarize important findings on the positive and negative regulation of Osx's transcriptional activity through protein-protein interactions in expression of its target genes during osteoblast differentiation. In particular, the identification of the histone demethylase NO66 as an Osx-interacting protein, which negatively regulates Osx activity opens further avenues in studying epigenetic control of Osx target genes during differentiation and maturation of osteoblasts.

Multiple postnatal craniofacial anomalies are characterized by conditional loss of polycystic kidney disease 2 (Pkd2)

Polycystin 2 (Pkd2), which belongs to the transient receptor potential family, plays a critical role in development. Pkd2 is mainly localized in the primary cilia, which also function as mechanoreceptors in many cells that influence multiple biological processes including Ca(2+) influx, chemical activity and signalling pathways. Mutations in many cilia proteins result in craniofacial abnormalities. Orofacial tissues constantly receive mechanical forces and are known to develop and grow through intricate signalling pathways. Here we investigate the role of Pkd2, whose role remains unclear in craniofacial development and growth. In order to determine the role of Pkd2 in craniofacial development, we located expression in craniofacial tissues and analysed mice with conditional deletion of Pkd2 in neural crest-derived cells, using Wnt1Cre mice. Pkd2 mutants showed many signs of mechanical trauma such as fractured molar roots, distorted incisors, alveolar bone loss and compressed temporomandibular joints, in addition to abnormal skull shapes. Significantly, mutants showed no indication of any of these phenotypes at embryonic stages when heads perceive no significant mechanical stress in utero. The results suggest that Pkd2 is likely to play a critical role in craniofacial growth as a mechanoreceptor. Pkd2 is also identified as one of the genes responsible for autosomal dominant polycystic kidney disease (ADPKD). Since facial anomalies have never been identified in ADPKD patients, we carried out three-dimensional photography of patient faces and analysed these using dense surface modelling. This analysis revealed specific characteristics of ADPKD patient faces, some of which correlated with those of the mutant mice.

Two distinct processes of bone-like tissue formation by dental pulp cells after tooth transplantation

Dental pulp is involved in the formation of bone-like tissue in response to external stimuli. However, the origin of osteoblast-like cells constructing this tissue and the mechanism of their induction remain unknown. We therefore evaluated pulp mineralization induced by transplantation of a green fluorescent protein (GFP)-labeled tooth into a GFP-negative hypodermis of host rats. Five days after the transplantation, the upper pulp cavity became necrotic; however, cell-rich hard tissue was observed adjacent to dentin at the root apex. At 10 days, woven bone-like tissue was formed apart from the dentin in the upper pulp. After 20 days, these hard tissues expanded and became histologically similar to bone. GFP immunoreactivity was detected in the hard tissue-forming cells within the root apex as well as in the upper pulp. Furthermore, immunohistochemical observation of α-smooth muscle actin, a marker for undifferentiated cells, showed a positive reaction in cells surrounding this bone-like tissue within the upper pulp but not in those within the root apex. Immunoreactivities of Smad4, Runx2, and Osterix were detected in the hard tissue-forming cells within both areas. These results collectively suggest that the dental pulp contains various types of osteoblast progenitors and that these cells might thus induce bone-like tissue in severely injured pulp.

Analysis of the contribution of nonresident progenitor cells and hematopoietic cells to reparative dentinogenesis using parabiosis model in mice

INTRODUCTION

The aim of this study was to analyze the contribution of nonresident progenitor/stem cells and hematopoietic cells to reparative dentinogenesis.

METHODS

Parabiosis was established between C57BL/6-TgN(ACTbEGFP)10sb/J transgenic mice (GFP+) and C57BL/6 wild-type mice (GFP-) to ensure blood cross-circulation between animals. Reparative dentinogenesis was stimulated by pulp exposures and capping on the first maxillary molar in the GFP- mice. Histologic sections of injured molars from GFP- mice were analyzed by epifluorescence microscopy to examine the contributions of GFP+ cells (nonresident progenitor cells and hematopoietic cells originating from GFP+ mice) to reparative dentinogenesis.

RESULTS

GFP+ cells were detected in close association with reparative dentin formed at the site of pulp exposure in the maxillary first molars of the GFP- mice.

CONCLUSIONS

The present study suggests the participation of the nonresident progenitor cells and hematopoietic cells in reparative dentinogenesis.

A feasibility study for the analysis of reparative dentinogenesis in pOBCol3.6GFPtpz transgenic mice

AIM

To examine the feasibility of using the pOBCol3.6GFPtpz [3.6-green fluorescent protein (GFP)] transgenic mice as an in vivo model for studying the biological sequence of events during pulp healing and reparative dentinogenesis.

METHODOLOGY

Pulp exposures were created in the first maxillary molar of 12-16-week-old 3.6-GFP transgenic mice with CD1 and C57/Bl6 genetic background. Direct pulp capping on exposed teeth was performed using mineral trioxide aggregate followed by restoration with a light-cured adhesive system (AS) and composite resin. In control teeth, the AS was placed in direct contact with the pulp. Animals were euthanized at various time points after pulp exposure and capping. The maxillary arch was isolated, fixed and processed for histological and epifluorescence analysis to examine reparative dentinogenesis.

