Dentine proteoglycans: composition, ultrastructure and functions

A model of the early mineralization process of mantle dentin

The aim of this study was to investigate the relationship between proteoglycans (PGs) and collagen fibrils at the early mineralization process of mantle dentin. Ten first molar dental germs of rats were removed and fixed in glutaraldehyde/formaldehyde in cacodylate buffer and post-fixed in osmium tetroxide. The samples were dehydrated and embedded in epoxy resin. Ultrathin sections were contrasted and analyzed in TEM before and after treatment with EDTA, chondroitinases AC and ABC. After EDTA treatment, a electrondense substance associated with collagen fibril was removed, and did not stain again. A high magnification of these areas showed globular structures with 15 nm diameter surrounding collagen fibrils. In advanced mineralization areas, collagen fibrils showed a banded pattern and at high magnification the fibrils presented a light 10 nm ring inside and a dark 10 nm ring outside. After chondroitinase treatment, the electrondense substance associated with collagen fibrils was removed, showing a banded pattern of clear and dark areas along them. From morphological data, the authors proposed a model of interaction between PGs and collagen fibrils, where glicosaminoglycans chains are inside the fibrils, while the protein core remains outside. That stereochemical arrangement would start the crystal nucleation.

Immunohistochemical identification of type I and type III collagen and chondroitin sulphate in human pre-dentine: a correlative FEI-SEM/TEM study

AIM

To identify type I- (I-CF) and type III-collagen fibrils (III-CF) and chondroitin 4/6 sulphate (CS) within human pre-dentine by means of a correlative analysis under field emission in-lens-scanning electron microscopy (FEI-SEM) and transmission electron microscopy (TEM).

METHODOLOGY

Human-extracted teeth were obtained and submitted to either a pre-embedding or a post-embedding immunolabelling procedure using monoclonal primary antibodies anti-I-CF, anti-III-CF and anti-CS. Gold-conjugated secondary antibodies were coupled to primary antibodies to visualize labelling under the electron beam. Correlative labelling patterns were obtained for I-CF and CS under both FEI-SEM and TEM.

RESULTS

Field emission in lens-SEM analysis revealed an intricate three-dimensional network of I-CF and CS clarifying the intimate relationship between the two main components of the pre-dentine organic matrix. TEM analysis revealed odontoblasts exhibiting intracellular labelling for CS, which became more intense and diffuse over the pre-dentine organic matrix. The same diffuse immunoreaction was revealed for I-CF, whereas a weak immunolocalization of III-CF was found scattered throughout the pre-dentine layer and over the collagen fibrils.

CONCLUSIONS

Both the pre- and post-embedding immunohistochemical approaches have led to the visualization of CF- and CS-labelling distribution within the pre-dentine layer, adding further knowledge on the elucidation of collagen-proteoglycans interaction in the organic matrix of human dental roots.

Removal of noncollagenous components affects dentin bonding

The structural integrity of fibrillar type I collagen is critical for effective dentin bonding. Since most noncollagenous matrix components in dentin are closely associated with collagen, we hypothesized that they may also contribute to dentin bonding. To test this hypothesis, bovine dentin was acid-etched, treated with chondroitinase ABC (C-ABC), endo-beta-galactosidase (Endo-beta), or trypsin. Controls were prepared in the same manner but without the enzymes. All control and experimental specimens were then bonded with One-Step. Bond strength data were analyzed by one-way ANOVA and Fisher's PLSD test (p < 0.05). When dentin was treated with C-ABC or trypsin, bond strengths significantly decreased for the rewetted groups (p < 0.05). The treatment with Endo-beta showed no effects on bond strengths (p > 0.05). When the treated dentin surfaces were observed under SEM, the C-ABC and trypsin treated groups revealed significant loss of collagen fibril architecture. The results indicate that chondroitin sulfate glycosaminoglycans and trypsin-digestible noncollagenous proteins play roles in maintaining the open dimensions of the collagen fibril scaffold, which is essential for optimal dentin bonding.

Dentin matrix protein 1 induces cytodifferentiation of dental pulp stem cells into odontoblasts

Odontoblasts are postmitotic cells that differentiate from the dental papilla. These cells are responsible for producing the calcified dentin matrix. The pulp-odontoblast interphase contains undifferentiated mesenchymal stem cells, which have the ability to cytodifferentiate into odontoblast-like cells in response to specific signaling molecules. Dentin matrix protein 1 (DMP1) is one of the dentin noncollagenous extracellular matrix proteins that has been implicated in regulation of mineralization. In this study, we have examined the potential role of DMP1 in inducing cytodifferentiation of dental pulp stem cells into odontoblast-like cells and formation of reparative dentin in a rat model. Cavities were drilled and pulps exposed in maxillary first molars. Collagen matrix impregnated with recombinant DMP1 was implanted directly in Group 1, while calcium hydroxide, a commonly used pulp-capping agent was implanted in group 2, collagen matrix that was not impregnated with rDMP1 was implanted directly in group 3, which served as control. Each of these three groups was subdivided into two subgroups, A for 2 weeks time duration and B for 4 weeks duration. At the end of the time period the maxillae were excised, tissues were processed for histological and immunohistochemical evaluations. The results showed that DMP1 could act as a morphogen on undifferentiated mesenchymal cells present in the dentin-pulp complex. These differentiated cells had the potential to regenerate dentin-like tissue, which was confirmed by the presence of collagenous matrix, odontoblast specific markers and calcified deposits.

