Ultrastructure of a new generation of odontoblasts in grafted coronal tissues of mouse molar tooth germs

Differentiation potential of dental papilla, dental pulp, and apical papilla progenitor cells

INTRODUCTION

Regenerative endodontic procedures use the differentiation potential of embryonic and adult pulp progenitor cell populations to reconstitute dental structures.

METHODS

An in-depth search of the literature was accomplished to review biologic knowledge from basic research on tooth morphogenesis and differentiation, root development, dentin-pulp regeneration, pulp revascularization and apexification, experimental and clinical studies on the dentinogenic differentiation potential of progenitor cells in the embryonic dental papilla, dental pulp, and associated mesenchymal tissues of the developing root.

RESULTS

Odontogenic potential is determined during early tooth morphogenesis in the odontogenic mesenchyme. Progenitor cells from the odontogenic mesenchyme give rise to primary dentin-forming cells (odontoblasts) in the presence of stage-specific enamel epithelium and/or basement membrane and tertiary dentin-forming cells (odontoblast-like cells) in experimental conditions. The specificity of odontogenic mesenchymal cells to form tertiary dentin might be related to the repertoire of signaling pathways operated by the temporospatial pattern of epithelial-mesenchymal interactions during tooth formation. Dental papilla cells isolated from tooth germs before the onset of odontoblast differentiation have not shown any competence to become odontoblasts in the absence of enamel epithelium. On the other hand, the specificity of progenitor cells in the mesenchymal cell populations of the developing root apex remains to be determined.

CONCLUSIONS

It seems evident that the dental pulp might be only used as a source of progenitor cells with dentinogenic competence for the regeneration of the dentin-pulp complex. The nature of dental or apical papilla progenitor cells in terms of their specificity for dentin regeneration has to be first characterized.

Preliminary Study of Tissue-Engineered Odontogenesis in the Canine Jaw

PURPOSE

We previously reported a method for the development of tissue-engineered tooth. However, 1 drawback of the procedure was the inability to determine whether the tooth would function when transplanted in the jaw because it was formed in the omentum of the abdomen. Therefore, the present study was designed to evaluate whether transplantation of dissociated odontogenic cells could induce tissue-engineered odontogenesis in the canine jaw.

MATERIALS AND METHODS

Cells were harvested from canine first molar tooth buds and the resulting heterogeneous cell population was seeded on a biodegradable polymer. These constructs were then transplanted into the same sockets after extracting the tooth buds. After transplantation, we evaluated the transplanted constructs using dental x-ray, micro-computed tomography, histology, and immunohistochemistry.

RESULTS

After 24 weeks, micro-x-ray computed tomography showed regenerated hard tissues in the jaw, and hematoxylin and eosin staining showed tubular dentin and bone. In the regenerated tissue, osteopontin, osteonectin, and osteocalcin antibodies stained the dentinal matrix. However, enamel tissue and dental-root formation were not observed.

CONCLUSION

These data show for the first time the formation of dentin and bone from dissociated odontogenic cells in the canine jaw.

Development of transplanted pulp tissue containing epithelial sheath into a tooth-like structure

The aim of these studies was to find out whether intact neonatal pulp tissue containing residual epithelial cells can induce the development of a tooth-like structure in situ. First maxillary neonatal hamster molar pulps containing adhering undifferentiated epithelial cells were transplanted submucosally in the oral cavity of recipient mothers for periods ranging from 2-8 weeks and the tissues were then processed for light microscopy. Developing tooth-like structures containing mineralised tubular dentine, predentine and a vascularised pulp-like chamber lined with functional odontoblast-like cells were observed in the specimens within 2 weeks of transplantation. Enamel and root formation were not observed. These data indicate that neonatal dental pulp tissues containing epithelial cell remnants have the capacity to develop into tooth-like structures and that this could be the explanation for the development of tooth-like structures sometimes observed in infants after extraction of a natal tooth.