RESULTS

Analysis of teeth immediately after pulp exposure revealed absence of odontoblasts expressing 3.6-GFP at the injury site. Evidence of reparative dentinogenesis was apparent at 4 weeks in 3.6-GFP mice in CD1 background and at 8 weeks in 3.6-GFP mice with C57/Bl6 background. The reparative dentine with both groups contained newly formed atubular-mineralized tissue resembling a dentine bridge and/or osteodentine that was lined by cells expressing 3.6-GFP as well as 3.6-GFP expressing cells embedded within the atubular matrix.

CONCLUSION

This study was conducted in a few animals and did not allow statistical analysis. The results revealed that the 3.6-GFP transgenic animals provide a unique model for direct analysis of cellular and molecular mechanisms of pulp repair and tertiary dentinogenesis in vivo. The study also shows the effects of the capping material and the genetic background of the mice in the sequence and timing of reparative dentinogenesis.

Behaviour of rat-cultured dental pulp cells in three-dimensional collagen type-1 gel in vitro and in vivo

The purpose of this study was to investigate the growth and differentiation potential of dental pulp cells (DPCs) in three-dimensional (3-D) collagen type-1 scaffold in vitro and in vivo. Third passage DPCs were cultured in a 3-D collagen and expression of both bone- or dentin-related mRNA (alkaline phosphatase (ALP), bone sialoprotein (BSP) and osteopontin (OPN)) and morphological changes evaluated in vitro. In the in vivo study, two types of grafts were transplanted into the rectus abdominus muscles of rats and harvested after 7 days: DPCs in α-minimal essential medium and DPCs mixed with a collagen gel. ALP, BSP and OPN were used as primary antibodies for immunohistochemical study. Histological and immunohistochemical results showed that DPCs in collagen gel were spindle shaped and showed significantly greater expression of ALP, BSP and OPN in vitro than the controls. Transplanted DPCs in collagen type-1 gel showed greater positive immunoreactivity for ALP, BSP and OPN than the controls. It was concluded that the collagen gel scaffold encouraged the differentiation of DPCs into osteoblastic cells.

Reparative dentinogenesis induced by mineral trioxide aggregate: a review from the biological and physicochemical points of view

This paper aims to review the biological and physicochemical properties of mineral trioxide aggregate (MTA) with respect to its ability to induce reparative dentinogenesis, which involves complex cellular and molecular events leading to hard-tissue repair by newly differentiated odontoblast-like cells. Compared with that of calcium hydroxide-based materials, MTA is more efficient at inducing reparative dentinogenesis in vivo. The available literature suggests that the action of MTA is attributable to the natural wound healing process of exposed pulps, although MTA can stimulate hard-tissue-forming cells to induce matrix formation and mineralization in vitro. Physicochemical analyses have revealed that MTA not only acts as a "calcium hydroxide-releasing" material, but also interacts with phosphate-containing fluids to form apatite precipitates. MTA also shows better sealing ability and structural stability, but less potent antimicrobial activity compared with that of calcium hydroxide. The clinical outcome of direct pulp capping and pulpotomy with MTA appears quite favorable, although the number of controled prospective studies is still limited. Attempts are being conducted to improve the properties of MTA by the addition of setting accelerators and the development of new calcium silicate-based materials.

Effects of single and cyclical local injections of basic fibroblast growth factor on cancellous bone defects in rabbits

BACKGROUND

Local administration of basic fibroblast growth factor (bFGF) has anabolic effects on bone formation. A delivery system for local treatment is required to increase efficacy because of its short half-life. However, little is known about the effects of cyclical local injection of bFGF. We evaluated the effects of single and cyclical local injection of bFGF at a cancellous bone defect in the femoral condyle in rabbits.

METHODS

Using the "vehicle only" as a control, a single low dose (40 microg), single high dose (120 microg), or cyclical low dose (40 microg, three times) of bFGF was injected percutaneously into a bone defect implanted with a gelatin sponge. The rabbits were killed at 4 weeks after surgery and the femurs were harvested for evaluation.

RESULTS

Both single and cyclical administration of bFGF dose-dependently increased the amount of new bone formation in the bone defect using radiographs (P < 0.01) and bone mineral density (BMD) measurements (P < 0.01) compared to controls. However, only high-dose bFGF injection significantly increased the cancellous bone volume at the bone defect (P < 0.05) compared to controls, using bone histomorphometry. Cyclical injection of bFGF significantly increased the number of runt-related transcription factor-2 (Runx2)-positive cells compared to single low- and high-dose bFGF administration (P < 0.01 and P < 0.05, respectively), and single high-dose and cyclical administration significantly increased the number of osteopontin-positive cells compared to controls (P < 0.01), based on immunohistochemical analysis.

