Delta-notch signaling in odontogenesis: correlation with cytodifferentiation and evidence for feedback regulation

Recent data suggest that dental cells utilize the evolutonarily conserved Notch-mediated intercellular signaling pathway to regulate their fates. Here we report on the expression and regulation of Delta1, a transmembrane ligand of the Notch receptors, during mouse odontogenesis. Delta1 is weakly expressed in dental epithelium during tooth initiation and morphogenesis, but during cytodifferentiation, expression is upregulated in the epithelium-derived ameloblasts and the mesenchyme-derived odontoblasts. The expression pattern of Delta1 in ameloblasts and odontoblasts is complementary to Notch1, Notch2, and Notch3 expression in adjacent epithelial and mesenchymal cells. Notch1 and Notch2 are upregulated in explants of dental mesenchyme adjacent to implanted cells expressing Delta1, suggesting that feedback regulation by Delta-Notch signaling ensures the spatial segregation of Notch receptors and ligands. TGFbeta1 and BMPs induce Delta1 expression in dental mesenchyme explants at the stage at which Delta1 is upregulated in vivo, but not at earlier stages. In contrast to the Notch family receptors and their ligand Jagged1, expression of Delta1 in the tooth germ is not affected by epithelial-mesenchymal interactions, showing that the Notch receptors and their two ligands Jagged1 and Delta1 are subject to different regulations.

A novel organ culture method to study the function of human odontoblasts in vitro: gelatinase expression by odontoblasts is differentially regulated by TGF-beta1

Odontoblasts cannot be cultured by traditional cell culture methods, thus restricting in vitro studies. Here we present an organ culture method for human odonto-blasts that utilizes the pulp chamber as a culture crucible. Crowns of human third molars were dissected, pulp was gently removed, and the odontoblasts attached to and in the walls of the pulp chambers were cultured in serum-free OPTI-MEM medium, or DMEM/Ham's F12 medium containing 10% serum. Pulp tissues were cultured separately. Cell content and morphology were analyzed by SEM, and the removed pulps were examined by light microscopy. Proteins secreted into the medium with or without TGF-beta1 supplementation were metabolically labeled with [35S]methionine, and the total protein content was assessed by TCA precipitation and SDS-PAGE/fluorography. To assess the role of gelatinolytic enzymes on dentin matrix remodeling, we used enzymography to analyze the effect of TGF-beta1 on gelatinase A and B expression. SEM revealed odontoblasts in pulp chambers after 5 days of culture, with only few or no fibroblasts, and no alterations in the odontoblast cell morphology or differences between the cells cultured in serum-free and serum-containing media. Rarely were any odontoblasts present in pulp tissue. Radiolabeling revealed protein synthesis and secretion until day 6 in both the odontoblast and pulp cultures, with no marked differences between TGF-beta1-treated and control cultures. The level of gelatinase A remained constant up to 7 days, while gelatinase B expression was always low and decreased with time in culture. However, gelatinase B levels were markedly increased upon TGF-beta1 treatment of cells and remained high to day 7. The results suggest that this method provides a novel technique for the study of human odontoblasts in vitro and that odontoblasts can be cultured even in serum-free conditions.

Odontoblast differentiation: a response to environmental calcium?

The response of the dental pulp to calcium hydroxide has been well described but the process of pulpal repair leading to dentinal bridge formation appears complex and the mechanisms remain incompletely understood. Through the precise regulation of the free calcium ion in the cytosol, cells have been able to utilize anions such as phosphates for a wide range of activities such as energy production (oxidative phosphorylation). As anions are abundant in the cytosol, intracellular levels of calcium ions are kept low, several orders of magnitude less than that of the surrounding extracellular matrix. Consequently, cells are able to use calcium ions for the regulation of many cellular events. The binding of extracellular molecules such as cytokines, hormones or antibodies, with receptors on the plasma membrane may result in short- or long-term modifications to cellular metabolism, including the mechanisms of intracellular calcium homeostasis. Cell survival depends upon the ability to adapt to changes in the cell's micro-environment. Adaptation in turn results in altered cellular activity that may be interpreted as showing that the cell has become more or less specialised. In some instances this may include the resumption of mitotic activity. If the rate or magnitude of change exceeds a cell's adaptive capacity, the cell dies. Responses of cells to alterations in their environment are reviewed as they may provide an explanation for the success of calcium hydroxide in facilitating pulpal repair and the differentiation of odontoblasts.