Fibromodulin-deficient Mice Display Impaired Collagen Fibrillogenesis in Predentin as Well as Altered Dentin Mineralization and Enamel Formation

To determine the functions of fibromodulin (Fmod), a small leucine-rich keratan sulfate proteoglycan in tooth formation, we investigated the distribution of Fmod in dental tissues by immunohistochemistry and characterized the dental phenotype of 1-day-old Fmod-deficient mice using light and transmission electron microscopy. Immunohistochemistry was also used to compare the relative protein expression of dentin sialoprotein (DSP), dentin matrix protein-1 (DMP 1), bone sialoprotein (BSP), and osteopontin (OPN) between Fmod-deficient mice and wild-type mice. In normal mice and rats, Fmod immunostaining was mostly detected in the distal cell bodies of odontoblasts and in the stratum intermedium and was weaker in odontoblast processes and predentin. The absence of Fmod impaired dentin mineralization, increased the diameter of the collagen fibrils throughout the whole predentin, and delayed enamel formation. Immunohistochemistry provides evidence for compensatory mechanisms in Fmod-deficient mice. Staining for DSP and OPN was decreased in molars, whereas DMP 1 and BSP were enhanced. In the incisors, labeling for DSP, DMP 1, and BSP was strongly increased in the pulp and odontoblasts, whereas OPN staining was decreased. Positive staining was also seen for DMP 1 and BSP in secretory ameloblasts. Together these studies indicate that Fmod restricts collagen fibrillogenesis in predentin while promoting dentin mineralization and the early stages of enamel formation. (J Histochem Cytochem 54:525-537, 2006)

Targeted disruption of two small leucine-rich proteoglycans, biglycan and decorin, excerpts divergent effects on enamel and dentin formation

Small leucine-rich proteoglycans have been suggested to affect mineralization of dental hard tissues. To determine the functions of two of these small proteoglycans during the early stages of tooth formation, we characterized the dental phenotypes of biglycan (BGN KO) and decorin deficient (DCN KO) mice and compared them to that of wild type mice. Each targeted gene disruption resulted in specific effects on dentin and enamel formation. Dentin was hypomineralized in both knock out mice, although the effect was more prominent in the absence of decorin. Enamel formation was dramatically increased in newborn biglycan knockout mice but delayed in absence of decorin. Increased enamel formation in the former case resulted from an upregulation of amelogenin synthesis whereas delayed enamel formation in the later case was most probably an indirect consequence of the high porosity of the underlying dentin. Enamelin expression was unchanged in BGN KO, and reduced in DCN KO. Dentin sialoprotein (DSP), a member of the family of phosphorylated extracellular matrix proteins that play a role in dentinogenesis, was overexpressed in BGN-KO odontoblasts and in the sub-odontoblastic layer. In contrast, a decreased expression of DSP was detected in DCN KO. Dentin matrix protein-1 (DMP-1), bone sialoprotein (BSP) and osteopontin (OPN) were upregulated in BGN KO and downregulated in the DCN KO. Despite the strong effects induced by these deficiencies in newborn mice, no significant difference was detected between the three genotypes in adult mice, suggesting that the effects reported here in newborn mice are transient and subjected to self-repair.

Distributions of magnesium and sulphur in defect layers of incisor dentine in magnesium-deficient rats

This study examined the mechanism governing the occurrence of defect layers of incisor dentine in Mg-deficient rats by X-ray microanalysis. Young (5 weeks of age) Wistar male rats were pair-fed semi-synthetic diets containing either control (0.05% Mg) (N = 8) or Mg-deficient (0.001% Mg) (N = 8) ingredients for 17 days. All animals were time marked with a combination of 0.1 mol nitrilotriacetato lead and 0.1 mol nitrilotriacetato zinc (2mg Pb/kg body weight) on days 0, 7 and 14 into incisor dentine. Blood samples were obtained on days 10 and 17 in order to measure Ca, Mg, P and alkaline phosphatase activity levels in serum; moreover, hypomagnesaemia and hypercalcaemia were confirmed. After the 17th day, rats were sacrificed humanely under anaesthesia and mandibular incisors were removed. Dentine formation of right mandibular incisors was assessed (time marking lines); furthermore, Ca, P, Mg and sulphur (S) concentrations as well as Ca/P molar ratio were determined in left mandibular incisors based on contiguous measurement points at 2 microm intervals from dentine pulp to dentine of the lingual aspect via X-ray analysis. Additionally, proteoglycan distribution was observed in other Mg-deficient rat dentine. These findings demonstrated decreases in body weight, incisor formation and incisor length in Mg-deficient rats. Mg and S levels increased in the defect layers, whereas proteoglycan decreased. This phenomenon was possibly attributable to condensation of Mg and S contents consequent to decreased dentine formation during Mg-deficiency and a transient increase in Mg due to transport of Mg as a result of inhibition of cell proliferation in soft tissues.

