Morphological and structural studies of early mineral formation in enamel of rat incisors by electron spectroscopic imaging (ESI) and electron spectroscopic diffraction (ESD)

Preferential and selective degradation and removal of amelogenin adsorbed on hydroxyapatites by MMP20 and KLK4 in vitro

The hardest tooth enamel tissue develops from a soft layer of protein-rich matrix, predominated by amelogenin that is secreted by epithelial ameloblasts in the secretory stage of tooth enamel development. During enamel formation, a well-controlled progressive removal of matrix proteins by resident proteases, Matrix metalloproteinase 20 (MMP20), and kallikrein 4 (KLK4), will provide space for the apatite crystals to grow. To better understand the role of amelogenin degradation in enamel biomineralization, the present study was conducted to investigate how the adsorption of amelogenin to hydroxyapatite (HAP) crystals affects its degradation by enamel proteinases, MMP20 and KLK4. Equal quantities of amelogenins confirmed by protein assays before digestions, either adsorbed to HAP or in solution, were incubated with MMP20 or KLK4. The digested samples collected at different time points were analyzed by spectrophotometry, SDS-PAGE, high performance liquid chromatography (HPLC), and LC-MALDI MS/MS. We found that majority of amelogenin adsorbed on HAP was released into the surrounding solution by enzymatic processing (88% for MMP20 and 98% for KLK4). The results show that as compared with amelogenin in solution, the HAP-bound amelogenin was hydrolyzed by both MMP20 and KLK4 at significantly higher rates. Using LC-MALDI MS/MS, more accessible cleavage sites and hydrolytic fragments from MMP20/KLK4 digestion were identified for the amelogenin adsorbed on HAP crystals as compared to the amelogenin in solution. These results suggest that the adsorption of amelogenin to HAP results in their preferential and selective degradation and removal from HAP by MMP20 and KLK4 in vitro. Based on these findings, a new degradation model related to enamel crystal growth is proposed.

Quantitatively and kinetically identifying binding motifs of amelogenin proteins to mineral crystals through biochemical and spectroscopic assays

Tooth enamel is the hardest tissue in vertebrate animals. Consisting of millions of carbonated hydroxyapatite crystals, this highly mineralized tissue develops from a protein matrix in which amelogenin is the predominant component. The enamel matrix proteins are eventually and completely degraded and removed by proteinases to form mineral-enriched tooth enamel. Identification of the apatite-binding motifs in amelogenin is critical for understanding the amelogenin-crystal interactions and amelogenin-proteinases interactions during tooth enamel biomineralization. A stepwise strategy is introduced to kinetically and quantitatively identify the crystal-binding motifs in amelogenin, including a peptide screening assay, a competitive adsorption assay, and a kinetic-binding assay using amelogenin and gene-engineered amelogenin mutants. A modified enzyme-linked immunosorbent assay on crystal surfaces is also applied to compare binding amounts of amelogenin and its mutants on different planes of apatite crystals. We describe the detailed protocols for these assays and provide the considerations for these experiments in this chapter.

General principle of ordered apatitic crystal formation in enamel and collagen rich hard tissues

The biomineralization processes in different hard tissues like enamel, circumpulpal dentine, epiphyseal growth plates were analyzed morphologically and ultrastructurally by an energy filtering transmission electron microscope. In the primary stage of crystal formation Ca- and phosphate-ions accumulate at charged sites, "active sites", along the fiber matrix-molecules of the extracellular matrix. After exceeding the critical radius for nucleation, crystal nuclei appear that develop to "chains" of stable nanometer-sized paracrystalline particles. In the latest studies of small area electron diffraction it was found that in the earliest stage of crystal formation these mineral chains show a parallel orientation in the direction of the c-axis of apatite. This was supported by a texture of the 002 reflection in the corresponding diffraction patterns. Since apatite is bipolar in this direction crystal growth would be in like manner in both directions. Thus the center-to-center distances between nucleating sites along the matrix macromolecules show with the chains of nanometer islands the same process of biomineralization in the different mineralizing hard tissue systems. This way of crystal formation might be a general principle of apatitic biomineralization.