Attachment formation following replantation of cultured cells into periodontal defects--a study in minipigs

Regeneration processes in the periodontium occur by the interaction of different cell populations. It is known that these cells are also capable of forming new periodontal tissue after culture in vitro. The present study investigated whether replanted cultured cells from the periodontium could contribute to attachment formation. Primary cell cultures from alveolar bone and periodontal ligament were obtained from 11 minipigs. Experimentally induced furaction and interdental defects (n = 168) were treated in groups: (a) flap surgery, replantation of alveolar bone cells, and covering of the defects with Teflon membranes (ABC group); (b) flap surgery, replantation of periodontal ligament cells and membranes (PLC group); (c) flap surgery, bone gelatin (carrier material) and membranes (BG group); (d) flap surgery and membranes (NBG group); (e) flap surgery (FS group); and (f) no treatment (NT group). The defects were clinically and histologically (polyfluorochrome labeling) assessed after 10, 30, and 90 days. In the ABC group, initial calcified tissue formation at the roots was apparent after only 8 days. Marked new formation of cementum and alveolar bone and the development of a new attachment were observed after 90 days. In the BG and the NBG groups, wound healing varied depending on membrane healing and the morphology of the defects, which led to significantly poorer and variable results. Similar results were found in the PLC group, although some defects showed extensive cementum and bone formation. Defects in the FS and the NT groups healed largely by epithelialization. The study shows that replantation of cultured alveolar bone cells leads to formation of new cementum and bone, which, in turn, leads to formation of a new attachment. It is likely that the cells stabilize the tissue formation in the defect or on the root surface in the early phase of wound healing and prevent epithelial downgrowth. Results also show that regeneration in the periodontium is determined by the availability of (precursor) cells capable of forming calcified tissues.

Immortalized mouse odontoblast cell line MO6-G3 application for in vitro biocompatibility testing

PURPOSE

This study was designed to determine the usefulness of an established stable immortalized mouse odontoblast cell line (MO6-G3) for dental material biocompatibility testing. Using a standard toxicity assay based on cell respiratory activity, the response to MO6-G3 cells was compared to the mouse fibroblastic cell line, L929, presently used for dental materials testing. The dental resin monomer TEGDMA was used as the dental material for the assay.

MATERIALS AND METHODS

Cell lines (1 x 10(3)/well) were plated in 96 well culture plates and grown in DMEM supplemented with 10% FCS, 100 units/ml each of penicillin and streptomycin, and 50 micrograms/ml ascorbic acid in an atmosphere of 95% air and 5% CO2. Cells were exposed to TEGDMA resin monomer covering a dose range of 1 x 10(-6) to 0.5 x 10(-3) M. Unexposed control cells, as well as cells exposed to the DMSO vehicle in which the TEGDMA was dissolved, were included in all assays. Cytotoxicity was evaluated by determining cell respiratory activity spectrophotometrically using the tetrazolium compound WST-1.

RESULTS

Statistical analysis by ANOVA using Tukey's method for pair wise comparisons as the post hoc test indicated toxic effects of TEGDMA at 1 x 10(-5) M in the odontoblast cell line MO6-G3. By contrast, the monomer produced no toxic effects on the L929 fibroblast cell line after 24 hours of exposure, over the entire concentration range tested. Furthermore, MO6-G3 cells exposed to a concentration of 0.5 x 10(-3) M were unable to recover from the effects of the exposure 48 hours after removal of the resin. MO6-G3 cells exposed to 1 x 10(-4) and 0.5 x 10(-4) TEGDMA recovered 40-50% and 75-80% of control respiratory activity respectively, 48 hours after removal of the resin. Respiratory activity by L929 cells exposed to all TEGDMA concentrations tested was not different from the vehicle control 48 hours after removal of the resin.

Temperature rise in pulpal chamber during fabrication of provisional resinous crowns

STATEMENT OF PROBLEM

The heat generated during the exothermic polymerization reaction of autopolymerizing resinous materials and the heat generated by ultraviolet lamps during irradiation of photopolymerizing resinous materials could cause pulpal damage when a direct technique is used to fabricate provisional restorations. This could occur if temperature elevations overcome the physiological heat dissipating mechanisms of the dental-periodontal system.

