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PubMed2006

Formation of the dentino-enamel interface in enamelysin (MMP-20)-deficient mouse incisors.

An anomalous dentino-enamel junction (DEJ), manifested by delamination of the enamel layer, was reported in enamelysin [matrix metalloproteinase-20 (MMP-20)] knockout (KO) mice. To better understand the possible role of MMP-20 in the formation of the DEJ, we performed transmission electron microscopy (TEM) studies of the DEJ at early stages of tooth morphogenesis in KO mice. Our TEM analysis revealed that in the incisors from KO mice the mantle dentin is hypomineralized at the onset of enamel mineralization. At this early stage, TEM revealed no apparent differences in nascent aprismatic enamel between the KO mice and the controls. Hypomineralized mantle dentin was also observed in the incisors from KO mice, as assessed by back-scattered SEM at the secretory and early maturation stages, but not in the late-maturation stage, suggesting that the mineralization of mantle dentin is not completely arrested, but rather postponed. Histological studies indicate that the organic content in the initial enamel layer remains very high throughout amelogenesis. These results imply that MMP-20 is involved in the regulation of mineralization in mantle dentin and demonstrate the complex nature of DEJ formation. They also suggest that the structural and functional properties of the DEJ are determined during the initial mineralization stages.

Amelogenesis↗
PubMed2006

Assembly and processing of an engineered amelogenin proteolytic product (rP148).

The purpose of this study was to express, characterize, and investigate the self-assembly of a recombinant porcine amelogenin lacking the hydrophilic 24 C-terminal amino acids (rP148). To gain further insight into the function of amelogenin processing during enamel mineralization, this protein was also used as a substrate to examine the action of matrix metalloproteinase-20 (MMP-20). The assembly properties of rP148 were monitored by dynamic light scattering (DLS). In general, rP148 molecules assemble into monomers, dimers, oligomers, and some nanosphere-like particles. Depending on the solution conditions, large aggregates were also observed. Matrix metalloproteinase-20 cleaved the rP148 molecule at a few sites, creating a number of different products, including the tyrosine-rich amelogenin polypeptide (TRAP). Our data suggest that although rP148 self-assembles into small particles, its assembly properties are different from those of the full-length rP172, indicating that the C-terminal 24 amino acids play a critical role in nanosphere assembly. We further demonstrate that MMP-20 digests rP148 in a manner that generates a similar proteolytic pattern, as would be expected to occur in vivo.

Amelogenin↗
PubMed2006

Formation of acellular cementum-like layers, with and without extrinsic fiber insertion, along inert bone surfaces of aging c-Src gene knockout mice.

To investigate the long-term effects of c-src deficiency on skeletal and dental tissues, we examined the lower jaws and long bones of c-src gene knockout (c-src KO) mice by histological and histochemical methods. Numerous multinucleated osteoclasts were distributed throughout the mandible in 5-wk-old c-src KO mice, but by 14 wk they had almost completely disappeared from the alveolar bone, leaving tartrate-resistant acid phosphatase (TRAP)-positive layers along the bone surface. Deposition of osteopontin-positive mineralized tissue, reminiscent of acellular afibrillar cementum (AAC), was confirmed along the TRAP-positive bone surface at 14 wk. The layer progressively thickened up to 21 months. A comparable mineralized layer was noted along the trabeculae of long bones as thickened cement lines. In the periostin-rich areas of jaw bones, but not in the long bones, portions of AAC-like mineralized layers were often replaced with and/or covered by acellular extrinsic fiber cementum (AEFC)-like tissue. These data suggest that the deposition of AAC-like mineralized tissue is a general phenomenon that may occur along inert or slowly remodeling bone surfaces under conditions characterized by reduced bone-resorbing activity, whereas the induction of AEFC-like tissue seems to be associated with the expression of certain molecules that are particularly abundant in the microenvironment of the periodontal ligament.

Aging↗
PubMed2004

The involvement of platelet-derived growth factor receptors and insulin-like growth factor-I receptors signaling during mineralized nodule formation by human periodontal ligament cells.