Stimulation of proliferation and differentiation of dog dental pulp cells in serum-free culture medium by insulin-like growth factor

Insulin, insulin-like growth factors (IGF) I and II are considered to play an important part in the growth and differentiation of dental pulp cells. The present study examined the effects of these factors on pulp cells in serum-free culture conditions. The DNA content and alkaline phosphatase (ALPase) activity of dog pulp cells increased when they were cultured in a serum-free medium supplemented with transferrin, yolk lipoprotein and basic fibrobrast growth factor (TYF medium). The pulp cells produced type I collagen but not type III, suggesting that they might proliferate and differentiate into odontoblast-like cells in a serum-free culture. Both IGF-I and IGF-II enhanced the ALPase activity of pulp cells cultured in TYF medium to an equivalent level, but a higher concentration of IGF-II was necessary to produce a similar effect on DNA synthesis to that of IGF-I. Insulin dose-dependently enhanced DNA synthesis and increased ALPase activity, but its effects were weaker than those of the IGFs. These findings suggest that IGF-I might have a primary role in the growth and differentiation of pulp cells.

Effects of recombinant basic fibroblast growth factor, insulin-like growth factor-II and transforming growth factor-beta 1 on dog dental pulp cells in vivo

The effects of recombinant basic fibroblast growth factor (bFGF), insulin-like growth factor (IGF)-II and transforming growth factor (TGF)-beta 1 on dental pulp cells were investigated by light and transmission electron microscopy after their implantation for 1 and 3 weeks at central sites of mechanically exposed pulps in dog molar and canine teeth. The implants were Millipore filters that have been soaked with solutions containing 100 or 500 ng/ml of bFGF or IGF-II or 100 ng/ml of TGF-beta 1. Control filters were soaked with dog albumin. No changes in cell organization or matrix synthesis were seen after implantation of control filters. Groups of columnar, polarized cells with numerous mitochondria and Golgi elements or elongated cells unassociated with any matrix deposition were demonstrated after 1 or 3 weeks, respectively, in close proximity to the filters that had been soaked with bFGF solution; at a distance from these implants enhanced formation of an osteotypic matrix was seen beneath the exposure site. No particular response was found in close proximity to the filters that had been soaked with IGF-II solution after 1 or 3 weeks implantation but thick zones of osteodentine were found beneath the exposure site and at adjacent circumferential dentine sites. Numerous elongated, polarized cells with long cytoplasmic extensions invading the filter pores were consistently seen after 1 week in close proximity to the filters that had been soaked with TGF-beta 1 solution. After 3 weeks implantation of these filters, deposition of a tubular matrix surrounding the implants was seen in association with the highly elongated odontoblast-like cells, while enhancement of circumferential dentine formation was also found at adjacent peripheral sites. These experiments demonstrate that TGF-beta 1 when implanted for short term periods at central pulp sites exerted dentine-specific effects, inducing differentiation of odontoblast-like cells and stimulating primary odontoblasts. Implantation of bFGF and IGF-II did not result in reparative dentine formation, but did stimulate osteotypical matrix deposition at a distance from the implants.

Induction of odontoblast-like cell differentiation in dog dental pulps after in vivo implantation of dentine matrix components

The effects of dentine extracellular matrix components on dental mesenchymal cells were studied by light and transmission electron microscopy after their implantation at central sites of mechanically exposed pulps in dog molar teeth. The implants were Millipore filters that had been soaked with solutions containing 30 or 300 micrograms/ml of an EDTA-soluble fraction of rabbit incisor dentine. Control filters were soaked with dog albumin or phosphate buffered saline. Columnar, polarized cells were consistently seen after 8 days in close proximity to the filters coated with both concentrations of dentine matrix components. Characteristic features of these polarized cells included widened cisternae of the rough endoplasmic reticulum, a rich microfilamentous network in the long cytoplasmic extensions invading the filter pores and numerous cytoplasmic bodies. These cells also showed evidence of functional as well as cytological differentiation. Polarized processing of secretory granules could be observed after 8 days' implantation, and also the presence of matrix vesicles and deposition of a fine, collagenous matrix into the filters apically to the distal end of the cytoplasmic processes. After 24 days' implantation, secretion of a tubular matrix could be consistently seen in association with the odontoblast-like cells. No changes in cell organization or matrix synthesis were seen after implantation of control filters. These studies demonstrate that bioactive components present in the EDTA-soluble dentine matrix fraction are able to directly induce cell polarization and apical secretion of tubular matrix when implanted in contact with dental pulp cells at sites remote from the odontoblast layer.