PURPOSE

This in vitro study compared the rise in temperatures in the pulpal chamber during fabrication of provisional complete veneer crowns by direct method with different autopolymerizing and photopolymerizing resins. The effect of curing resinous crowns in different matrices, such as a polyvinyl siloxane impression and a vaccuum-formed polypropylene sheet, was also evaluated.

RESULTS

The results demonstrated that the amount of heat generated during resin polymerization and transmitted to the pulpal chamber could be damaging to pulpal tissues including odontoblasts. When curing of provisional resinous crowns was performed in the polyvinyl siloxane impression, significantly lower temperatures were recorded compared with curing in the vacuum-formed polypropylene sheet.

CONCLUSIONS

To prevent pulpal damage, effective cooling procedures are strongly recommended when directly fabricating resinous provisional crowns.

The effect of two sucrose diets on formation of dentin and predentin in growing rats

The effect of two high-sucrose diets on dentinal caries, dentin formation, and the predentin width was studied in Sprague-Dawley rats. Rats were weaned at the age of 3 weeks and for 4 weeks fed a non-cariogenic commercial rat food (R36) for control, a high-sucrose Stephan-Harris (S-H) diet, or a new high-sucrose (sR36) diet in which most of the barley and wheat flour of the control R36 diet were replaced by sucrose. The areas of dentinal caries, the areas of dentin formation, and the width of predentin and dentin were quantified. Both high-sucrose diets induced dentinal caries, and both reduced dentin formation and increased the width of predentin compared with the control diet. Moreover, rats fed the S-H high-sucrose diet showed significantly greater progression of caries and reduction of dentin formation relative to rats fed the new high-sucrose diet, sR36. The high-sucrose diet thus was a substrate for caries-inducing microbes and a significant, but possibly not the exclusive, substrate for host modulation of odontoblast function.

Enamel matrix, cementum development and regeneration

Studies during the last 20 years have indicated that enamel-related proteins are involved in the formation of cementum. In the present article, this relation is further explored. Attention is called to the fact that coronal acellular extrinsic fiber cementum is formed on the enamel surface in a number of species. The composition of the enamel matrix proteins and the expression of these proteins during root formation are briefly reviewed. The dominating constituent of the enamel matrix, amelogenin, is shown by means of immunohistochemistry to be expressed in human teeth during root formation. Amelogenin was also found to be present in Tomes' granular layer of human teeth. When mesenchymal cells of the dental follicle were exposed to the enamel matrix a non-cellular hard tissue matrix was formed at the enamel surface. Application of porcine enamel matrix in experimental cavities in the roots of incisors of monkeys induced formation of acellular cementum that was well attached to the dentin. In control cavities without enamel matrix, a cellular, poorly attached hard tissue was formed. The present studies provide additional support to the idea that enamel matrix proteins are involved in the formation of acellular cementum and also that they have the potential to induce regeneration of the same type of cementum.

Measurements of cytosolic free Ca2+ concentrations in odontoblasts

To investigate the responsiveness of odontoblasts to electrical and mechanical stimuli, the concentrations of cytosolic free Ca2+ ([Ca2+]i) were measured using fura-2 microfluorometry in slice preparations of rat dental pulp. [Ca2+]i under resting conditions (basal [Ca2+]i) were 258.0 +/- 37.7 nM (mean +/- S.E., n = 113). The basal [Ca2+]i of the odontoblasts with remarkably active responses to stimuli were 221.0 +/- 81.1 nM (mean +/- S.E., range: 16-685 nM). Depolarization by high extracellular K+ concentration ([K+]o) caused a rapid increase in [Ca2+]i. The depolarization-induced increase in [Ca2+]i was enhanced by caffeine. The intense depolarization elicited [Ca2+]i oscillation in odontoblasts, which was enhanced by caffeine and suppressed by dantrolene. Hypotonic stimulation also induced an increase in [Ca2+]i of odontoblasts. The results indicate that the odontoblast possesses voltage-dependent Ca2+ channels, caffeine-sensitive Ca2+ stores and mechanosensitive cation channels.

