Formation of octacalcium phosphate in vitro

Effect of air-abrasion pretreatment with three bioactive materials on enamel susceptibility to erosion by artificial gastric juice

OBJECTIVE

The purpose of the study was to investigate the protective effect of three in-office preventive treatments with bioactive materials against enamel erosion induced by artificial gastric juice similar to that found in gastroesophageal reflux disease (GERD) patients. The treatments included air-abrasion of enamel with a fluoride-containing bioactive glass (BioMinF®), Bioglass 45S5 (ProSylc) and nano-hydroxyapatite (MI Pearls) to test enamel susceptibility following an erosive challenge.

METHODS

Enamel surface loss was evaluated using confocal microscopy, while changes in enamel surface roughness and morphology were also investigated after the treatments. SEM and EDS were used to observe formation of apatite crystals on enamel and to detect alterations in mineral composition. In Group 1 (negative control) the specimens did not receive any treatment; Group 2 specimens (positive control) treated with 0.4 % SnF₂, while in Groups 3-5 enamel was air-abraded with BioMinF®, ProSylc and MI Pearls, respectively.

RESULTS

All the experimental groups reduced significantly enamel surface loss compared to the negative control group (p < 0.05), except for the MI Pearls treatment (p > 0.05). The most protective behavior against erosion presented the treatment with SnF₂. BioMinF induced the larger amount of apatite crystals on the enamel surface, followed by ProSylc.

SIGNIFICANCE

BioMinF and ProSylc treatments may be beneficial against dental erosion induced by gastric juice in GERD patients, while MI Pearls treatment may not suitable for this indication. Both materials promote formation of apatite crystals on enamel in acidic conditions protecting the surface from the erosion. The tested treatments may be useful in GERD patients who cannot comply with at-home therapies with SnF₂.

Ultrastructure of early amelogenesis in wild-type, Amelx-/-, and Enam-/- mice: enamel ribbon initiation on dentin mineral and ribbon orientation by ameloblasts

INTRODUCTION

Dental enamel is comprised of highly organized, oriented apatite crystals, but how they form is unclear.

METHODS

We used focused ion beam (FIB) scanning electron microscopy (SEM) to investigate early enamel formation in 7-week-old incisors from wild-type, Amelx^-/-, and Enam^-/- C56BL/6 mice. FIB surface imaging scans thicker samples so that the thin enamel ribbons do not pass as readily out of the plane of section, and generates serial images by a mill and view approach for computerized tomography.

RESULTS

We demonstrate that wild-type enamel ribbons initiate on dentin mineral on the sides and tips of mineralized collagen fibers, and extend in clusters from dentin to the ameloblast membrane. The clustering suggested that groups of enamel ribbons were initiated and then extended by finger-like membrane processes as they retracted back into the ameloblast distal membrane. These findings support the conclusions that no organic nucleator is necessary for enamel ribbon initiation (although no ribbons form in the Enam^-/- mice), and that enamel ribbons elongate along the ameloblast membrane and orient in the direction of its retrograde movement. Tomographic reconstruction videos revealed a complex of ameloblast membrane processes and invaginations associated with intercellular junctions proximal to the mineralization front and also highlighted interproximal extracellular enamel matrix accumulations proximal to the interrod growth sites, which we propose are important for expanding the interrod matrix and extending interrod enamel ribbons. Amelx^-/- mice produce oriented enamel ribbons, but the ribbons fuse into fan-like structures. The matrix does not expand sufficiently to support formation of the Tomes process or establish rod and interrod organization.

CONCLUSION

Amelogenin does not directly nucleate, shape, or orient enamel ribbons, but separates and supports the enamel ribbons, and expands the enamel matrix to accommodate continued ribbon elongation, retrograde ameloblast movement, and rod/interrod organization.

