Temporal and spatial expression of c-jun and jun-B proto-oncogenes in pulp cells involved with reparative dentinogenesis after cavity preparation of rat molars

Cannabidiol Rescues TNF-α-Inhibited Proliferation, Migration, and Osteogenic/Odontogenic Differentiation of Dental Pulp Stem Cells

Strategies to promote dental pulp stem cells (DPSCs) functions including proliferation, migration, pro-angiogenic effects, and odontogenic/osteogenic differentiation are in urgent need to restore pulpitis-damaged dentin/pulp regeneration and DPSCs-based bone tissue engineering applications. Cannabidiol (CBD), an active component of Cannabis sativa has shown anti-inflammation, chemotactic, anti-microbial, and tissue regenerative potentials. Based on these facts, this study aimed to analyze the effect of CBD on DPSCs proliferation, migration, and osteogenic/odontogenic differentiation in basal and inflammatory conditions. Highly pure DPSCs with characteristics of mesenchymal stem cells (MSCs) were successfully isolated, as indicated by the results of flowcytometry and multi-lineage (osteogenic, adipogenic, and chondrogenic) differentiation potentials. Among the concentration tested (0.1-12.5 µM), CBD (2.5 μM) showed the highest anabolic effect on the proliferation and osteogenic/odontogenic differentiation of DPSCs. Pro-angiogenic growth factor VEGF mRNA expression was robustly higher in CBD-treated DPSCs. CBD also prompted the migration of DPSCs and CBD receptor CB1 and CB2 expression in DPSCs. TNF-α inhibited the viability, migration, and osteogenic/odontogenic differentiation of DPSCs and CBD reversed these effects. CBD alleviated the TNF-α-upregulated expression of pro-inflammatory cytokines TNF-α, interleukin (IL)-1β, and IL-6 in DPSCs. In conclusion, our results indicate the possible application of CBD on DPSCs-based dentin/pulp and bone regeneration.

Autophagy-Related Protein MAP1LC3C Plays a Crucial Role in Odontogenic Differentiation of Human Dental Pulp Cells

BACKGROUND

Autophagy plays important roles in odontogenic differentiation of dental pulp cells (DPCs) in the developmental stage of tooth bud. Few studies have reported the role of autophagy during reparative dentin formation process. The objective of this study was to discover gene expression pattern correlated to autophagy and their role during odontogenic differentiation process in DPCs.

METHODS

After tooth cavities were prepared on the mesial surface of lower first molar crown of rats. Odontogenic differentiation and reparative dentin formation were assessed based on detection of morphology change with hematoxylin and eosin staining.

RESULTS

After tooth cavities were prepared on the mesial surface of lower first molar crown of rats, odontogenic differentiation and reparative dentin formation were assessed based on detection of morphology change with hematoxylin and eosin staining and dentin sialophosphoprotein (DSPP), whereas autophagy inhibitor 3-methyladenine (3MA) reversed. Results of quantitative polymerized chain reaction array of autophagosome formation related genes revealed that GABARAPL2 was prominently upregulated while expression of other ATG8 family members were moderately increased after tooth cavity preparation. In addition, human DPCs incubated in differentiation medium predominantly upregulated MAP1LC3C, which selectively decreased by 3MA but not by autophagy enhancer trehalose. Knock-down of MAP1LC3C using shRNA resulted in strong downregulation of dentin matrix protein 1 and DSPP as well-known odontogenic marker compared to knock-down of MAP1LC3B during odontogenic differentiation process of human DPCs.

CONCLUSION

Our results suggest that MAP1LC3C plays a crucial role in odontogenic differentiation of human DPCs via regulating autophagic flux.