The crystalline pattern of calcium in different topographical regions of the otoconial membrane. An electron probe and spectroscopic diffraction study

Quantitative electron probe microanalysis and electron spectroscopic diffraction analysis was used to determine the gradient of distribution of calcium and its crystalline pattern at different levels (lower gelatinous membrane, upper gelatinous membrane and otoliths) in the otoconial membrane of adult OF1 mice. Our quantitative electron probe microanalytical data, obtained with scanning-transmission electron microscopy, indicated that there was a gradient in calcium concentration which increased from the vestibular surface towards the otoliths. Differences between the three regions of the otoconial membrane were statistically significant in both the utricle and saccule. Our results with electron spectroscopic diffraction revealed an increasing crystalline development from the lower gelatinous membrane towards the otoliths. Our findings with both techniques suggest that the gelatinous membrane is involved in the maturation and crystallization of the otoliths.

High resolution electron microscopy: structure and growth mechanisms of human dentin crystals

Biological crystal formation was postulated to begin by a nucleation process. Such processes have been demonstrated for human amelogenesis and bone mineralization. The aim of this study was to confirm if such mechanisms occur during dentin crystal formation. The structure of human fetal dentin crystals and the earliest stages of mineral growth were followed by High Resolution Electron Microscopy (HREM) associated with digitalized image analysis. Micrographs of the mineralization front were first digitalized, and selected areas were transformed in the reciprocal space by Fast Fourier Transform. The resulting diffractograms were compared with computer-simulated diffractograms and used to determine the orientation of crystals. Dentin crystals, found close to the mineralization front, show a structure closely related to that of hydroxyapatite (HA), as determined by comparison of HREM images with simulated images. These crystals present numerous structural defects such as dislocations and grain boundaries. These defects appear to be present in dentin crystals at an early stage of growth. We have also observed nanometer-sized particles in mineralization areas. Calculated diffractograms of these areas show significant similarities with HA diffraction patterns, and in one case, their structure could be correlated to HA structure through an image simulation process. These nanometer-sized particles could be related to the nucleation process, and their growth, orientation, and formation appear to be mediated by extracellular matrix components.

Structural relationship between the primary crystal formations and the matrix macromolecules in different hard tissues. Discussion of a general principle

For many years we have investigated the earliest crystal formations of different developing hard tissues (matrix vesicle, bone, dentine, enamel, etc.) by different electron microscopic measurements. It was observed that primarily Ca-phosphate (apatite) "chains," composed of nanometer sized particles (dots, islands), exist, which coalesce rapidly to needles. For the mineralization of collagen (e.g., bone, dentine) the center to center distances between the dots in the mineral chains represent the distances between nucleating sites, so-called "active sites" of collagen which bind primarily Ca for a subsequent nucleation. For the mineralization of noncollagen macromolecules (e.g., enamel) the same principle of mineral nucleation at such "active sites" exists being represented indirectly by corresponding center to center distances between the dots in the mineral chains.

Microradiography and fluorescence microscopy of bone remodeling on the basal crypt of permanent mandibular premolars in dogs during eruption

Alveolar bone of erupting teeth was studied in order to define the types of calcified tissues deposited as well as the rate of tooth growth. The third (P3) and fourth (P4) mandibular premolars of 30 dogs aged 12-24 weeks were analyzed by microradiography and microscopy in fluorescent and ordinary light. The bone plate separating P3 and P4 from the mandibular canal presented a complex arrangement of lamellar and woven bone, and even of chondroid tissue. During the pre-eruptive phase, this plate shifted towards the base of the mandible by means of selective resorption and apposition activities. As soon as the furcation was formed, bone apposition appeared on the alveolar side and became the main activity under P3 at the outset of eruption. Under the roots of P4 it occurred 4 weeks later. Dynamic morphometry in fluorescence microscopy showed that eruption progressed faster than the radicular growth. The formation of interradicular bone underwent the same acceleration as the eruption. However, though the tissues were formed at a high rate, it cannot be inferred therefrom that they are responsible for tooth shifting. They might just fill the space left by the erupting tooth.