Osteocalcin expression in young and aged dental pulps as determined by RT-PCR

Dental pulps were obtained from third molars of young adults (17-25 yr) or from molar teeth of individuals > 50 yr of age and examined for the expression of osteocalcin (OC) mRNA by RT-PCR. OC was selected as a determinant of pulp vitality, because it has long been associated with the production of hard tissue matrix in teeth and bone. For comparative purposes, the expression of OC in each pulp was normalized relative to its housekeeping gene-product GAPDH by the establishment of a OC/GAPDH ratio. This study demonstrated that OC expression, presumably by cells of odontoblast lineage, does not diminish relative to the extant cell population. Our findings suggest, despite a reduction in volume and cell numbers, that the pulps of aging teeth retain a capacity for dentin deposition and a potential for caries and trauma resistance.

Development, maturation, and aging of the alveolar bone. New insights

Osteoblasts and bone tissue of the mandibular and maxillary alveolar processes substantially differ from osteoblasts and bone in other parts of the skeleton. These differences are apparent during embryonic development, maturation, and aging of these bones. The cellular and molecular basis for these differences is still not clear, but it is unfolding at record speed.

The carboxyl-terminal domain of phosphophoryn contains unique extended triplet amino acid repeat sequences forming ordered carboxyl-phosphate interaction ridges that may be essential in the biomineralization process

Phosphophoryns (PPs), a family of Asp and Ser(P)-rich dentin proteins, are considered to be archetypal regulators of several aspects of extracellular matrix (ECM) biomineralization. We have cloned a rat incisor PP gene, Dmp2, from our odontoblast cDNA library and localized it to mouse chromosome 5q21 within 2 centimorgans of Dmp1, another tooth-specific ECM protein. The carboxyl-terminal region of Dmp2 protein (60 residue % Ser, 31 residue % Asp) is divided into two domains, one with unique repetitive blocks of [DSS]n,3</=14, the other with [SD]m = 2,3. Conformational analysis shows the phosphorylated form of the [DS*S*]n repeats to have a unique structure with well defined ridges of phosphates and carboxyls available for counter ion binding. The [S*D]m domains have different phosphate and carboxylate interaction edges and thus different calcium ion and apatite surface binding properties. These two domains and the colocalization of Dmp1 and Dmp2 genes at a position equivalent to the dentinogenesis imperfecta type II location on human 4q21 all suggest that the PPs are indeed involved in some aspect of ECM mineralization.

An in vitro model of human dental pulp repair

Pulp tissue responds to dentin injury by laying down reactionary dentin secreted by existing odontoblasts or reparative dentin elaborated by odontoblast-like cells that differentiated from precursor cells in the absence of inner dental epithelium and basement membrane. Furthermore, growth factors or active dentin matrix components are fundamental signals involved in odontoblast differentiation. In vitro, dental pulp cells cultured under various conditions are able to express typical markers of differentiation, but no culture system can re-create pulp response to dentin drilling. This paper reports the behavior of thick slices from human teeth drilled immediately after extraction and cultured from 3 days to 1 month. Results show that the damaged pulp beneath the cavity is able to develop, in vitro, some typical aspects correlated to tissue healing, evidenced by cell proliferation (BrdU-positive cells), neovascularization (positive with antitype-IV collagen antibodies), and the presence of functional (3H proline-positive) cuboidal cells close to the injured area. After 30 days of culture, elongated spindle-shaped cells can be seen aligned along the edges of the relevant dentin walls, whereas sound functional odontoblasts are well-preserved beneath healthy areas. This tissue recovery leads us to believe that such a culture model will be a useful system for testing factors regulating pulp repair.

Gene expression and localization of amelogenin in the rat incisor

Gene expression and localization of amelogenin were studied in the developing rat incisor by the methods of in situ hybridization and immunohistochemistry. ISH revealed the first expression of amelogenin mRNA in the inner enamel epithelium of the cervical loop. The signals were clearly observed in pre-ameloblasts in the region bordering on predentin formation and became more intense toward the cells on the initial enamel matrix secretion. The maximal signals were found in the cytoplasm of secretory ameloblasts. From the terminal secretion zone, the signals then became gradually weaker toward the incisal edge but were still evident in the cytoplasm of shortening, transitional ameloblasts and those at the early maturation stage. No signals were found in the cells of the stratum intermedium and stellate reticulum throughout amelogenesis. Immunohistochemistry by means of an antibody against amelogenin C-telopeptide consisting of 12 amino acids revealed immunoreaction in the secretory ameloblasts reacting to the ISH. When a polyclonal antibody against amelogenin was used, immunoreaction was found in the distal ends of ruffle-ended ameloblasts (RA) in the maturation zone. Those results indicated that amelogenin is synthesized by ameloblastic cells from the inner enamel epithelium to the early maturation stage and is then resorbed by the RA.

