The influence of fluoride on the uptake of radiosulphate by rat incisor odontoblasts in vitro

Altered cell motility and attachment with titanium surface modifications

BACKGROUND

Titanium implants are widely used in dentistry to replace lost teeth. Various surface modifications have been used to improve implant retention and osseointegration. This study is designed to compare the ability of three titanium surfaces to promote cell attachment and cell motility of cells relevant to periodontal tissues.

METHODS

Three clinically relevant surfaces were tested: 1) machined titanium; 2) a titanium surface roughened through acid etching (dual thermal-etched titanium [DTET]); and 3) a titanium surface roughened with nanometer-scale calcium phosphate deposition (nanoscale calcium phosphate-impregnated titanium [NCPIT]). Cell attachment and migration were examined for four cell types: rat osteosarcoma cells, human osteoblasts, and gingival and periodontal ligament (PDL) fibroblasts.

RESULTS

All four cell types attached to each of the three titanium surfaces equally by 2 hours, and the PDL and gingival fibroblasts generally displayed less attachment than the osteosarcoma cells and osteoblasts. The cells displayed differential motility and long-term attachment to each of the titanium surfaces. Osteosarcoma cells displayed preferential motility on NCPIT, whereas PDL fibroblasts were more motile on machined titanium, and gingival fibroblasts moved more rapidly on both DTET and NCPIT. Osteoblasts displayed little motility on any of the titanium surfaces and lost viability on NCPIT after 24 hours. Gingival fibroblasts lost attachment to machined titanium.

CONCLUSIONS

Periodontal cells displayed differential motility and long-term attachment to titanium surfaces. Selective modification of titanium surface properties in various regions of an implant may be useful in guiding specific cell populations to specific locations where they might best aid in osseointegration and soft tissue remodeling.

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

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

Odontoblast transport of sulphate--the in vitro influence of fluoride

The present study reports the development of a culture system for the analysis of 35S-sulphate release from odontoblasts in vitro. Pulpless longitudinally split rat incisors were cultured in supplemented minimum essential medium (alphaMEM) with 20 microCi 35S-sulphate per ml, 20 microCi 3H-mannitol per ml for 1h. Teeth were then transferred to fresh unlabelled media and aliquots of media were removed and the level of 35S-sulphate 3H-mannitol determined. Results indicated a two phase release of 35S-sulphate into the media, and comparison with pulp tissue indicated a specific release pattern. Transport of sulphate is essential for correct synthesis and glycosylation of macromolecules such as proteoglycans (PG). Previous studies have shown that post-translational modifications of these proteins can be influenced by excess fluoride, resulting in decreased sulphation and elongation of glycosaminoglycan (GAG) chains. Therefore the influence of fluoride on sulphate transport, using the optimised culture system was also investigated. Inclusion of 6mM fluoride during pulse labelling caused a significant decrease of 35S-sulphate (P<0.0001) during the initial release phase. Inclusion of 3 and 6mM fluoride only in the post-labelling incubation media resulted in a significant decrease in the release of 35S-sulphate (P<0.0001), during the total time course. The influence of fluoride was not dose dependent. Inclusion of a specific chloride channel blocker SITS, into the culture system indicated that 35S-sulphate transport may in part be via this route. Fluoride would therefore appear to influence the transport of 35S-sulphate across the odontoblast membrane, potentially via a chloride channel.

The influence of fluoride on the cellular morphology and synthetic activity of the rat dentine-pulp complex in vitro