Site-specific effect of ascorbic acid deficiency on the structure and function of odontoblasts in the teeth of osteogenic disorder rat in vivo

The influence of chronic L-ascorbic acid (AsA) deficiency on dentinogenesis was examined in Osteogenic Disorder Shionogi (ODS) rat, which bear inborn lack of L-gulonolactone oxidase. Weanling male rats were kept on AsA-free diet for 4 weeks until all suffered from scurvy. Control rats were given AsA in drinking water. The dentin of molars and incisors of the scorbutic rats was thinner than that in control, except for the crown-analogue (enamel-related) of incisors. Predentin in scorbutic molars showed irregular thickness, and was almost lacking in roots. In the root-analogue (cementum-related) region of scorbutic incisors, dentin displayed metachromatic incremental lines, and the thickened predentin contained collagen fibrils of irregular diameter. The odontoblasts facing the affected regions contained dilated rough endoplasmic reticulum cisternae. In the crown-analogue of scorbutic incisors, however, dentin, predentin, and odontoblasts were comparable to those of controls. These data indicate that AsA deficiency differentially affects the synthetic and/or secretory activity of odontoblasts in ODS rat teeth in a site-specific manner. The regional differences implicate the presence of putative local factor(s) in the crown-analogue of incisors that might have compensated for AsA deficiency. The odontoblasts in the crown-analogue of incisors may have different requirements for AsA from those in molars and the root-analogue of incisors.

Immunocytochemical analysis of dentin: A double-labeling technique

Immunocytochemical analysis is a fundamental and selective technique for identifying different molecular components of human dental structure. The hypothesis tested here is that the application of different etching solutions on dentin does not hinder collagen fibrils and proteoglycans from maintaining their immunochemical antigenicity. Human dentin disks were treated with 0.5M of EDTA, citric acid, maleic acid, or phosphoric acid (for 15 or 30 s). A double-immunolabeling technique was performed to identify, simultaneously, collagen fibrils and chondroitin sulfate. The use of different acids resulted in different degrees of labeling. Maleic and citric acids revealed a diffuse and intense labeling for both collagen fibrils and proteoglycans. The use of phosphoric acid on dentin showed a massive coagulation of the proteoglycans (15 s) or very low labeling (30 s). These data clarify that the use of acids on dentin components is able to modify their antigenicity. Moreover, the double-labeling immunocytochemical technique allows understanding of the spatial relationships between the collagen fibrils and proteoglycans of the dentin matrix.

Influence of dentinal polyelectrolytes on wet demineralized dentin, a bonding substrate

The objective of this study was to show the influence of dissolved dentinal polyelectrolytes on the characteristics of dentin (bonding substrate) demineralized by citric acid in the absence or presence of ferric chloride. The demineralizing agent was an aqueous mixture of 0, 1, 3, or 10% ferric chloride in 10% citric acid (10-0, 10-1, 10-3, 10-10, respectively). The hypothesis was that the concentration of dissolved dentinal noncollagenous substances, mainly polyelectrolytes soluble in water, must be decreased by their aggregation with ferric ions, which changes the characteristics of demineralized dentin, the rates of demineralization, and dehydration. Cervical bovine dentin was prepared in 3 x 2 x 2-mm blocks, each weighing 20.0 +/- 0.5 mg. The rate of demineralization was investigated by measuring the weight loss resulting from demineralization by immersion in 10 mL of conditioner at 2-h intervals. The dehydration rate of wet demineralized dentin was determined using two methods: (1) weight loss in a desiccator under 263 Pa pressure and (2) differential scanning calorimetry (DSC). Twenty, 12, 8, and 4 h were required to complete demineralization of the blocks with the 10-0, 10-1, 10-3, and 10-10 solutions, respectively. The 10-10 wet demineralized dentin showed the highest rate of dehydration, followed in descending order by the 10-3, 10-1, and 10-0 specimens. Ferric chloride in dentin conditioners provided both a higher rate of dentin demineralization and a higher dehydration rate of wet demineralized dentin. These results suggest that in the presence of ferric chloride, a decreasing amount of dissolved polyelectrolytes aggregated with ferric ions in the substrates may increase the permeability of dentin to water and citric acid. Improvement of monomer permeability is essential to the preparation of good hybridized dentin, providing a more stable and reliable bonding and also protecting the dentin and pulp from infection. A further study of bonding substrates is required in order to understand the role of hybridized dentin in improved dental treatment.