Inability of calcium hydroxide to induce reparative dentinogenesis at non-peripheral sites of dog dental pulp

The ability of 2 calcium hydroxide-containing materials to induce initiation of reparative dentinogenesis was tested at sites remote from the dentinogenically-active regions of the pulp periphery. Pieces of the cements, Dycal and Life, were implanted in central parenchymal sites of dog dental pulps for periods of 6, 14 and 42 days, respectively. Similar pieces were placed in peripheral capping sites as controls. The responses were analyzed by light and transmission electron microscopy. Induction of tubular dentin matrix lined with elongated and polarized odontoblast-like cells was only seen at peripheral capping sites. In response to the centrally implanted cements, only atubular hard tissue with lining fibroblast-like cells was deposited.

Effect of mechanical removal of the pulp upon the retention of odontoblasts around the pulp chamber of human third molars

The pulp chambers of 11 freshly extracted human third molars were exposed by cutting off the roots apical to the cervical margin and the pulps were either removed with forceps and discarded or left in situ. The teeth were fixed, demineralized, divided longitudinally, embedded in resin and 2-micron sections stained with toluidine blue were examined by light microscopy. In pulp-removed specimens the percentage retention of the odontoblast layer with the predentine varied near the longitudinal division but when sectioned deeper all six specimens displayed 100% retention. The intactness of the retained odontoblast layer was mostly good as judged by the mutual close apposition of the distal ends of the cell bodies and their relation to the predentine. The retention of the odontoblast layer with the predentine may be due to the distribution of fibronectin, which others have shown is present between odontoblasts, and between odontoblasts and predentine, but lacking beneath the odontoblast layer.

Endocytotic functions of ameloblasts and odontoblasts: immunocytochemical and tracer studies on the uptake of plasma proteins

BACKGROUND

Biochemical, (immuno)cytochemical, and radioautographic data accumulated over several years have lead to the view that ameloblasts carry out both secretory and degradative functions throughout amelogenesis. Whereas it has been assumed that maturation stage ameloblasts endocytose aged enamel proteins from the enamel layer, the origin of the newly formed ones detected in the endosomal/lysosomal compartment of ameloblasts from all stages remains to be elucidated. One possible source is from secretory products released ectopically along basolateral surfaces.

METHODS

To test this hypothesis, we have investigated, using colloidal gold immunocytochemistry, whether plasma proteins (albumin and alpha 2HS-glycoprotein) found in the interstitial fluid are endocytosed by rat incisor ameloblasts and other cells from hard and soft tissues. Rat albumin, tagged with dinitrophenol, was injected intravenously to trace the movement of this protein.

RESULTS

Plasma proteins were immunodetected along the baso-lateral surfaces and in multivesicular bodies of ameloblasts where enamel proteins were also found. By 2 hours following intravenous administration of dinitrophenylated albumin, the tracer had left the blood and diffused into the enamel organ and between odontoblasts and osteoblasts. The tracer was also found in multivesicular bodies of all cells examined.

CONCLUSIONS

The uptake of albumin by many different cell types suggests that this process is not restricted to ameloblasts and likely occurs in a nonselective manner. Hence, baso-lateral uptake in ameloblasts may play a role not only in the continuous removal of plasma proteins leaking from the blood, but also of enamel proteins 'dumped' laterally between these cells. Likewise, odontoblasts may use the same mechanism to internalize some of the plasma proteins and any enamel protein that diffuse toward them.