Exposure to high fluoride concentrations in the immediate environment of the tissue is recognized to result in the post-translational modification of non-collagenous dentine extracellular matrix (ECM) components, potentially altering dentine mineralization. However, less is known about the effects of fluoride exposure on the morphology or metabolism of the cells associated with the dentine-pulp complex. This study examined the effects of fluoride exposure at defined concentrations on the cellular morphology and ECM synthetic activities of odontoblasts and pulpal fibroblasts by the culture of tooth sections from male Wistar rat incisors in Trowel-type cultures for up to 14 days, in the presence and absence of 6mM sodium fluoride. Histomorphometric analysis of the dentine-pulp complex of sodium fluoride-exposed tooth sections demonstrated no obvious gross morphological differences with respect to the odontoblasts and pulpal fibroblasts throughout the 14-day culture period, in comparison with unexposed tooth sections. No significant differences in odontoblast and pulpal fibroblast cell numbers were determined in the absence and presence of fluoride. Image analysis examination of odontoblast cytoplasmic:nuclear (C/N) ratios also showed no significant differences in fluoride-exposed and unexposed tooth sections, although reductions in the C/N ratios of pulpal fibroblasts were evident in fluoride-exposed sections at days 10 and 14. No significant differences in predentine width were observed in fluoride-exposed and unexposed tooth sections over the 14-day culture period. Autoradiography following [3H]proline incorporation into the dentine-pulp complex demonstrated inhibition of collagen synthesis, particularly by the odontoblasts in tooth sections exposed to 6mM sodium fluoride. These findings, in association with those from previous studies, imply that dentine ECM alterations may contribute to the altered mineralization of dentine during fluorosis, rather than secretory-related changes in odontoblast morphology.

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 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.

An in vitro approach for the study of dentinogenesis by organ culture of the dentine-pulp complex from rat incisor teeth

Culture of the developing dental tissues has contributed to understanding of developmental processes during early odontogenesis. However, to understand fully the mechanisms involved during dentinogenesis and tissue repair there is a need to develop culture models for the dentine-pulp complex from more mature dental tissues. This study describes the development of a system for the organ culture of mature rodent teeth. Slices of incisors from 28-day-old rats were embedded in a semisolid, agar-based medium and cultured on floating Millipore filters at the liquid-gas interface for up to 14 days. Preservation of cell and tissue morphology was observed throughout the entire dentine-pulp complex after each culture period and autoradiographic studies showed that the odontoblasts were actively synthesizing and secreting extracellular matrix during culture. Transmission electron microscopy confirmed that the phenotypic morphology of the odontoblasts had been maintained during culture. These results demonstrate that the dentine-pulp complex from mature rodent tissues can be cultured successfully for substantial periods of time and will provide a useful model for the study of dentinogenesis and tissue repair.

Quantitative immunohistochemical evidence of a functional gradient of chondroitin 4-sulphate/dermatan sulphate, developmentally regulated in the predentine of rat incisor

A quantitative examination was carried out on the early and mature stages of dentinogenesis in the rat incisor, using a post-embedding immunogold labelling with an anti-chondroitin 4 sulphate/dermatan sulphate antibody (2B6). At a very early stage of predentine formation, before polarizing odontoblasts have established junctional complexes, immunolabelling was weak. In contrast, when polarized odontoblasts established distal junctional complexes, immunolabelling in predentine was uniform and threefold denser than in initial predentine. The same gold particle density was found in the non-mineralized mantle dentine. During circumpulpal dentine formation, a gradient was seen in predentine, a larger number of gold particles being scored in the proximal zone compared with the distal region adjacent to the mineralization front. In circumpulpal dentine, some labelling was found within the lumen of the tubules and in the bordering dentine around the tubules. A few particles were also detected in intertubular matrix after demineralization. Together, these data provide evidence for a developmentally regulated gradient during the transition between mantle and circumpulpal dentine, and also in a more mature part of the tooth, a functional gradient that probably plays a role in the process of mineralization.

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.

The influence of fluoride on the adsorption of proteoglycans and glycosaminoglycans to hydroxyapatite

Proteoglycans and their spatial arms, the glycosaminoglycans, are known to interact with hydroxyapatite, and are considered to have a role in the regulation of mineralization. This study investigates the interactive mechanisms, with particular attention directed at the influence of fluoride on the adsorption process. Proteoglycans and glycosaminoglycans were adsorbed to hydroxyapatite in the presence of fluoride (1-20 ppm range). The adsorbates included a chondroitin 4-sulfate-rich proteoglycan extracted from rat incisor dentine, hyaluronan, chondroitin 4-sulfate, and dermatan sulfate. The order of glycosaminoglycan in decreasing affinity for hydroxyapatite was chondroitin 4-sulfate, dermatan sulfate, and hyaluronan, and the individual glycosaminoglycans showed different responses to the presence of fluoride. Graded increases in fluoride (1-4 ppm) led to 5-40% reduction of glycosaminoglycan adsorption to hydroxyapatite. The proteoglycan showed less affinity for hydroxyapatite, and demonstrated a reduction in adsorption of up to 22% with 20 ppm fluoride. The inhibitory effect of fluoride indicated an electrostatic mechanism, presumably via the calcium sites in the hydroxyapatite lattice.