Phospholipids in amelogenesis and dentinogenesis

Phospholipids have been identified in enamel and dentin. Before demineralization, a group of phospholipids extracted by lipid solvents was associated with cell membranes and is therefore closely related to cell growth and intracellular regulations. After demineralization, a second group of phospholipids, associated with the extracellular matrix, was extracted; this group is probably linked to the mineralized phase. Using imidazole-osmium tetroxide fixation of rat incisors, we stained cellular unsaturated fatty acids, so that we could visualize the membrane domains, coated pits, and endocytic inclusions. Filipin, a probe for cholesterol, varied in density along the plasma membrane of secretory ameloblasts, and allowed us to visualize membrane remnants inside the forming enamel. With respect to phospholipids located in the extracellular matrix, the malachite-green-glutaraldehyde (MGA) method or iodoplatinate (IP) reaction retains and visualizes enamel and dentin phospholipids. In predentin, aggregates appearing as granules and filaments, or liposome-like structures, were located in the spaces between collagen fibrils. In dentin, organic envelopes coating the crystals, also named "crystal-ghost" structures, outlined groups of collagen fibrils. Histochemical data provided evidence that phospholipids are co-distributed or interact with proteoglycans. Radioautography after IP reaction established that [3H] choline was detected in dentin as early as 30 min after the intravenous injection of the labeled precursor, before any labeling was seen in odontoblasts and predentin. This suggests that blood-serum-labeled phospholipids pass between odontoblasts, cross the distal permeable junctional complex, and diffuse in dentin prior to any cellular uptake and phospholipid synthesis. Pharmacologically and genetically induced pathology also supports the suggestion that phospholipids play an important role in the formation and mineralization of dental tissues.

Soluble silk-like organic matrix in the nacreous layer of the bivalve Pinctada maxima

Nacre organic matrix has been conventionally classified as both 'water-soluble' and 'water-insoluble', based on its solubility in aqueous solutions after decalcification with acid or EDTA. Some characteristics (aspartic acid-rich, silk-fibroin-like content) were specifically attributed to either one or the other. The comparative study on the technique of extraction (extraction with water alone vs. demineralization with EDTA) presented here, seems to reveal that this generally accepted classification may need to be reconsidered. Actually, the nondecalcified soluble organic matrix, extracted in ultra-pure water, displays many of the characteristics of what until now has been called 'insoluble matrix'. We present the results obtained on this extract and on a conventional EDTA-soluble matrix, with various characterization methods: fractionation by size-exclusion and anion-exchange HPLC, amino acid analysis, glycosaminoglycan and calcium quantification, SDS/PAGE and FTIR spectroscopy. We propose that the model for the interlamellar matrix sheets of nacre given by Nakahara [In: Biomineralization and Biological Metal Accumulation, Westbroek, P. & deJong, E.W., eds, (1983) pp. 225-230. Reidel, Dordrecht, Holland] and Weiner and Traub [Phil. Trans. R. Soc. Lond. B (1984) 304, 425-434] may no longer be valid. The most recent model, proposed by Levi-Kalisman et al. [J. Struct. Biol. (2001) 135, 8-17], seemed to be more in accordance with our findings.

Biglycan is a repressor of amelogenin expression and enamel formation: an emerging hypothesis

The authors suggest that biglycan acts as a repressor of the expression of amelogenin in the two unique groups of cells involved in amelogenin synthesis, namely, the secretory ameloblasts and odontoblasts.

Dentin proteoglycans: an immunocytochemical FEISEM study

Dentin proteoglycans are fundamental constituents of the dentin matrix and are distributed ubiquitously both in dentin and cement. They have several important functional properties; in particular, they have a fundamental role in the maintenance and the correct stabilization of collagen fibers. The use of phosphoric acid on dentin, as proposed in most common dental adhesive systems to establish a reliable bond, may affect the molecular structure of proteoglycans. The aim of this study was to evaluate, after the application of EDTA or phosphoric acid on dentin, the dentin proteoglycans with an immunocytochemical approach with high resolution SEM. For this purpose, dentin disks obtained from recently extracted human molars were etched with a 35% water solution of phosphoric acid for 15 s, 30 s, and 60 s. Control specimens were conditioned with EDTA. Specimens were immunolabeled with a monoclonal antibody antichondroitin sulfate and visualized with a gold-conjugated secondary antibody. Conditioning dentin with EDTA resulted in a distinct labeling of the proteoglycans, as visualized on branching fibrillar structures in the order of 10-20 nm. The use of 35% phosphoric acid on dentin revealed a coagulation of proteoglycans after etching for 15 s while a very low labeling signal was detectable after 30 s. No labeling was obtained after etching dentin with phosphoric acid for 60 s. These results suggest that the use of 35% phosphoric acid on dentin is able to produce significant structural modifications of the dentin proteoglycans even after short application times. Additionally, when applied on the dentin surface for more than 30 s, phosphoric acid produces a dramatic decrease in proteoglycans' antigenicity, probably due to structural modifications of the three-dimensional conformation of these molecules.