Characteristics and effects of calcified degenerative zones on the formation of hard tissue barriers in amputated canine dental pulp

The purpose of this research was to study under undecalcified conditions the presence, ultrastructural features, and contributions of the degenerative zone beneath the necrotic zone and whether it had effects on the formation of reparative dentin in canine incisors and premolars. The research was conducted over a period of 14 days after experimental pulpotomy using calcium hydroxide as a pulp-capping agent. On the first day following pulp exposure and capping with calcium hydroxide, electron-dense spherical bodies were observed under the necrotic zone. Energy dispersive X-ray point analysis confirmed that these electron-dense deposits contained calcium and phosphorus. By the third day, varying amounts of minute von Kossa-positive granules could be observed light-microscopically between the two zones of necrosis and underlying vital pulp tissue. Migration and proliferation of pulpal cells, most probably mesenchymal cells, were observed adjacent to the von Kossa-positive zone. The ultrastructure of the von Kossa-positive zone consisted of degenerated cells, electron-dense spherical bodies, and electron-dense shortened dilating fibrils. By the seventh day, short cylindrical-shaped cells collected at the coronal end of the vital pulp tissue. By the fourteenth day, the specimens having a uniform von Kossa-positive zone exhibited rapid differentiation of odontoblasts and tubular dentin formation. In contrast, only some specimens having an irregular von Kossa-positive zone exhibited osteodentin formation and the beginning of odontoblast differentiation beneath the osteodentin. These findings suggest that this calcified degenerative zone has an important effect on the reparative process of pulp tissue after pulpotomy.

Dynamics of the pulpo-dentin complex

Dentin has a relatively high water content due to its tubular structure. Once dentin is exposed, this intratubular water is free to move in response to thermal, osmotic, evaporative, or tactile stimuli. Fluid shifts across dentin are thought to cause sufficient shear forces on odontoblasts, nerve endings, nearby fibroblasts, and blood vessels to cause significant mechanical irritation, disruption, or damage, depending on the magnitude of the fluid shift. Even in the absence of fluid shifts, the water-filled tubules provide diffusion channels for noxious (i.e., bacterial products) substances which diffuse inward toward the pulp, where they can activate the immune system, provide chemotactic stimuli, cytokine production, and produce pain and pulpal inflammation. Viewed from this perspective, dentin is a poor barrier to external irritants. However, pulpal tissues react to these challenges by increasing the activity of nerves, blood vessels, the immune system, and interstitial fluid turnover, to make the exposed dentin less permeable either physiologically, via increased outward fluid flow, or microscopically, by lining tubules with proteins, mineral deposits, or tertiary dentin, thereby enhancing the barrier properties of dentin, and providing additional protection to pulpal tissues. These reactions involve dentin and pulp, both in the initiation of the processes and in their resolution. These responses of the dental pulp to irritation of dentin demonstrate the dynamic nature of the pulpo-dentin complex.

Cell biology of calcified tissues: experimental models of differentiation and mechanisms by which local and systemic factors exert their effects

Interpretation of the cell biology literature, as it relates to formation and mineralization of calcifying tissues, is complicated by the plethora of models available. Some culture models use heterogeneous populations of cells while others use relatively homogeneous populations. The issues are further confused by comparison of monolayer and three dimensional cultures. In addition, transformed and nontransformed cell lines are also used. As little clinical data about the age and sex of the original donor for many of these cell lines is lacking, it is impossible to know where in the cell lineage the cells were when they were isolated, yet this information can have a direct impact on the data obtained and their interpretation. Furthermore, many responses are attributed to the cell, while much of the effect observed may be targeted to the matrix. These issues are discussed and a potential mechanism explaining how cells can modulate events in the matrix nongenomically is presented.

Progressive changes in the pulpo-dentinal complex and their clinical consequences

With changes in the age structure and oral health in the population, changes in the pulpo-dentinal complex are becoming more relevant clinically. Age-related changes in the structure of dentine and pulp are reviewed. The influence of these changes on restorative dentistry are considered with particular emphasis on endodontics and the use of adhesive restorative materials.

Neurotensin-like immunoreactivity in odontoblasts and their processes in rat maxillary molar teeth and the effect of pulpotomy

A strong neurotensin-like immunoreactivity (NT-like IR) was detected in the odontoblast cells of the rat teeth. 4 h after a partial pulpotomy performed in two maxillary molar teeth a decreased NT-like IR was observed in the odontoblast layer located at the vicinity of the lesion together with edema and nuclear pyknosis. NT-like IR became further decreased after 24 h. After 7 days NT-like IR had almost fully disappeared with signs of necrosis of the dental pulp and infiltration of polymorphonuclear lymphocytes. It seems possible that NT like peptides in the odontoblast cell layer may play a role, e.g., in dentinogenesis and/or nociception.