Dental Pulp Stem Cells: Advances to Applications

Dental pulp stem cells (DPSCs) have a high capacity for differentiation and the ability to regenerate a dentin/pulp-like complex. Numerous studies have provided evidence of DPSCs’ differentiation capacity, such as in neurogenesis, adipogenesis, osteogenesis, chondrogenesis, angiogenesis, and dentinogenesis. The molecular mechanisms and functions of DPSCs’ differentiation process are affected by growth factors and scaffolds. For example, growth factors such as basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF-β), nerve growth factor (NGF), platelet-derived growth factor (PDGF), and bone morphogenic proteins (BMPs) influence DPSC fate, including in differentiation, cell proliferation, and wound healing. In addition, several types of scaffolds, such as collagen, hydrogel, decellularized bioscaffold, and nanofibrous spongy microspheres, have been used to characterize DPSC cellular attachment, migration, proliferation, differentiation, and functions. An appropriate combination of growth factors and scaffolds can enhance the differentiation capacity of DPSCs, in terms of optimizing not only dental-related expression but also dental pulp morphology. For a cell-based clinical approach, focus has been placed on the tissue engineering triad [cells/bioactive molecules (growth factors)/scaffolds] to characterize DPSCs. It is clear that a deep understanding of the mechanisms of stem cells, including their aging, self-renewal, microenvironmental homeostasis, and differentiation correlated with cell activity, the energy for which is provided from mitochondria, should provide new approaches for DPSC research and therapeutics. Mitochondrial functions and dynamics are related to the direction of stem cell differentiation, including glycolysis, oxidative phosphorylation, mitochondrial metabolism, mitochondrial transcription factor A (TFAM), mitochondrial elongation, and mitochondrial fusion and fission proteins. This review summarizes the effects of major growth factors and scaffolds for regenerating dentin/pulp-like complexes, as well as elucidating mitochondrial properties of DPSCs for the development of advanced applications research.

Pulp Stem Cell-Mediated Functional Pulp Regeneration

The preservation of vital dental pulp with vasculature and nerve components remains one of the most significant challenges in modern dentistry. Due to the immense potential for neurovascularization, mesenchymal stem cell (MSC) transplantation has shown emerging promise in regenerative medicine and dental translational practice. Actually, pulp mesenchymal stem cells, including postnatal dental pulp stem cells (from permanent teeth) and stem cells from human exfoliated deciduous teeth, possess unique properties based on their origins from neural crest or glial cells. Furthermore, they reside in a neurovascular niche and have the potential for neurogenesis, angiogenesis, and neurovascular inductive activity. According to current pulp regeneration strategies, pulp stem cell-mediated approaches to regeneration have demonstrated convincing evidence that they can rebuild the complex histologic structure of native pulp in situ with highly organized physiologic patterns or even achieve de novo regeneration of complete dental pulp tissues. More importantly, recent clinical studies emphasized in situ neurovascularization outcomes in successful regeneration of vitalized pulp via pulp stem cell transplantation. In this review, we summarize recent breakthroughs in pulp stem cell-mediated pulp regeneration, emphasizing the crucial achievement of neurovascularization. This functional pulp regeneration represents an innovative and promising approach for future regenerative endodontics.

Concentration-dependent effect of bleaching agents on the immunolabelling of interleukin-6, interleukin-17 and CD5-positive cells in the dental pulp

AIM

To evaluate lymphocyte-like cell activation (CD5-positive cells) and the expression of interleukin (IL)-6 and IL-17 in the pulp after tooth bleaching with two concentrations of hydrogen peroxide (H₂ O₂ ).

METHODOLOGY

The right and left maxillary molars from 40 rats were treated randomly with bleaching gel with 20% H₂ O₂ (BLUE group, 1 application of 50 min), 35% H₂ O₂ (MAXX group, three applications of 15 min), or placebo gel (control). After 2 and 30 days, the rats were killed (n = 10), and the jaws were processed for histological and immunohistochemistry analysis of the pulp tissue. The scores of inflammation and immunolabelling (IL-6/IL-17) were submitted to Mann-Whitney and Kruskal-Wallis followed Dunn tests, respectively; anova tests were used for comparisons of number of CD5-positive cells and pulp chamber area values (P < 0.05).

RESULTS

At 2 days, 60% of specimens of the BLUE group were associated with moderate inflammation in pulp horns, and in the MAXX group with necrosis (P < 0.05). At 30 days, the pulp was organized, and tertiary dentine was formed. The MAXX group had superior immunolabelling of IL-17 at 2 days differing significantly from other groups (P < 0.05). At 2 days, 90% of the specimens of the BLUE group had moderate immunolabelling of IL-6, and 50% of the MAXX group had severe immunolabelling, both significantly different from the control (P < 0.05). There was no significant difference between the groups at 30 days (P > 0.05). CD5-positive cells were present at 2 and 30 days, particularly in the bleached groups (P < 0.05), without significant difference between time periods (P > 0.05).

CONCLUSIONS

IL-6 and IL-17 participated in inflammation in the pulp tissue of rats after tooth bleaching, particularly at 2 days. The immunolabelling was greater with increasing H₂ O₂ concentration. This process was accompanied by the prolonged activation of CD5-positive cells.