In situ atomic force microscopy of partially demineralized human dentin collagen fibrils

Dentin collagen fibrils were studied in situ by atomic force microscopy (AFM). New data on size distribution and the axial repeat distance of hydrated and dehydrated collagen type I fibrils are presented. Polished dentin disks from third molars were partially demineralized with citric acid, leaving proteins and the collagen matrix. At this stage collagen fibrils were not resolved by AFM, but after exposure to NaOCl(aq) for 100-240 s, and presumably due to the removal of noncollagenous proteins, individual collagen fibrils and the fibril network of dentin connected to the mineralized substrate were revealed. High-aspect-ratio silicon tips in tapping mode were used to image the soft fibril network. Hydrated fibrils showed three distinct groups of diameters: 100, 91, and 83 nm and a narrow distribution of the axial repeat distance at 67 nm. Dehydration resulted in a broad distribution of the fibril diameters between 75 and 105 nm and a division of the axial repeat distance into three groups at 67, 62, and 57 nm. Subfibrillar features (4 nm) were observed on hydrated and dehydrated fibrils. The gap depth between the thick and thin repeating segments of the fibrils varied from 3 to 7 nm. Phase mode revealed mineral particles on the transition from the gap to the overlap zone of the fibrils. This method appears to be a powerful tool for the analysis of fibrillar collagen structures in calcified tissues and may aid in understanding the differences in collagen affected by chemical treatments or by diseases.

Proteoglycans in dentinogenesis

The predominant proteoglycans present in predentin and dentin are the chondroitin-sulphate-rich decorin and biglycan and the keratan-sulphate-rich lumican and fibromodulin. These are small, interstitial, leucine-rich proteoglycans which have recently been shown to exist in gradients across the predentin. Antibodies recognizing chondroitin sulphate show a decreasing gradient from the pulpal aspect toward the mineralizing front, the converse being true for keratan sulphate. Antidecorin shows an increase toward the mineralization front. Evidence from biochemical, autoradiographic, and immunohistochemical studies implies that such changes may be brought about by gradients of metalloproteinases. This offers the possibility that the proteoglycans organize the collagen network for receipt of phosphoproteins and phospholipids, the former being evident only at the onset of dentin formation. The suggestion is raised that glycosaminoglycan-depleted leucine-rich protein cores act as sequester points for receipt of phosphoproteins in particular. The rigid, spatially oriented glycosaminoglycan chains on decorin and biglycan are known to bind calcium and may feature directly in mineral initiation.

A transmission electron microscope study using vitrified ice sections of predentin: structural changes in the dentin collagenous matrix prior to mineralization

The assembly of the collagenous organic matrix prior to mineralization is a key step in the formation of bones and teeth. This process was studied in the predentin of continuously forming rat incisors, using unstained vitrified ice sections examined in the transmission electron microscope. Progressing from the odontoblast surface to the mineralization front, the collagen fibrils thicken to ultimately form a dense network, and their repeat D-spacings and banding patterns vary. Using immunolocalization, the most abundant noncollagenous protein in dentin, phosphophoryn, was mapped to the boundaries between the gap and overlap zones along the fibrils nearest the mineralization front. It thus appears that the premineralized collagen matrix undergoes dynamic changes in its structure. These may be mediated by the addition and interaction with the highly anionic noncollagenous proteins associated with collagen. These changes presumably create a collagenous framework that is able to mineralize.

Expression of the small leucine-rich proteoglycan osteoadherin/osteomodulin in human dental pulp and developing rat teeth

Because the extracellular matrices of dentin and bone are composed mainly of type I collagen, their characteristics are determined by the nature of noncollagenous proteins (NCPs). Among these NCPs, some proteoglycans (PGs) belong to the small leucine-rich proteoglycans (SLRPs). Recently, osteoadherin (OSAD) has been described as a new member of this family, that is expressed by mature bovine osteoblasts. Here, we report the expression of OSAD messenger RNA (mRNA) in human dental tissues and during the development of rat molars, using in situ hybridization. For this purpose, we constructed a probe for OSAD mRNA transcripts from human odontoblast cells cultured in vitro. Our results indicate that the mature human odontoblasts overexpress the OSAD gene as compared with cells present in the pulp core. In rat developing molars, mRNA transcripts were first detected in alveolar bone in 19-day-old embryos. At the same age, no signal was detected in any cell of the first molar. In more mature teeth (newborn and 2-day-old rats), OSAD expression starts in the polarized odontoblasts and increases in the secretory and mature odontoblasts, respectively. Interestingly, a similar pattern of expression was observed in the ameloblast layer responsible for the deposition of enamel mineralized matrix. Together, these results lead us to speculate that OSAD may be implicated in biomineralization processes.