Effect of fluoride on the morphology of calcium phosphate crystals grown on acid-etched human enamel

The aim of this study was to examine the effect of fluoride ion concentration on the morphology of calcium phosphate crystals grown on acid-etched enamel as a model for tooth enamel erosion. Samples were immersed in calcification solution for 16 h and changes in crystal morphology were monitored by field emission scanning electron microscopy. Without fluoride, plate-like octacalcium phosphate crystals (20 nm thick, 2-10 microm wide) were formed. With 1-10 mg/l fluoride, arrays of denser needle-like nanocrystals (20-30 nm wide, >500 nm in length) were formed. We conclude that there is a minimal fluoride concentration (1 mg/l) that dramatically affects the morphology of calcium phosphate crystals grown on etched enamel in vitro.

Fluoride incorporation in hydroxyapatite/gelatin nanocomposite

The incorporation of fluoride ion into hydroxyapatite (HAp)/gelatin (GEL) nanocomposite was investigated. The F(-) ion incorporation into OH(- )site of HAp phase was an energetically active process, which could be confirmed from the spray solution reaction. The precursors of Ca(2+) in water and phosphates in aqueous gelatin were mixed in the humidified air chamber by air spray, and then the precipitates were aged in a reactor. The F(-) ion precursor was dissolved in the starting solution of Ca(OH)(2) in water, and the resulted Ca(OH, F)(2) complex droplets induced the formation of stable fluoroapatite (F, OH)Ap. The reaction kinetics could be assumed from TEM morphology with ED, XRD and FT-IR analysis.

Organic-inorganic interaction and the growth mechanism of hydroxyapatite crystals in gelatin matrices between 37 and 80 degrees C

The crystal development of hydroxyapatite[HAp] phase in gelatin[GEL] matrices was investigated in the temperature range 37 to 80 degrees C by using X-ray diffraction, scanning electron microscopy(SEM), thermoanalytical measurement(DT/TGA), Fourier-Transformed Infra-Red(FT-IR) spectroscopy, and transmission electron microscopy(TEM) with electron diffraction(ED). It was found that during the coprecipitation of apatite phase in GEL matrices and the next aging process the crystallites were formed and developed through the two reaction mechanisms of organic-inorganic interaction between apatite phase and GEL molecules, and thermodynamic reaction for the crystal growing. The analytical evidences showed that there was a definite competition between these two mechanisms with the reaction temperature. Below 50 degrees C the crystal development of HAp was greatly suppressed by the existence of the GEL molecules, indicating the heterogeneous nucleation by the supposed number of carboxyl groups in GEL. Above 50 degrees C the effective organic components as a template for the heterogeneous nucleation of apatite crystallites were greatly degraded and so more amount of inorganic ions could be favorably accredited on the preexisting crystallites in virtue of the limited nucleation chance, finally resulting in the crystal growth. At higher temperature pretty big HAp crystals were developed with the depletion of the organics to be bound with crystallites in the slurry solution. Presumably it is believed that the poisoning of the functional groups in GEL molecules was vigorously occurred in the phosphoric acid environment above approximately 50 degrees C.

Mineral acquisition rates in developing enamel on maxillary and mandibular incisors of rats and mice: implications to extracellular acid loading as apatite crystals mature

The formation rates of mineral in developing enamel were determined by microweighing of incisors of mice and rats. Computations indicated that a large excess of hydrogen ions would result from creating apatite at the calculated rates. Enamel organ cells (ameloblasts), therefore, likely excrete bicarbonate ions to prevent pH in fluid bathing enamel from becoming too acidic.

INTRODUCTION

Protons (H+) are generated whenever calcium and phosphate ions combine directly from aqueous solutions to form hydroxyapatite. Enamel is susceptible to potential acid loading during development because the amount of fluid bathing this tissue is small and its buffering capacity is low. The epithelial cells covering this tissue are also believed to form permeability barriers at times during the maturation stage when crystals grow at their fastest rates. The goal of this study was to measure the bulk weight of mineral present in rodent enamel at specific times in development and estimate the amount of acid potentially formed as the apatite crystals mature.