BACKGROUND AND OBJECTIVE: Periodontal ligament cells are regarded to have the capacity to differentiate into cementoblasts or osteoblasts, and are capable of forming a mineralized nodule in vitro. However, the precise mechanisms are unclear. Here we evaluated the possible involvement of growth factor receptors, such as the platelet-derived growth factor receptor (PDGFR), insulin-like growth factor-I receptor (IGF-IR), and epidermal growth factor receptor (EGFR) on periodontal ligament cells and their ligands during periodontal ligament cells differentiation in vitro. METHODS: Human periodontal ligament cells were differentiated via culturing in the presence of dexamethasone, ascorbic acid, and beta-glycerophosphate for mineralized nodule formation, characterized by von Kossa staining. Expressions of receptors and their ligands were analyzed by flow cytometry/reverse transcription-polymerase chain reaction. RESULTS: During the differentiation, PDGFR-alpha was held at a lower level compared with the control. PDGFR-beta, however, was maintained at a slightly higher level that was reversed to the control level when mineralized nodules formed. In contrast, IGF-IR and EGFR were not substantially different from the control. The mineralized nodule formation was strongly inhibited by a PDGFR kinase blocker (AG1295 and AG1296), partially inhibited by an IGF-IR kinase blocker (I-Ome-AG538 and AG1024), and not inhibited by an EGFR kinase blocker (AG99). PDGF-A, PDGF-C, PDGF-D, IGF-I, and IGF-II, but not PDGF-B, were expressed on the control as well as dexamethasone/ascorbic acid-treated periodontal ligament cells during mineralized nodule formation; however, the pattern of their expressions was quite different. CONCLUSION: These findings suggest that a pathway of PDGFs/PDGFR and IGFs/IGF-IR on periodontal ligament cells are involved during mineralized nodule formation, and that PDGFs and IGFs expressed by periodontal ligament cells may contribute to the formation.

Adolescent↗
PubMed1982

The effect of fluoride in the remineralization of enamel caries and caries-like lesions in vitro.

Remineralization occurs naturally during the formation of a carious lesion in human dental enamel and is seen in the form of two of the four classical histological zones of the lesion. Exposure of small lesions to a synthetic calcifying in vitro results in a significant increase in remineralization of the lesion. The degree of remineralization achieved depends upon the presence of fluoride ions in the calcifying fluid and its degree of supersaturation. It appears that only low levels of fluoride are required to trigger the mechanism of remineralization; raising the fluoride level further does not result in a greater degree of remineralization. The calcium ion concentration of the calcifying fluid is critical with respect to determining which components are supersaturated and which in turn determines the degree of remineralization achieved. With levels of 3.0 mM calcium only the superficial region of the lesion is remineralized whereas with a calcium concentration of 1.0 mM, remineralization occurs in depth. It is hoped that studies on remineralization phenomena will provide further insight into the mechanisms of caries formation and be the basis for a more powerful and effective form of caries prevention.

Calcium↗
PubMed1991

An epidemiologic estimate of the critical period during which human maxillary central incisors are most susceptible to fluorosis.

The temporal relation between a declining fluorosis gradient and an abrupt downward shift in community drinking water fluoride concentration was evaluated through multiple correlation analysis to determine the critical time frame during which developing maxillary central incisors are most susceptible to fluoride challenge. Fluorosis data were scrutinized through a time-related series of epidemiologic "windows" or time frames of varying lengths. The placement of these time frames was in turn related to the presumed start of enamel mineralization (at birth), and ranged from zero to 60 months later. In this way, the susceptibility of developing enamel to changes in water fluoride concentration was localized. The greatest risk was associated with a four-month critical period commencing at 22 months following birth. The risk of fluorosis from exposures to a fluoride challenge acting during shorter periods was better localized than risk associated with longer exposures. We concluded (1) that human maxillary central incisors are most susceptible to fluorosis during a critical period of as little as four months' duration, commencing at 22 months of age; and (2) that for these incisors, fluoride exposure during the months prior to this period carries less risk than continued exposure for up to 36 months beyond this critical time.

Aging↗
PubMed1999

Mechanism and timing of fluoride effects on developing enamel.

Fluoride appears to specifically interact with mineralizing tissues, causing an alteration of the mineralization process. In enamel, fluorosis results in a subsurface hypomineralization. This hypomineralized enamel appears to be directly related to a delay in the removal of amelogenins at the early-maturation stage of enamel formation. The specific cause for this delay is not known, although existing evidence points to reduced proteolytic activity of proteinases that hydrolyze amelogenin. This delay in hydrolysis of amelogenins could be due to a direct effect of fluoride on proteinase secretion or proteolytic activity, or to a reduced effectiveness of the proteinase due to other changes in the protein or mineral of the fluorosed enamel matrix. The formation of dental fluorosis is highly dependent on the dose, duration, and timing of fluoride exposure. The early-maturation stage of enamel formation appears to be particularly sensitive to the effects of fluoride on enamel formation. Although the risk of enamel fluorosis is minimal with exposure only during the secretory stage, this risk is greatest when exposure occurs in both secretory and maturation stages of enamel formation. The risk of fluorosis appears to be best related to the total cumulative fluoride exposure to the developing dentition.