Differences in the pattern of lanthanum diffusion into predentine and dentine in mouse incisors and molars

Lanthanum nitrate was either perfused intravascularly or segments of mouse tooth were immersed in a fixative solution containing the tracer. The tracer deposits were examined in young (8-day-old) and older (8-week-old) mouse incisors and molars, demineralized or undemineralized. Lanthanum passed the distal junctional complex of odontoblasts and appeared in the predentine of incisors as large electron-dense stellate aggregates, 40-70 nm in diameter, and in molars as round, 20-40 nm dots. In dentine, tracer deposits were detected at three locations. Near the predentine dentine junction, the tracer densely stained a band 0.5-2.5 microm in width, also termed metadentine; in the inner circumpulpal dentine, the staining was weaker or lacking in an area extending 5-7 microm from the predentine-dentine junction; in outer circumpulpal dentine, lateral diffusion had occurred in porosities of intertubular dentine. Lanthanum impregnated the walls of dentine tubules and a peritubular-like dentine. In contrast, the mantle dentine was never stained. These differences in the pattern of diffusion prove that lanthanum staining is age-dependent and varies between mouse incisors and molars, independently of tissue processing. Architectural properties and driving flux are involved in the transport and localization of lanthanum in predentine and dentine.

Odontoblast commitment and differentiation

Histological and cytological organization confer specificity to the odontoblasts. These postmitotic, neural crest derived, polarized cells are aligned in a single layer at the periphery of the dental pulp and secrete the organic components of predentin-dentin. The developmental history of these cells demands a cascade of epigenetic signalling events comprising the acquisition of odontogenic potential by neural crest cells, their patterning in the developing jaws, the initiation of odontogenesis through interaction with the oral epithelium, commitment, and tooth-specific spatial distribution of competent preodontoblasts able to overtly differentiate. Recent experimental investigations are critically summarized, many open questions are stressed, and current hypotheses concerning the control of terminal odontoblast differentiation are outlined.Key words: odontoblast, neural crest, oral ectoderm, differentiation.

Potassium is involved in apatite biomineralization

The biogenetic formation of mineral crystals, one aspect of biomineralization, is a multistep process of apatite formation throughout the growth of dentin tissue. An important step is the transformation of the non-mineralized predentin matrix to mineralizing dentin matrix and its biological control. In this study, the high capacity of elemental mapping is combined with single x-ray point measurements to elucidate whether special elements are involved in initiation or regulation of mineral nucleation. Directly at the mineralization front, micro-areas with a strong co-enrichment of phosphorus (e.g., as phosphate) and potassium are found. During the beginning of the calcium enrichment and the subsequent apatite mineral formation in the characteristic micro-areas, the content of potassium decreases significantly. These findings indicate that potassium is involved in the process of dentin mineralization.

Proteoglycans (PGs) in the larval amphibian tooth as visualized by cuprolinic blue

Proteoglycans (PGs) were localized in the predentine and dentine of young larvae from the urodelan species Salamandra salamandra. After cuprolinic blue (CB) staining at the critical electrolyte concentration of 0.1 M MgCl2, CB-positive, electron dense filaments with considerable variations in length and width were found in the collagen-free zone adjacent to the odontoblast processes (length up to 1.3 microns, width 21 nm), in predentine (660 nm/3.2 nm), in dentine around (20 nm/9 nm) and in the dentine tubules (35-150 nm/8 nm). Size classes very likely represent different PGs.

Immunolocalization of glycosaminoglycans in ageing, healthy and periodontally diseased human cementum

The distribution of glycosaminoglycans in the extracellular matrix of human cementum was investigated in periodontally involved and periodontal disease-free teeth separated into eight different age groups (from 12 to 90 years), to investigate possible changes in the distribution of glycosaminoglycan species associated with ageing and periodontal disease. A standard indirect immunoperoxidase technique was used, with a panel of monoclonal antibodies, 2B6, 3B3, 5D4, and 7D4, that recognize epitopes in chondroitin-4-sulphate/dermatan sulphate (C-4S/DS), chondroitin-6-sulphate (C-6S), keratan sulphate (KS) and a novel sulphated chondroitin sulphate (CS) epitope, respectively. Intense positive staining for C4-S/DS was observed at the margins and lumina of almost all the lacunae and canaliculi in cellular cementum in all sections. Immunoreactivity to C6-S, KS and novel CS epitopes was limited to a proportion of lacunae and canaliculi in all sections, although C6-S and the novel CS epitopes were more widely distributed than KS. In acellular cementum, there was no demonstrable staining for any of the glycosaminoglycans except where periodontal ligament (Sharpey's) fibres insert; periodontal ligament fibres inserting in cellular cementum also demonstrated positive immunoreactivity. In addition, the cementoblasts on the outer root surface, as well as the pericellular areas around a proportion of these cells, demonstrated positive immunoreactivity. These results indicate that glycosaminoglycan species present in human cementum include C4-S, DS, C6-S, and novel sulphated CS epitopes. KS is also present in cementum but is limited to a more restricted proportion of lacunae and canaliculi. Regional differences in the distribution of glycosaminoglycans exist between the two cementum types, but no qualitative differences in that distribution were observed between the various age groups or between periodontally involved and periodontal disease-free teeth. The immunoreactivity observed in a proportion of lacunae after staining for C6-S, KS, and novel sulphated CS epitopes could suggest the existence of different cementocyte subpopulations.