The influence of fluoride on proteoglycan structure using a rat odontoblast in vitro system

Using an in vitro rat incisor odontoblast system, the effect of fluoride on proteoglycans was investigated at both the metabolic and structural level. Incisors were removed from 4-week-old rats, split longitudinally, and the pulps removed. Teeth were incubated at 37 degrees C, 5% CO2 in Eagle's Minimum Essential Medium containing 35S-sulfate for 7 hours in the presence of 0 mM, 3 mM, or 6 mM sodium fluoride. Teeth were demineralized in EDTA, proteoglycan was extracted from the residue with 4 M guanidinium chloride, and further purified by anion exchange chromatography. Uptake of radiolabel was monitored by liquid scintillation counting. The resultant products were examined by cellulose acetate electrophoresis, SDS-PAGE, chondroitinase digestion, and amino acid analysis. Differential effects of fluoride were observed in both metabolism and biochemical characterization of proteoglycans following incubation at the two concentrations. Fluoride decreased uptake of the radiolabel but led to an accumulation of glycosaminoglycan within the proteoglycan of the matrix. Chondroitin sulfate was the predominant glycosaminoglycan identified, with the additional presence of dermatan sulfate and heparan sulfate identified. Dermatan sulfate levels increased in 3 mM-treated teeth. Fluoride-treated proteoglycans had a reduced molecular weight (200-90K to 180-79K); this reduction is primarily a result of smaller glycosaminoglycan chains, with limited reduction in the size of the core protein of 6 mM-treated teeth occurring. Such alterations in the biochemical metabolism and hence structure and function of proteoglycan may be implicated in the hypomineralization seen in fluorosis.

Incorporation of (35S)sulfate and (3H)glucosamine into glycoaminoglycans in rat incisor predentine and dentine: comparison by autoradiography of fixation by rapid-freezing, freeze-substitution, and aldehyde fixation

At 4 and 24 hours after injection of (35S)sulfate, there were 22-67% fewer silver grains on conventionally fixed sections than on sections fixed by cryotechniques. Differences were smaller when (3H) glucosamine was chosen as precursor. The number of silver grains increased between 4 and 24 hours in predentine and in a 30-micron band of dentine that had already undergone mineralization. Only a few grains were observed, however, in the 5-micron dentine band located at the mineralization front. This suggests that glycoaminoglycans, which in predentine have a space-filling role, facilitate transport and diffusion and inhibit mineralization, may limit crystal growth in dentine once a certain degree of apatite formation has been reached. All these properties are correlated with the structural and functional properties of the tissues.

Short-term effects of fluoride on biosynthesis of enamel-matrix proteins and dentine collagens and on mineralization during hamster tooth-germ development in organ culture

The effect of various concentrations of fluoride (F-) on cell proliferation, matrix formation and mineralization was examined in hamster molar tooth germs in premineralizing and mineralizing stages. The exposure lasted 16 h (mineralizing stages) and 24 h (premineralizing stages) and the F- levels ranged from 2.63 microM to 2.63 mM; [3H]-thymidine, [3H]-proline, 45Ca and 32PO4 were used as markers for cell proliferation, matrix formation and mineralization, respectively. The proline-labelled amelogenins were isolated by sequential extraction with water and formic acid and their nature examined by SDS-urea-polyacrylamide electrophoresis. Digestion by collagenase was used to assess the amount of proline incorporated into collagens. F- in concentrations up to 1.31 mM inhibited neither biosynthesis of DNA and amelogenins, nor synthesis of collagens and their hydroxylation. Amelogenins extracted from F- induced, non-mineralizing enamel matrix had the same electrophoretic mobility and the same degree of phosphorylation as amelogenins from normal, mineralizing enamel. However, F- increased the uptake of 45Ca and TCA-soluble 32P dose-dependently, starting with 52 microM. Thus, interference with secretion of enamel matrix by F- takes place at much lower concentrations than required to inhibit biosynthesis of enamel matrix.