MATERIALS AND METHODS

Upper and lower jaws of mice and rats were freeze-dried, and the enamel layers on the incisors were partitioned into a series of 0.5 mm (mouse) or 1.0 mm (rat) strips along the length of each tooth. The strips were weighed on a microbalance, ashed at 575 degrees C for 18-24 h to remove organic material, and reweighed to determine the actual mineral weight for each strip.

RESULTS AND CONCLUSIONS

The data indicated that, despite differences in gross sizes and shapes of maxillary and mandibular incisors in rats and mice, the overall pattern and rates of mineral acquisition were remarkably similar. This included sharply increasing rates of mineral acquisition between the secretory and maturation stages, with peak levels approaching 40 microg/mm tooth length. Computer modeling indicated that quantities of H+ ions potentially generated as apatite crystals grew during the maturation stage greatly exceeded local buffering capacity of enamel fluid and matrix proteins. In other systems, bicarbonate ions are excreted to neutralize highly acidic materials generated extracellularly. Data from this study indicate that ameloblasts, and perhaps cells in other apatite-based hard tissues, use similar bicarbonate release mechanisms to control excess acid arising from mineral formation.

Interactions of amelogenins with octacalcium phosphate crystal faces are dose dependent

Amelogenins, the major protein components of the enamel extracellular matrix, are postulated to be involved in controlling the elongated and oriented growth of enamel carbonated apatite crystals. In order to clarify the functional role of amelogenin during the early stage of enamel biomineralization, octacalcium phosphate (OCP) crystals, known to be potent precursors of hydroxyapatite, were grown in 1-10% (w/w) native bovine and two recombinant murine amelogenins. Amelogenins were solution-like at 1% and formed gel at 10%, while 5% amelogenins became gel after reaction and it was inhomogeneous and porous. Morphological changes of OCP crystals were evaluated as the function of amelogenin concentration by analyzing the mean values of length, width, thickness, their reduction ratios (L/Lc, W/Wc, T/Tc) as well as L/W and W/T ratios. Length, width, and thickness decreased in a does-dependent manner. Length decreased almost linearly in 1%-10%, whereas width decreased drastically in 1%-5% while the decrease from 5% to 10% was small. As a result, elongated morphology of OCP crystal was most emphasized in 5% bovine amelogenins and rM166 and 2%-5% rM179. The size reduction was in the order of W/Wc < L/Lc < T/Tc. We therefore concluded that amelogenin interaction with crystal faces was in the order (010) > (001) > (100). At all concentrations, W/ Wc was significantly the smallest. This indicated that the primary role of amelogenin was to decrease the width of OCP by blocking the hydrophobic (010) faces. We suggest that the drastic decrease of crystal width is the result of interaction of the densely packed nanospheres in 5%-10% amelogenin.

Elongated growth of octacalcium phosphate crystals in recombinant amelogenin gels under controlled ionic flow

Amelogenin proteins constitute the primary structural entity of the extracellular protein framework of the developing enamel matrix. Recent data on the interactions of amelogenin with calcium phosphate crystals support the hypothesis that amelogenins control the oriented and elongated growth of enamel carbonate apatite crystals. To exploit further the molecular mechanisms involved in amelogenin-calcium phosphate mineral interactions, we conducted in vitro experiments to examine the effect of amelogenin on synthetic octacalcium phosphate (OCP) crystals. A 10% (wt/vol) recombinant murine amelogenin (rM179, rM166) gel was constructed with nanospheres of about 10- to 20-nm diameter, as observed by atomic force microscopy. The growth of OCP was modulated uniquely in 10% rM179 and rM166 amelogenin gels, regardless of the presence of the hydrophilic C-terminal residues. Fibrous crystals grew with large length-to-width ratio and small width-to-thickness ratio. Both rM179 and rM166 enhanced the growth of elongated OCP crystals, suggesting a relationship to the initial elongated growth of enamel crystals.