Immunochemical localization of the small leucine-rich proteoglycan lumican in human predentine and dentine

The immunocalization of the small interstitial leucine-rich keratan sulphate proteoglycan lumican was studied in human teeth using a polyclonal antibody towards lumican and a standard indirect peroxidase technique. There was intense labelling for lumican in predentine, with immunopositivity also observed in and around the tubules, around the odontoblasts and in the pulp with a fibrillar distribution. The previous reported ability of lumican to control collagen fibrillogenesis and it putative function in collagen spacing suggest potential roles for lumican in predentine in relation to mineralization.

Extracellular matrix in tooth cementum and mantle dentin: localization of osteopontin and other noncollagenous proteins, plasma proteins, and glycoconjugates by electron microscopy

BACKGROUND

Noncollagenous proteins (NCPs) are considered to have multiple functions related to the formation, turnover, and repair of the collagen-based mineralized tissues. Collectively, they comprise a class of generally acidic, mineral-binding proteins showing extensive posttranslational modifications, including glycosylation, phosphorylation, and sulfation. METHODS. We have used colloidal-gold immunocytochemistry and lectin-gold cytochemistry, together with transmission electron microscopy, to examine the organic matrix composition of tooth cementum and the subjacent mantle dentin in rodent molar teeth. Molars were processed for immunocytochemistry using antibodies against osteopontin (OPN), osteocalcin (OC), bone sialoprotein (BSP), bone acidic glycoprotein-75 (BAG-75), albumin (ALB), and alpha 2HS-glycoprotein (alpha 2HS-GP), or for glycoconjugate cytochemistry using lectin-gold complexes.

RESULTS

Ultrastructurally, at the advancing root edge in developing molars, OPN and BSP initially were associated with small calcification foci in the mantle dentin. With progressing mineralization, OC and alpha 2HS-GP appeared diffusely distributed throughout the calcified mantle dentin, and diminished as a gradient toward the circumpulpal dentin. Immediately following disruption of Hertwig's epithelial root sheath, cementum deposition commenced at the root surface occasionally with the appearance of a cement line rich in OPN. Cementum matrix proper contained abundant OPN, BSP, OC, and alpha 2HS-GP, but no or little BAG-75 or ALB. Protein immunolabeling, as well as lectin labeling for beta-D-galactose and N-acetyl-neuraminic acid and/or N-glycolyl-neuraminic acid, both being prominent sugars of certain NCPs, was primarily concentrated between, and at the surface of, collagen fibrils in acellular extrinsic fiber cementum. OPN, BSP, OC, and alpha 2HS-GP were also prominent components of cellular cementum and of Sharpey's fibers. In cellular cementum, laminae limitantes sometimes present delimiting cementocyte lacunae and cell process-containing canaliculi were also rich in OPN. Along the root surface, occasional cementoblasts exhibited intracellular labeling for OPN over the Golgi apparatus and secretory granules.

CONCLUSIONS

We have identified OPN, BSP, OC, and alpha 2HS-GP as being prominent organic constituents of both mantle dentin and acellular and cellular cementum, and, have elucidated the details of their distribution at the ultrastructural level. The temporal appearance and spatial distribution of these organic moieties in the teeth root are similar to those seen during bone formation and are consistent with proposals that certain NCPs may be involved in regulating calcification and/or participating in cell-matrix and matrix-matrix/mineral adhesion events.

Dual incorporation of (35S)sulfate into dentin proteoglycans acting as mineralization promotors in rat molars and predentin proteoglycans

Autoradiographic investigations were carried out 0.5, 1, 2, 4, 24, 48, 72, and 120 hours after the injection of a single dose of [35S]-sulfate on undemineralized molars of 7-15-day-old rats. In predentin, labeling was detected at 0.5 hours. Silver grain density reached a plateau value between 1 and 24 hours, then decreased and disappeared 120 hours after injection. In dentin, the mineralization front started to be labeled as early as 0.5 hours after injection. Labeling increased at the dentin edge between 1 and 2 hours, reached a maxima at 4 hours, then started to decrease, the labeled band seen 24 hours after injection being further incorporated into dentin. This band stood at constant distance from the dentin-enamel junction with stable grain density, even at 120 hours. This investigation proves the existence of two distinct groups of [35S]-labeled proteoglycans, one exclusively related to predentin and disappearing with time, and the second one located in dentin behaves as a stable component. The fact that an early labeling appeared at the mineralization front which was further incorporated into dentin, confirms that dentin proteoglycans constitute an individual group of molecules that are not derived from predentin proteoglycans, and act as mineralization promotors.