Long-term (8 days) effects of exposure to low concentrations of fluoride on enamel formation in hamster tooth-germs in organ culture in vitro

Second maxillary molars of 3-4-day-old hamsters were cultured for 7-8 days in the continuous presence of fluoride (F-) or chloride in concentrations between 2.63 microM and 1.31 mM. For biochemical study, explants were labelled during the last 24 h of culture with a triple label of [3H]-proline, 45Ca and 32PO4. The 3H-labelled presumptive amelogenins were separated from the 3H-labelled dentine collagens by a three-step extraction procedure. Histologically, chronic exposure to F- had no obvious effects below 26.3 microM; at 26.3 microM of F-, a non-mineralizing enamel matrix was observed besides that of a normal mineralizing enamel. From 52 microM of F- onwards, only a non-mineralizing enamel matrix was found in decreasing amounts extracellularly as F- concentrations increased. Except for the presence of globular dentine, dentinogenesis was not obviously affected by F-. Biochemically, total synthesis of presumptive amelogenins was hardly disturbed, but their solubility was changed by chronic F- treatment; more amelogenins became formic-acid soluble at the expense of water-soluble amelogenins. Chronic exposure to F- decreased the water-soluble amelogenin fraction according to a logarithmic function of the medium F- concentration. By extrapolation, it was calculated that concentrations higher than 1-2 microM of F- affect amelogenesis in vitro. Synthesis of dentine collagen was not affected by chronic exposure to F- in vitro. Chronic exposure to F- decreased uptake of 45Ca and to a less extent trichloroacetic acid-soluble 32PO4. Chronic F- exposure may inhibit energy production in the enamel organ resulting in an impairment of enamel matrix secretion as well as that of a trans-epithelial transport mechanism for calcium.

Effect of dissociative extraction with 1.5 M calcium chloride on proteoglycans in rat-incisor predentine visualized with cuprolinic blue

Treatment of rat-incisor predentine for 48 h in a 1.5 M CaCl2 solution halved the number and size of the proteoglycan precipitates visualized using a critical MgCl2 concentration and cuprolinic blue staining. About 75 per cent of the aggregable proteoglycans were extracted; 25 per cent resisted extraction. Treatment with CaCl2 also completely abolished the electron density of the amorphous ground substance, an observation consistent with the possible existence of two classes of proteoglycans in predentine.

The ultrastructure of the rat incisor odontoblast in organ culture

Extracted incisors were dissected free of adherent soft tissue, split by a longitudinal incision lingually and the pulp sac carefully removed. The predentine-dentine pieces with attached odontoblasts were incubated for up to 24 h in culture medium in 5 per cent CO2, humid air at 37 degrees C. The cultures were recovered and processed for light microscopy and transmission electron microscopy. Examination of the explants showed a homogeneous layer of odontoblasts attached to the dentinal surface. The odontoblasts had a normal structure and nuclear polarity similar to that seen in vivo. There were no apparent degenerative changes.

Electron microscopic visualization of proteoglycans in rat incisor predentine and dentine with cuprolinic blue

Cuprolinic blue in a MgCl2-critical electrolyte concentration solution allows a larger surface representation of proteoglycans than other methods. Glycoaminoglycans in predentine were ribbon-like structures 14 nm thick and 60 nm long, with radiating filaments (3-4 nm) located in spaces between collagen fibres. Glycoaminoglycans and collagen together displayed longitudinal and orthogonal relationships. The ground substance was slightly electron dense. In dentine, the precipitate was less in quantity and consisted of rounded granules (15 nm in diameter, and filaments/3 nm in width) closely associated with the surfaces of collagen fibres.

Autoradiographic visualization of glycoproteins and glycosaminoglycans in the epithelio-mesenchymal interface of developing mouse tooth germ

The turnover of basement membrane macromolecules during tooth morphogenesis and odontoblast differentiation was examined by light microscopic autoradiography using 3H-fucose, 35S-sulfate and 3H-glucosamine. Marked incorporation into the basement membrane was found throughout the progressive development. Pulse-chase experiments and prelabeling of tissue components indicated that glycoproteins and glycosaminoglycans in the dental basement membrane are mainly derived from the enamel epithelium. During odontoblast differentiation, incorporation was increased at the epitheliomesenchymal interface at the site of differentiating mesenchymal cells. From these sites the label also disappeared rapidly. This suggests that the active remodeling of extracellular matrix is related to odontoblast differentiation.