Immunolocalization of fibronectin during the early response of dog dental pulp to demineralized dentine or calcium hydroxide-containing cement

The role of fibronectin during the events initiating the post-developmental histogenesis of dentine was investigated by exposing the pulp to implants of autogenous demineralized dentine or calcium hydroxide-containing cement for short periods. Implants exposed for 3 days were processed for immunoelectron-microscopic analysis of fibronectin adsorption on to their surfaces. The localization of fibronectin in the critical area of interaction was examined by immunofluorescence 6, 14 and 21 days after implantation. Heavy adsorption of fibronectin on to the dentine implants and the crystalline structures that had been deposited on the cement implants was demonstrated. Positive fluorescence was consistently seen around dentine implants. Strongly immunopositive fibroblast-like cells and weakly reactive, differentiating odontoblast-like cells were found in association with the implanted matrix. Uncalcified matrix secreted by the polarized or non-polarized cells was consistently rich in fibronectin. Fibroblast-like cells exhibiting intense immunoreaction only at 14 and 21 days were mainly associated with the crystalline precipitates on the cement surfaces or within the surrounding pulp. The findings indicate that the specific inductive effects of demineralized dentine on pulp cells are initiated by exposure of the pulp to a fibronectin-containing surface; adhesion of pulp cells and synthesis of a fibronectin-rich matrix characterize the development of new dentine. The reparative response to non-specific inductive influences such as calcium hydroxide seems to be mediated by progressive enhancement of fibronectin synthesis in pulp cells.

ED-A region-containing isoform of cellular fibronectin is present in dentin matrix in dentinogenesis imperfecta associated with osteogenesis imperfecta

To elucidate the defective dentin formation in osteogenesis imperfecta (OI), we analyzed the expression of selected fibronectin (FN) isoforms in the dentin matrix of a patient with dentinogenesis imperfecta (DI) associated with OI, and in normal teeth. Frozen tooth sections were immunostained with three monoclonal antibodies (MAbs). The MAb recognizing the major cell-binding region (f-33), shared by plasma FN (pFN) and cellular FN (cFN), stained the pulp of normal adult permanent teeth intensely, while no reactivity was present in predentin, (demineralized) dentin, or dental cementum. The periodontal ligament stained unevenly. The dentin matrix of the patient with OI displayed reactive zones, alternating layerwise or concentrically with non-reactive ones. Staining throughout the connective tissue of adult oral mucosa, analyzed for the form of FN present, was intense, and in dermis, which was also studied, it was moderate. Reactivities in dental tissues with the MAb specific for the ED-A region (IST-9), included in cFN but not pFN, were similar to those with MAb f-33. The mucosal connective tissue stained weakly and dermis was negative, except that nerves and endothelia of some large blood vessels stained clearly. The MAb specific for the ED-B segment (BC-1), also included in cFN only, did not stain any of the tissues analyzed. The results suggest that, unlike mucosal and dermal FNs, FNs in the dental tissues are largely cellular, and also that dentin formation in OI may be completed by successive generations of pulpal fibroblasts differentiated into hard-tissue-forming cells.

The effect of dietary xylitol on dentin formation in ovariectomized rats

Twenty-nine 3-month-old female Wistar rats were labeled by means of a single intraperitoneal tetracycline injection. Nineteen animals were subsequently ovariectomized, whereas a control group of 10 animals underwent sham operations. All the animals received the basal diet, and 10 of the ovariectomized animals were given an additional dietary xylitol supplementation (5%). Three months later the animals were killed by decapitation, and dentinal apposition on the molars was measured. The results indicate that supplementation of the diet with 5% xylitol had an attenuating effect on the enhanced dentin formation caused by ovariectomy, but the mechanism remains unsolved.

Mechanisms controlling secondary initiation of dentinogenesis: a review

Dentinogenesis can be initiated secondarily as an intrinsic ability of the dental pulp to repair, or after interaction of pulp cells with specific exogenous inductive factors. In the present article the basic developmental aspects, highlighting the mechanism by which dentinogenesis is initiated during tooth development, are discussed. Furthermore, clinical and experimental observations concerning the events taking place during secondary initiation of dentine formation, as part of exposed or non-exposed pulp tissue repair, or as a result of dentine matrix or other chemical-pulp cell interactions, are reviewed. Discussion includes hypotheses relating to the crucial biological steps leading to expression of odontoblastic-like cell phenotype and secondary initiation of dentine histogenesis.