Tissue engineering: Dentin - pulp complex regeneration approaches (A review)

Dental pulp is a highly specialized tissue that preserves teeth. It is important to maintain the capabilities of dental pulp before a pulpectomy by creating a local restoration of the dentin-pulp complex from residual dental pulp. The articles identified were selected by two reviewers based on entry and exit criteria. All relevant articles indexed in PubMed, Springer, Science Direct, and Scopus with no limitations from 1961 to 2016 were searched. Factors investigated in the selected articles included the following key words: Dentin-Pulp Complex, Regeneration, Tissue Engineering, Scaffold, Stem Cell, and Growth Factors. Of the 233 abstracts retrieved, the papers which were selected had evaluated the clinical aspects of the application of dentin-pulp regeneration. Generally, this study has introduced a new approach to provoke the regeneration of the dentin-pulp complex after a pulpectomy, so that exogenous growth factors and the scaffold are able to induce cells and blood vessels from the residual dental pulp in the tooth root canal. This study further presents a new strategy for local regeneration therapy of the dentin-pulp complex. This review summarizes the current knowledge of the potential beneficial effects derived from the interaction of dental materials with the dentin-pulp complex as well as potential future developments in this exciting field.

Parallels between Tooth Development and Repair: Conserved Molecular Mechanisms following Carious and Dental Injury

The reparative mechanisms that operate following carious and traumatic dental injury are critical for pulp survival and involve a series of highly conserved processes. It appears that these processes share genetic programs—linked to cytoskeletal organization, cell movement, and differentiation—that occur throughout embryogenesis. Reactionary dentin is secreted by surviving odontoblasts in response to moderate stimuli, leading to an increase in metabolic activity. In severe injury, necrotic odontoblasts are replaced by other pulp cells, which are able to differentiate into odontoblast-like cells and produce a reparative dentin. This complex process requires the collaborative efforts of cells of different lineage. The behavior of each of the contributing cell types during the phases of proliferation, migration, and matrix synthesis as well as details of how growth factors control wound cell activities are beginning to emerge. In this review, we discuss what is known about the molecular mechanisms involved in dental repair.

WNT6 promotes the migration and differentiation of human dental pulp cells partly through c-Jun N-terminal kinase signaling pathway

INTRODUCTION

During the dental pulp repair process, human dental pulp cells (HDPCs) migrate to injury sites where they may differentiate into odontoblastlike cells. WNT6 plays a role in dental development and can activate a noncanonical pathway including the c-Jun N-terminal kinase (JNK) pathway. The mechanism of WNT6 in dental pulp repair is still unknown. The purpose of this study was to explore the potential role of the WNT6/JNK signaling pathway in the promotion of cell migration and the differentiation of HDPCs.

METHODS

The third passage of HDPCs were cultured in vitro and treated with WNT6 conditioned medium with or without the pretreatment of JNK inhibitor SP600125. The activation of JNK was detected by Western blot, the expression of c-Jun was quantified by reverse-transcription polymerase chain reaction, the migration of HDPCs was determined by wound healing and transwell migration assays, and the differentiation of HDPCs was investigated using alkaline phosphatase staining and alizarin red staining. The expression of odontogenesis-related genes such as Runt-related transcription factor 2, dentin sialophosphoprotein, and dentin matrix protein 1 was quantified.

RESULTS

WNT6 activates the JNK pathway in HDPCs and enhances cell migration, mineralization nodule formation, and alkaline phosphatase activation. WNT6 also increases the expression of Runt-related transcription factor 2, dentin sialophosphoprotein, and dentin matrix protein messenger RNA in HDPCs. Blockage of the JNK pathway in HDPCs decreases but does not completely abolish the cell migration and differentiation capacity induced by WNT6.

CONCLUSIONS

WNT6 activates the JNK signaling pathway in HDPCs, leading to migration and differentiation.

Stem cells in dentistry--part I: stem cell sources

Stem cells can self-renew and produce different cell types, thus providing new strategies to regenerate missing tissues and treat diseases. In the field of dentistry, adult mesenchymal stem/stromal cells (MSCs) have been identified in several oral and maxillofacial tissues, which suggests that the oral tissues are a rich source of stem cells, and oral stem and mucosal cells are expected to provide an ideal source for genetically reprogrammed cells such as induced pluripotent stem (iPS) cells. Furthermore, oral tissues are expected to be not only a source but also a therapeutic target for stem cells, as stem cell and tissue engineering therapies in dentistry continue to attract increasing clinical interest. Part I of this review outlines various types of intra- and extra-oral tissue-derived stem cells with regard to clinical availability and applications in dentistry. Additionally, appropriate sources of stem cells for regenerative dentistry are discussed with regard to differentiation capacity, accessibility and possible immunomodulatory properties.