Amelogenesis↗
PubMed1980

Laboratory studies on the demineralization and remineralization of human enamel in relation to caries mechanisms.

These studies demonstrate that a significant degree of remineralization of both natural and artificial lesions can be achieved in vitro. Changing the calcium concentration of the synthetic calcifying fluid had a significant effect on both the degree of remineralization achieved and the site of the lesion affected. When the calcifying fluid contained 3 mM calcium, remineralization occurred mainly in the superficial part of the lesion. When a 1 mM calcium calcifying fluid was used on adjacent halves of the same lesion, remineralization occurred throughout the entire depth of the lesion. Under these conditions there was a highly significant reduction in lesion size accompanied by an increase in orientated mineral crystals. The crystals were found to have diameters several times larger than those found in the sound enamel, most probably produced as a result of crystal growth. These results have significance in terms of both mechanisms of hard tissue repair and the possible eventual development of an agent for caries prevention.

Dental Caries↗
PubMed1997

A light microscopic and ultrastructural examination of calcified dental tissues of horses: 1. The occlusal surface and enamel thickness.

Gross and microscopic examinations were undertaken on 46 cheek (molar and premolar) and 4 incisor equine teeth that were fractured, or sectioned either with a lathe or diamond saw. Specimens were examined without treatment, after decalcification or acid etching, utilising light, and scanning and transmission electron microscopy. In some horses, the occlusal surface of the teeth were covered with an organic pellicle. The occlusal surface of the underlying equine enamel contained different wear patterns, including polished areas, local fractures, wedge-shaped pits, striations and depressions. Occlusal dentine showed depressions whose depth was related to its occlusal surface area, with larger surface areas having deeper depressions. The thickness of equine enamel varied greatly throughout its folds in the transverse plane, and was thickest in areas where folds were parallel to the long axis of the maxilla and mandible. Enamel thickness remained constant in the longitudinal plane (throughout the length of the tooth). Peripheral enamel was more deeply infolded in lower than in upper cheek teeth and this appeared to compensate for the absence of infundibula (deep, cup-like enamel indentations that are partially filled with cement) in the lower cheek teeth.

Animals↗
PubMed1997

A light microscopic and ultrastructural examination of calcified dental tissues of horses: 2. Ultrastructural enamel findings.

Ultrastructural examinations of defined 3 equine enamel types termed equine (Eq.) Types 1, 2 and 3 enamel, according to the transverse appearance of their enamel prisms and the amount and appearance of their interprismatic enamel. Eq. Type 1 enamel contained alternating rows of oval shaped prisms and thick interprismatic enamel plates, and was found adjacent to the amelodentinal junction. Eq. Type 2 enamel consisted of circular, 'keyhole' to 'horseshoe' shaped prisms with little or no interprismatic enamel and was located adjacent to the amelocemental junction. Eq. Type 3 enamel was composed of rounded prisms surrounded by large amounts of interprismatic enamel and was inconsistently present in a thin layer at the amelodentinal and amelocemental junctions. Prism decussation was seen in the thickest peripheral enamel of the upper cheek teeth but was present throughout incisor enamel therefore making incisors highly resistant to cracking. Scanning electron microscopic examination showed enamel crystals to be cylindrical shaped on transverse section; however, on transmission electron microscopic examination these crystals had shapes, ranging from near oval to rectangular and formed small subunits, with crystals diverging from each other at various angles.

Animals↗
PubMed1997

A light microscopic and ultrastructural examination of calcified dental tissues of horses: 3. Dentine.

Ultrastructural examinations of equine dentine found that dentinal tubules extended from the amelodentinal junction towards the pulp forming primary curvatures. The number of dentinal tubules/unit area and their diameters increased significantly from the amelodentinal junction towards the pulp cavities, particularly in regular secondary dentine, but irregular secondary (tertiary) dentine contained no dentinal tubules. Dentinal tubules contained odontoblast processes that appeared to extend as far as the amelodentinal junction, but due to iatrogenic loss during specimen preparation, odontoblasts were seldom found in regular secondary dentine. In primary dentine, the dentinal tubules were surrounded by large amounts of peritubular dentine that increased in diameter from the amelodentinal junction towards the junction of primary and secondary dentine. The site of the dentinal tubule within the peritubular dentine varied at different dentinal sites. Peritubular dentine was present in primary dentine only and was surrounded by a thin layer of intertubular dentine. When acid etched, peritubular dentine gave dentine a honeycomb appearance adjacent to the junction of primary and secondary dentine.

Animals↗