Dentinogenesis

The formation of dentin, dentinogenesis, comprises a sophisticated interplay between several factors in the tissue, cellular as well as extracellular. Dentin may be regarded as a calcified connective tissue. In this respect, as well as in its mode of formation, it is closely related to bone. Using dentinogenesis as an experimental model to study biomineralization provides several practical advantages, and the results may be extrapolated to understand similar processes in other tissues, primarily bone. After describing dentin structure and composition, this review discusses items such as the morphology of dentinogenesis; the dentinogenically active odontoblast, transport, and concentrations of mineral ions; the constituents of the dentin organic matrix; and the presumed mechanisms involved in mineral formation.

Dentinogenic activity of allogenic plasma fibronectin on dog dental pulp

The response of ectomesenchymal cells of dog dental pulp to implantation of Millipore filters supplemented with bovine plasma fibronectin was evaluated after observation periods of one or four weeks. Two concentrations of plasma fibronectin were used (0.2 and 1 mg/mL). Experiments also included implants treated with control solutions (PBS or 1 mg/mL of dog albumin). Formation of a layer of elongated, polarized cells was demonstrated in direct contact with the implants treated with 1 mg/mL of plasma fibronectin solution, after one week post-operatively. Microfilamentous organization and orientation of rough endoplasmic reticulum was observed mainly in the supranuclear zone of the polarized cells. Implants treated with the same solution were consistently surrounded by a thick layer of dentinal matrix after four weeks of their exposure to pulp sites. Implants treated with control solutions or with the low concentration of fibronectin never showed any sign of cell polarization and matrix synthesis. These data provide evidence that the pulp cells can express their odontoblastic phenotype in response to a surface containing concentrated fibronectin (even allogenic), without the need of other molecules as exogenous inductive factors.

Short-term dentinogenic response of dog dental pulp tissue after its induction by demineralized or native dentine, or predentine

The events initiating the expression of odontoblastic potential by pulpal ectomesenchymal cells were investigated by exposing the pulp to demineralized, native and unmineralized autogenous dentine. The pulp responses to implants were histologically evaluated 3, 7 and 10 days postoperatively, while the surface structure of the newly mineralized matrices was examined 12 and 28 days after implantation. Differentiation of odontoblast-like cells in close proximity to the implanted matrix was consistently demonstrated after exposure to predentine. Scattered columnal cells undergoing polarization, characterized ultrastructurally by the orientation of their rough endoplasmic reticulum, were also found in direct contact with the demineralized dentine. However, in response to demineralized implants, groups of differentiated odontoblast-like cells were clearly seen only in association with a zone of matrix secreted in a polar, predentine-like pattern, indicating an asynchronous inductive influence of this type of implant on pulp cells. Further, the response of pulp cells to native dentine was characterized by the elaboration of a two-layered matrix (a fibrous and a polarly deposited matrix) before initiation of secondary dentinogenesis. Scanning electron microscopy of the newly deposited matrices revealed differences between the indirect matrix synthesis, observed in short-term response to implants of demineralized or native dentine, and the specific, dentinogenic function of the odontoblast-like cells. These observations indicate that the dentine-induced dentinogenesis is initiated by two mechanisms--direct induction of odontoblast-like cells as well as indirect matrix synthesis, which further controls cell polarization. Immobilization of the cells on implanted matrix seems to be the critical requirement for direct expression of the odontoblastic phenotype.

Expression of type I collagen pro-alpha 2 chain mRNA in adult human permanent teeth as revealed by in situ hybridization

The expression of the gene COL1A2, coding for the pro-alpha 2 chain of type I pro-collagen, was analyzed in fully developed human permanent teeth. The teeth were fixed with formalin, demineralized with EDTA for about ten weeks, and embedded in paraffin. Pro-alpha 2(I) mRNA was localized in the sections by in situ hybridization, with use of [35S)]-labeled single-stranded RNA probes. The amount of mRNA for pro-alpha 2(I) collagen chain, as indicated by the relative densities of silver grains and the grain counts per cell in autoradiography, was high in odontoblasts, whereas in pulpal fibroblasts it was low. High levels of pro-alpha 2(I)mRNA expression were also present in those odontoblasts which had elaborated new dentin matrix in response to dental caries. Expression in the periodontal ligament, including the cementoblast layer, was slightly stronger than that in odontoblasts. The intense expression of pro-alpha 2(I) mRNA in odontoblasts of adult teeth suggests that even after the completion of primary dentin formation, they continue to synthesize heterotrimeric type I collagen molecules. Cell type-specific differences in the expression of pro-alpha 2(I) mRNA imply that type I collagen probably plays a major role in the regulation of the structure and function of dental tissues. Finally, in situ hybridization enabled pro-alpha 2(I) collagen mRNA to be detected in tissue sections even after prolonged demineralization, and thus it proved to be a valuable technique for analysis of gene expression in adult dental tissues, as shown here for COL1A2.