Sequential expression of endothelial nitric oxide synthase, inducible nitric oxide synthase, and nitrotyrosine in odontoblasts and pulp cells during dentin repair after tooth preparation in rat molars

Nitric oxide (NO) stimulates osteoblast differentiation, but whether NO contributes to odontoblast differentiation during dentin repair is unknown. By using reverse transcription/polymerase chain reaction and immunostaining, we investigated the gene expression and/or immunolocalization of endothelial NO synthase (eNOS), inducible NOS (iNOS), and nitrotyrosine (a biomarker for NO-derived peroxinitrite), and alkaline phosphatase (ALP) and osteocalcin (early and terminal differentiation markers of odontoblasts, respectively) in dental pulp tissue after rat tooth preparation. At the early stage (1-3 days) post-preparation, markedly increased expression of iNOS and nitrotyrosine was found in odontoblasts and pulp cells beneath the cavity, whereas eNOS expression was significantly decreased. ALP mRNA expression was significantly increased after 1 day but decreased after 3 days, whereas ALP activity was weak in the dentin-pulp interface under the cavity after 1 day but strong after 3 days. Osteocalcin mRNA expression was significantly increased at this stage. At 7 days post-preparation, tertiary dentin was formed under the cavity. All the molecules studied were expressed at control levels in odontoblasts/pulp cells beneath the cavity. These findings show that abundant NO is released from odontoblasts and pulp cells at an early stage after tooth preparation and indicate that, after tooth preparation, the up-regulation of iNOS and nitrotyrosine in odontoblasts is synchronized with increased cellular expression of ALP and osteocalcin. Therefore, the NO synthesized by iNOS after tooth preparation probably participates in regulating odontoblast differentiation during tertiary dentinogenesis.

The impact of bioactive molecules to stimulate tooth repair and regeneration as part of restorative dentistry

After implantation in the exposed pulp, some molecules of the den-tin extracellular matrix induce the formation of a reparative dentinal bridge in the coronal pulp. In some cases, total occlusion of the root canal also is observed. This is the case for bone sialoprotein, bone morphogenetic protein-7, Dentonin (a fragment from matrix extracellular phosphoglycoprotein), and two small amelogenin gene splice products (A+4 and A-4). Cells implicated in the reparative process are recruited, proliferate, and differentiate into osteoblast-like and odontoblast-like cells. The same results may be obtained by direct implantation of odontoblast progenitor cell into the pulp.

Expression of osteocalcin and Jun D in the early period during reactionary dentin formation after tooth preparation in rat molars

We examined, in rats, the expression of osteocalcin and Jun D in the early stage of reactionary dentin formation after tooth preparation and the accompanying morphological changes. Reverse transcription/polymerase chain reaction analysis revealed strong expression of osteocalcin mRNA in pulp tissue at 2 and 3 days post-preparation compared with that in control teeth. Light microscopy demonstrated that, at the dentin-pulp interface, damaged odontoblasts were detached from the dentin matrix immediately after preparation, with neutrophils lining the dental surface after 1 day. After 2-3 days, differentiated odontoblasts appeared at the interface. Reactionary dentin with tubular structures was formed under the cavity after 10 days. Immunoelectron microscopy showed that trace amounts of osteocalcin were expressed in odontoblasts at 2 days post-preparation, and abundant osteocalcin was found in the highly developed Golgi apparatus and granules at 3 days post-preparation. Osteocalcin was also found on type I collagen fibrils in newly formed predentin. The existing dentinal tubules were filled with osteocalcin-coated type I collagen fibrils. We observed, by immunohistochemistry, that Jun D was temporally expressed in the nuclei of the odontoblasts at 1 and 2 days post-preparation. However, no Jun D was found in the dental pulp cells at any other time or in control teeth. Thus, osteocalcin expression is correlated with reactionary dentin formation, and Jun D is associated with osteocalcin expression in odontoblasts. Osteocalcin may also serve as an obturator of the dentinal tubules to protect dental pulp vitality against external irritants after preparation.