Postembedding colloidal-gold immunocytochemistry of noncollagenous extracellular matrix proteins in mineralized tissues

Immunocytochemistry is a powerful tool for investigating protein secretion, extracellular matrix assembly, and cell-matrix and matrix-matrix/mineral relationships. When applied to the tissues of bones (bone and calcified cartilage) and teeth (dentin, cementum, and enamel), where calcium phosphate-containing extracellular matrices are the predominant structural component related to their weight-bearing and masticatory roles, respectively, data from immunocytochemical studies have been prominent in advancing our understanding of mineralized tissue modeling and remodeling. The present review on the application of postembedding, colloidal-gold immunocytochemistry to mineralized tissues focuses on the advantages of this approach and relates them to conceptual, theoretical, and experimental data currently available discussing matrix-mineral interactions and extracellular matrix formation and turnover in these tissues. More specifically, data are summarized regarding the distribution and role of noncollagenous proteins in different mineralized tissues, particularly in the context of how they interface with mineral, and how this relationship might be affected by the various tissue-processing steps and immunocytochemical strategies commonly implemented to examine the distribution and function of tissue proteins. Furthermore, a technical discussion is presented that outlines several different possibilities for epitope exposure in mineralized tissues during preparation of thin sections for transmission electron microscopy. Cell biological concepts of protein secretion by cells of the mineralized tissues, and subsequent extracellular matrix assembly and organization, are illustrated by examples of high-resolution, colloidal-gold immunolabeling for osteopontin, bone sialoprotein, and osteocalcin in the collagen-based mineralized tissues and for enamel protein (amelogenin) in enamel.

Elemental distributions in predentine associated with dentine mineralization in rat incisor

Electron probe microanalysis was applied to study quantitatively and semi quantitatively the elemental concentrations and distributions that occur in predentine during the dentine mineralization of rat incisor. Apex regions of the continuously growing incisors were rapidly dissected and cryofixed in liquid nitrogen-cooled propane. Ultrathin cryosections were prepared from the dentine tissue. On the average in the extracellular predentine element concentrations of calcium and phosphorus were about 0.5% (w/w) and 0.5-1% (w/w), respectively; so the calcium content in the extracellular predentine is higher while the phosphorus content is much lower than in the odontoblast area. Due to the high content of glycosaminoglycans in the extracellular matrix the concentration of sulfur in the predentine was more than 1% (w/w); the potassium content was found in the range of 0.6-0.8% (w/w) which is quite high for an extracellular area and the concentrations of sodium and chlorine were higher than 2% (w/w). Elemental mapping analysis was carried out to demonstrate the distribution of some important elements at the predentine/dentine border during mineralization.

Lipids in predentine and dentine

Using two histochemical methods, malachite green-aldehyde and iodoplatinate, phospholipids were visualized in the predentine of rat incisors in the spaces located between collagen fibers and in dentine as needle-like structures located along individual or groups of mineralizing collagen fibers. The same staining pattern was seen with phospholipase A2-gold. Autoradiographic investigation using 3H choline as labelled precursor, visualized the incorporation of membrane-associated and extracellular choline-containing phosphatidyl choline and sphingomyelin. The cell and membrane-associated labelling decreased gradually between 24 and 4 days, whereas incorporation of the labelled precursor as stable extracellular matrix component was seen in dentine. In addition to these investigations, pharmacologically induced (suramine) and genetically (Krabbe's disease) lysosomal storage pathology was investigated. Defects due to lipid metabolism alterations were seen in predentine and/or in dentine. The major differences visualized here between the non-mineralized and mineralized compartments and interactions between phospholipids and proteoglycans, support the view that phospholipids as matrix components play an important role in the mechanisms of dentine formation and mineralization.

The proteoglycan skeleton of the Kurloff body as evidenced by cuprolinic blue staining

This study deals with the ultrastructure of the chondroitin sulphate proteoglycans of the Kurloff body, a large lysosome organelle, metachromatic towards Toluidine Blue, of a blood cell unique to the guinea pig and called the Kurloff cell. Splenic Kurloff cell from oestrogen-treated guinea pig cells were examined after staining with Cuprolinic Blue, a cationic phthalocyanine-like dye, in the presence of MgCl2 in a critical electrolyte concentration method. Better results were obtained when the fixation-staining by the glutaraldehyde Cuprinolinic Blue MgCl2 mixture was preceded by a glutaraldehyde pre-fixation. On light microscopy, Kurloff bodies generally exhibited an overall pink and glassy metachromasia, sometimes with additional darker metachromatic small dots at their peripheries. At the ultrastructural level, the metachromatic central matrix of the Kurloff body usually exhibited, as a major feature, a typical network pattern of ribbon-like or stellate electron-dense precipitates suggesting the presence of a skeleton of Cuprolinic Blue-reactive filamentous structures. Taking into account their high anionicity (as shown by the stability of the dye binding in the presence of 0.3 M MgCl2) and their susceptibility to chondroitinase ABC, these anionic structures were assumed to be related to the proteochondroitin-4-sulphate previously characterized as the only major sulphated glycoconjugate of the Kurloff cell.