CONCLUSIONS

These results suggest that high-dose injection of bFGF, at the very early stage of cancellous bone healing, is more effective in increasing cancellous bone volume, and cyclical injection of bFGF may stimulate osteoprogenitor cells.

Pulpal regeneration following allogenic tooth transplantation into mouse maxilla

Autogenic tooth transplantation is now a common procedure in dentistry for replacing a missing tooth. However, there are many difficulties in clinical application of allogenic tooth transplantation because of immunological rejection. This study aims to clarify pulpal regeneration following allogenic tooth transplantation into the mouse maxilla by immunohistochemistry for 5-bromo-2'-deoxyuridine (BrdU) and nestin, and by the histochemistry for tartrate-resistant acid phosphatase (TRAP). The upper right first molar (M1) of 2-week-old mice was extracted and allografted in the original socket in both the littermate and non-littermate after the extraction of M1. Tooth transplantation weakened the nestin-positive reactions in the pulp tissue that had shown immunoreactivity for nestin before operation. On postoperative Days 5-7, tertiary dentin formation commenced next to the preexisting dentin where nestin-positive odontoblast-like cells were arranged in all cases of the littermate group until Day 14, except for one case showing immunological rejection in the pulp chamber. In the non-littermate group, bone-like tissue formation occurred in the pulp chamber in addition to tertiary dentin formation until Day 14. The rate of tertiary dentin was 38%, and the rate of the mixed form of dentin and bone-like tissue formation was 23% (the remainder was immunological rejection). Interestingly, the periodontal tissue recovered even in the case of immunological rejection in which the pulp chamber was replaced by sparse connective tissue. These results suggest that the selection of littermate or non-littermate is decisive for the survival of odontoblast-lineage cells and that the immunological rejection does not influence the periodontal regeneration.

Inhibition of the terminal differentiation of odontoblasts and their transdifferentiation into osteoblasts in Runx2 transgenic mice

Runx2 is an essential transcription factor for bone and tooth development whose function in odontoblast differentiation remains to be clarified. To pursue this issue, we examined tooth development in Runx2 transgenic mice under the control of Col1a1 promoter (Tg(Col1a1-Runx2) mice). Endogenous Runx2 protein was detected in the nuclei of preodontoblasts, immature odontoblasts, mesenchymal cells in the dental sac, and osteoblasts, while transgene expression was detected in odontoblasts and osteoblasts. Odontoblasts in Tg(Col1a1-Runx2) mice lost their columnar shape and dentin was deposited around the odontoblasts, which were cuboid or flat in shape. The dentin in Tg(Col1a1-Runx2) mice was thin and possessed lacunae that contained odontoblasts and bone canaliculi-like structures, while predentin and dentinal tubules were absent. We examined the expression of dentin matrix protein genes, Col1a1 and dentin sialophosphoprotein (DSPP), by in situ hybridization, and dentin matrix proteins, osteocalcin, osteopontin, and dentin matrix protein 1 (DMP1) as well as an intermediate filament, nestin, by immunohistochemistry to characterize odontoblasts in Tg(Col1a1-Runx2) mice. Results showed Col1a1 expression was down-regulated, DSPP expression was lost, and nestin expression was severely decreased in the odontoblasts of Tg(Col1a1-Runx2) mice. Further, the expressions of osteocalcin, osteopontin, and DMP1 were up-regulated in odontoblasts, although the up-regulation of osteocalcin expression was transient. These findings indicate that Runx2 inhibits the terminal differentiation of odontoblasts, and that Runx2 induces transdifferentiation of odontoblasts into osteoblasts forming a bone structure. Thus, Runx2 expression has to be down-regulated during odontoblast differentiation to acquire full odontoblast differentiation for dentinogenesis.

Capacity of dental pulp differentiation in mouse molars as demonstrated by allogenic tooth transplantation

Dental pulp elaborates both bone and dentin under pathological conditions such as tooth replantation/transplantation. This study aims to clarify the capability of dental pulp to elaborate bone tissue in addition to dentin by allogenic tooth transplantation using immunohistochemistry and histochemistry. After extraction of the molars of 3-week-old mice, the roots and pulp floor were resected and immediately allografted into the sublingual region in a littermate. In addition, we studied the contribution of donor and host cells to the regenerated pulp tissue using a combination of allogenic tooth transplantation and lacZ transgenic ROSA26 mice. On Days 5-7, tubular dentin formation started next to the preexisting dentin at the pulp horn where nestin-positive odontoblast-like cells were arranged. Until Day 14, bone-like tissue formation occurred in the pulp chamber, where intense tartrate-resistant acid phosphatase-positive cells appeared. Furthermore, allogenic transplantation using ROSA26 mice clearly showed that both donor and host cells differentiated into osteoblast-like cells with the assistance of osteoclast-lineage cells, whereas newly differentiated odontoblasts were exclusively derived from donor cells. These results suggest that the odontoblast and osteoblast lineage cells reside in the dental pulp and that both donor and host cells contribute to bone-like tissue formation in the regenerated pulp tissue.