CELLS AND EXTRACELLULAR MATRICES OF DENTIN AND PULP: A BIOLOGICAL BASIS FOR REPAIR AND TISSUE ENGINEERING

Odontoblasts produce most of the extracellular matrix (ECM) components found in dentin and implicated in dentin mineralization. Major differences in the pulp ECM explain why pulp is normally a non-mineralized tissue. In vitro or in vivo, some dentin ECM molecules act as crystal nucleators and contribute to crystal growth, whereas others are mineralization inhibitors. After treatment of caries lesions of moderate progression, odontoblasts and cells from the sub-odontoblastic Höhl's layer are implicated in the formation of reactionary dentin. Healing of deeper lesions in contact with the pulp results in the formation of reparative dentin by pulp cells. The response to direct pulp-capping with materials such as calcium hydroxide is the formation of a dentinal bridge, resulting from the recruitment and proliferation of undifferentiated cells, which may be either stem cells or dedifferentiated and transdifferentiated mature cells. Once differentiated, the cells synthesize a matrix that undergoes mineralization. Animal models have been used to test the capacity of potentially bioactive molecules to promote pulp repair following their implantation into the pulp. ECM molecules induce either the formation of dentinal bridges or large areas of mineralization in the coronal pulp. They may also stimulate the total closure of the pulp in the root canal. In conclusion, some molecules found in dentin extracellular matrix may have potential in dental therapy as bioactive agents for pulp repair or tissue engineering.

Pulp biology: progress during the past 25 years

During the past 25 years there has been a rapid expansion in our knowledge of the dentine and pulp complex. This paper provides representative examples of important advances that researchers have made in this field. Topics to be considered include: differentiation of odontoblasts, dentine matrix proteins, extent of odontoblast processes, pulpal stem cells, apoptosis, interstitial fluid pressure in normal and inflamed pulps, class II antigen-presenting cells of the pulp, cytokines, antibodies, pulpal calcifications, tertiary dentine and pulpal inflammation associated with bacterial contamination of exposed dentine beneath restorations.

Transient co-localization of c-Jun N-terminal kinase and c-Jun with heat shock protein 70 in pulp cells during apoptosis

The c-Jun N-terminal kinase (JNK) pathway and heat-shock proteins (HSPs) are involved in stress-induced apoptosis. Here we examined the association of JNK, c-Jun, and anti-apoptotic HSPs with pulp apoptosis during wound healing. In normal pulp, c-Jun was activated only in a few pulp cells, but JNK was not. HSP70 was expressed in the cytoplasm of pulp cells. One day after injury, active JNK and c-Jun were detected in apoptotic pulp cells, whereas HSP70 was detected in non-apoptotic cells. We also found the translocation of HSP70 into nuclei of pulp cells, and co-localization with active JNK and c-Jun. Four days after injury, active JNK and c-Jun disappeared in pulp cells, and HSP70 was relocalized from nuclei to the cytoplasm. These results suggest that the JNK pathway may be one of the compartments inducing apoptosis in pulp cells, and that HSP70 may have an inhibitory role in the apoptosis of pulp cells during wound healing.

Differential induction of apoptosis by capping agents during pulp wound healing

Effects of capping agents on two waves of apoptosis during pulp wound healing were examined. After cavity preparation of rat molars, cavities were filled with calcium hydroxide, zing oxide eugenol cement, or 4-META/MMA-TBB resin (4MMT) and some were unfilled. One hour or 1 day after filling, we examined the distribution and the number of apoptotic cells by terminal deoxynucleotidyl transferase-mediated labeling (TUNEL) assay. One hour after filling, there were no differences in distribution patterns or the number of apoptotic odontoblasts among the four groups. One day after filling, we found differences in distribution patterns and the number of apoptotic pulp cells among the four groups. Especially in the 4MMT group, the distribution pattern of apoptotic cells was more broadly spread, and the number of apoptotic cells was significantly larger than those of other groups. These results suggest that capping agents may have an effect on pulp apoptosis and that 4MMT may actively induce apoptosis during pulp wound healing.