Enamel surface roughness and dental pulp response to coaxial carbon dioxide-neodymium: YAG laser irradiation

The purpose of this study was to evaluate the effect of a coaxial carbon dioxide/neodymium:yttrium aluminium garnet laser beam on enamel surface roughness and the dental pulps of mongrel dogs. In four dogs, four maxillary left posterior teeth were irradiated at 16 cm source-tooth distances. Two teeth were irradiated with 16 W CO2/16 W Nd:YAG and the remaining two with 16 W CO2/40 W Nd:YAG. Two maxillary right teeth were untreated controls. In addition, mandibular premolars were irradiated at the same distance and power levels, extracted, and analysed for surface roughness. Significant differences in surface roughness were found between control samples and either power level, but not between enamel surfaces at the two power levels. Maxillary teeth were removed at 10 days postoperatively, sectioned and stained (H & E). The reaction of pulpal cells to irradiation was scored. Data analysis revealed statistically significant differences between the control and lower power Nd:YAG groups and between the control and higher power Nd:YAG groups. The difference in pulpal response between both laser groups approached significance.

Immunohistochemical localization of the matrix glycoproteins--tenascin and the ED-sequence-containing form of cellular fibronectin--in human permanent teeth and periodontal ligament

The expression of two matrix glycoproteins, tenascin and cellular fibronectin (cFN), has been studied in fully developed human permanent teeth, periodontal ligament, and alveolar bone, in both frozen and paraffin-processed material. Polyclonal antibodies to tenascin and a monoclonal antibody recognizing the ED sequence specific to at least some forms of cFN were used. Staining for both tenascin and cFN was positive in the dental pulp, odontoblastic layer, cementoblast-pre-cementum zone, and on the periosteal as well as endosteal surfaces of the alveolar bone. In the periodontal ligament, cFN was evenly distributed, whereas tenascin was accumulated in the attachment zones. Pre-dentin stained for tenascin but not for cFN. Mineralized dentin and cementum were tenascin- and cFN-negative. The relative staining intensity for tenascin was greater than that for cFN in the cementoblast-pre-cementum layer and in the attachment zones of the periodontal ligament, whereas cFN stained more intensely in the pulp. In frozen material, antigenicities were well-preserved. Paraffin processing facilitated precise recognition of tissue morphology, but the antigenicity of cFN was lost. The co-expression of tenascin and cFN in the dental pulp, cementogenic zone, and on the surfaces of the alveolar bone may reflect the ability of the cells to deposit mineralized tissue matrices. The pronounced expression of tenascin in the interfaces between mineralized and non-mineralized tissues suggests that it is functionally associated with mechanical stress and may thus have at least two distinct functions. The relative amounts of the two matrix glycoproteins may contribute to regulation of tissue structure.

Differentiation of odontoblasts in mouse dental papillae recombined with normal or chemically-treated dentinal matrices

Separation and recombination experiments were made with manually or trypsin-dissociated dental papillae (day 15, 16, 17, 18 in utero and 2, 7, 14 postnatal) and manually isolated hard tissues of the third molar crown (14 postnatal days). Several series of hard tissues were further treated with citric acid, hyaluronidase or sodium hypochlorite. The recombinations were transplanted into the subcutaneous tissue of new-born mice. Grafts were removed 7, 14 and 21 days later and prepared for light and electron microscopy. Whatever the age of the papilla and whatever the treatment of the crowns, well-characterized odontoblasts differentiated and deposited new layers of tubular dentine, except when the recombined dental papilla was 15 days old. These findings indicate that odontoblasts are very early committed (since day 16 in utero) and that they may differentiate in dental papillae in contact with chemically altered dentinal matrices.