Characterization of cellular responses involved in reparative dentinogenesis in rat molars

During primary dentin formation, differentiating primary odontoblasts secrete an organic matrix, consisting principally of type I collagen and non-collagenous proteins, that is capable of mineralizing at its distal front. In contrast to ameloblasts that form enamel and undergo programmed cell death, primary odontoblasts remain metabolically active in a functional tooth. When dentin is exposed to caries or by operative procedures, and when exposed dentinal tubules are treated with therapeutic dental materials, the original population of odontoblasts is often injured and destroyed. The characteristics of the replacement pool of cells that form reparative dentin and the biologic mechanisms that modulate the formation of this matrix are poorly understood. Based on the hypothesis that events governing primary dentinogenesis are reiterated during dentin repair, the present study was designed to test whether cells that form reparative dentin are odontoblast-like. Cervical cavities were prepared in rat first molars to generate reparative dentin, and animals were killed at various time intervals. In situ hybridization with gene-specific riboprobes for collagen types I and III was used to study de novo synthesis by cells at the injured dentin-pulp interface. Polyclonal antibodies raised against dentin sialoprotein (DSP), a dentin-specific protein that marks the odontoblast phenotype, were used in immunohistochemical experiments. Data from our temporal and spatial analyses indicated that cells forming reparative dentin synthesize type I but not type III collagen and are immunopositive for DSP. Our results suggest that cells that form reparative dentin are odontoblast-like.

The early distribution and possible role of nerves during odontogenesis

Neural crest cells migrate along specific pathways to reach the mandibular and maxillary arches where they condense under specific areas of the ectoderm which will give rise to the primary and permanent dentition. In the mouse, the trigeminal ganglion becomes evident on E9 and the superior cervical sympathetic ganglion E13. Several studies have suggested that nerves in the vicinity of the developing teeth could influence the surrounding tissues and initiate tooth development, whereas other investigators have suggested that tooth development will proceed without an intact innervation. Innervation of the dental papilla has been reported as early as the cap stage in human teeth using an antibody to PGP 9.5. A large variety of putative neurotransmitters have been localized in the nerves of the dental pulp. Many of the putative neurotransmitters function in vasoregulation while others have unknown functions. A hypothesis is presented describing a possible signal transduction pathway between odontoblasts and nerve terminals.

Contributions of heterospecific tissue recombinations to odontogenesis

Determining the functions of cell surface and substrate adhesion molecules during heterospecific tissue recombinations constitutes a significant problem in biology. The developing tooth organ provides a fine model to pursue this problem, especially in the context of instructive epithelial-mesenchymal interactions. The interpretation of many experimental heterospecific tissue combinations is difficult because of the complexity of the embryonic systems. According to some authors, the expressed phenotype corresponds to the genotype of the epithelium; on the contrary, other studies have demonstrated the leading role of the mesenchyme. The importance of cranial neural crest in tooth morphogenesis has been established. Lizard teeth maintain a continuous morphogenetic field throughout life (polyphyodont). These tissues can be considered as biological models and are ideally suited to study neural crest cell differentiation. Oral cavity of chick embryos show a rudiment resembling the dental lamina of amphibian, reptilian and mammalian embryos, though further odontogenic interactions between the epithelial and mesenchymal tissues are not operating. Some authors have suggested that genes involved in tooth formation, which have remained silent in birds for more than 200 million years, can be activated by appropriate signals. Chick epithelium combined with mesenchyme from mouse molar tooth produced dental structures with differentiated ameloblasts depositing enamel matrix. Quail neural crest combined with lizard dental bud showed quail cells with odontoblastic processes. Combinants (quail ectoderm-lizard papilla, and quail ectoderm-rabbit embryo papilla) showed differentiated chimeric-tooth-like structures. However, controversy persists regarding the ability of avian epithelium to express the ameloblast phenotype and to secrete enamel protein.(ABSTRACT TRUNCATED AT 250 WORDS)