Application of bioactive molecules in pulp-capping situations

To evaluate the effects of bioactive molecules in pulpal wound healing, we carried out experiments using the rat upper molars as an in vivo model. Cavities were prepared on the mesial aspect, and pulp perforation was accomplished by the application of pressure with the tip of a steel probe. After the pulp-capping procedure, the cavities were filled with a glass-ionomer cement. Comparison was made between and among: (1) sham-operated controls with dentin and predentin fragments implanted in the pulp during perforation after 8, 14, and 28 days; (2) carrier without bioactive substance; (3) calcium hydroxide; (4) Bone Sialoprotein (BSP); (5) different concentrations of Bone Morphogenetic Protein-7 (BMP-7), also termed Osteogenic Protein-1 (OP-1); and (6) N-Acetyl Cysteine (NAC), an anti-oxidant agent preventing glutathione depletion. Histologic and morphometric comparison, carried out among the first 4 groups on demineralized tissue sections, indicated that, at 28 days after implantation, BSP was the most efficient bioactive molecule, inducing homogeneous and well-mineralized reparative dentin. BMP-7 gave reparative dentin of the osteodentin type in the coronal part of the pulp, and generated the formation of a homogeneous mineralized structure in the root canal. These findings indicate that the crown and radicular parts of the pulp bear their own specificity. Both BSP and BMP-7 were superior to calcium hydroxide in their mineralization-inducing properties, and displayed larger areas of mineralization containing fewer pulp tissue inclusions. The overall mineralization process to these molecules appeared to proceed by mechanisms that involved the recruitment of cells which differentiate into osteoblast-like cells, producing a mineralizing extracellular matrix. We also provide preliminary evidence that NAC induces reparative dentin formation in the rat molar model. Pulp-capping with bioactive molecules provides new prospects for dental therapy.

Primary and secondary induction of apoptosis in odontoblasts after cavity preparation of rat molars

The death regulation of damaged pulp cells after cavity preparation is not well-known. In this study, we examined whether apoptosis is associated with the death regulation of damaged pulp cells. In normal rat molars, terminal deoxynucleotidyl transferase-mediated labeling (TUNEL)-positive cells were not observed. Just after surgery, odontoblasts under cavities were TUNEL-positive, and these signals disappeared in six hours. One day after surgery, we found the reappearance of TUNEL-positive cells in the subodontoblastic region under cavities, and positive signals disappeared in four days. Ultrastructure of TUNEL-positive cells showed characteristics typical of apoptotic cells. Phagocytosis of apoptotic cells by scavenger cells was also observed. By immunohistochemistry, we also found Bcl-2-positive odontoblasts one day after surgery. These results suggest that two waves of apoptosis are induced in odontoblasts after cavity preparation, and that apoptotic cells must be eliminated before the initiation of reparative dentinogenesis.

Postnatal human dental pulp stem cells (DPSCs) in vitro and in vivo

Dentinal repair in the postnatal organism occurs through the activity of specialized cells, odontoblasts, that are thought to be maintained by an as yet undefined precursor population associated with pulp tissue. In this study, we isolated a clonogenic, rapidly proliferative population of cells from adult human dental pulp. These DPSCs were then compared with human bone marrow stromal cells (BMSCs), known precursors of osteoblasts. Although they share a similar immunophenotype in vitro, functional studies showed that DPSCs produced only sporadic, but densely calcified nodules, and did not form adipocytes, whereas BMSCs routinely calcified throughout the adherent cell layer with clusters of lipid-laden adipocytes. When DPSCs were transplanted into immunocompromised mice, they generated a dentin-like structure lined with human odontoblast-like cells that surrounded a pulp-like interstitial tissue. In contrast, BMSCs formed lamellar bone containing osteocytes and surface-lining osteoblasts, surrounding a fibrous vascular tissue with active hematopoiesis and adipocytes. This study isolates postnatal human DPSCs that have the ability to form a dentin/pulp-like complex.

Localization of transcription factor AP-1 family proteins in ameloblast nuclei of the rat incisor

We examined by immunocytochemistry the localization of the AP-1 family proteins c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, and Fra-2 in rat incisor ameloblasts. Most of the antibodies against AP-1 family proteins, except for c-Fos-specific antibody, labeled ameloblast nuclei. The labeling intensity of the c-Jun, JunD, and Fra-2 antibodies was stronger than that of JunB, FosB, and Fra-1. Antibody reactivities of c-Jun, JunD, and Fra-2 were greatly enhanced during or after the transition zone. Furthermore, c-Jun antibodies labeled maturation ameloblasts in a cyclic pattern, which was correlated with ameloblast modulation. Disruption of ameloblast modulation by colchicine injection resulted in greatly decreased reactivity of the c-Jun antibody in the ameloblast nuclei of the maturation zone. Phospho-specific antibodies to c-Jun labeled ameloblast nuclei only weakly throughout the secretion, transition, and maturation zones. These results suggest that the stage-specific localization of AP-1 in ameloblasts is closely related to tooth enamel formation.