A genetic model for the secretory stage of dental enamel formation

The revolution in genetics has rapidly increased our knowledge of human and mouse genes that are critical for the formation of dental enamel and helps us understand how enamel evolved. In this graphical review we focus on the roles of 41 genes that are essential for the secretory stage of amelogenesis when characteristic enamel mineral ribbons initiate on dentin and elongate to expand the enamel layer to the future surface of the tooth. Based upon ultrastructural analyses of genetically modified mice, we propose a molecular model explaining how a cell attachment apparatus including collagen 17, α6ß4 and αvß6 integrins, laminin 332, and secreted enamel proteins could attach to individual enamel mineral ribbons and mold their cross-sectional dimensions as they simultaneously elongate and orient them in the direction of the retrograde movement of the ameloblast membrane.

MMP20-generated amelogenin cleavage products prevent formation of fan-shaped enamel malformations

Dental enamel forms extracellularly as thin ribbons of amorphous calcium phosphate (ACP) that initiate on dentin mineral in close proximity to the ameloblast distal membrane. Secreted proteins are critical for this process. Enam-/- and Ambn-/- mice fail to form enamel. We characterize enamel ribbon formation in wild-type (WT), Amelx-/- and Mmp20-/- mouse mandibular incisors using focused ion beam scanning electron microscopy (FIB-SEM) in inverted backscatter mode. In Amelx-/- mice, initial enamel mineral ribbons extending from dentin are similar in form to those of WT mice. As early enamel development progresses, the Amelx-/- mineral ribbons develop multiple branches, resembling the staves of a Japanese fan. These striking fan-shaped structures cease growing after attaining ~ 20 µm of enamel thickness (WT is ~ 120 µm). The initial enamel mineral ribbons in Mmp20-/- mice, like those of the Amelx-/- and WT, extend from the dentin surface to the ameloblast membrane, but appear to be fewer in number and coated on their sides with organic material. Remarkably, Mmp20-/- mineral ribbons also form fan-like structures that extend to ~ 20 µm from the dentin surface. However, these fans are subsequently capped with a hard, disorganized outer mineral layer. Amelogenin cleavage products are the only matrix components absent in both Amelx-/- and Mmp20-/- mice. We conclude that MMP20 and amelogenin are not critical for enamel mineral ribbon initiation, orientation, or initial shape. The pathological fan-like plates in these mice may form from the lack of amelogenin cleavage products, which appear necessary to form ordered hydroxyapatite.

Requirement of integrin β3 for iron transportation during enamel formation

Rodent incisors exhibit pigmentation on their labial surfaces. Although previous studies have shown that this pigment is composed of iron, the existence of other elements has not been investigated. This study found that the lower incisors of CD61, also known as integrin β3, null mice (CD61(-/-)) lacked pigmentation. Although ameloblasts differentiated and formed enamel normally, no ferric ion accumulation was observed in maturation-stage ameloblasts in CD61(-/-) mice. Surface elements of control and CD61-/- lower incisors were compared by x-ray photoelectron spectroscopy (XPS). XPS analysis detected C, Ca, N, O, and P on the labial surfaces of lower incisors of both mice, whereas Fe was detected only in control samples. No peak of non-ferrous metal or other element was detected in either group. Quantitative RT-PCR analysis of 18 iron-transportation-related genes with mRNA from maturation-stage ameloblasts and ALC, a pre-ameloblastic cell line, was performed. The results suggested that CD61 regulates the expressions of Slc11a2 and Slc40a1, both of which are involved in iron transportation in epithelial tissues. These results suggested that the pigment on the labial surface of mouse incisors is composed of Fe and that both anemia and reduction of iron-transporting proteins may cause the loss of pigmentation in CD61(-/-) mice.

Cooperation of nectin-1 and nectin-3 is required for normal ameloblast function and crown shape development in mouse teeth

Nectins are immunoglobulin-like cell adhesion proteins and their interactions recruit various cell-cell junctions. Mutations in human NECTIN-1 cause an ectodermal dysplasia syndrome, but Nectin-1 null mice have only slight defects in teeth, suggesting compensation by other nectin(s). We observed overlapping expression of nectin-3 with nectin-1 and enamel abnormality in the nectin-3 mutant. We, therefore, generated nectin-1;nectin-3 compound mutants. However, all teeth developed and no significant dental abnormalities were observed before birth. At postnatal day 10, the upper molars of compound mutants exhibited conical crown shape and retarded enamel maturation. Nectin-1 was expressed in ameloblasts whereas nectin-3 was expressed in neighboring stratum intermedium cells at this stage. The immunohistochemical localization and electron microscopical observations indicated that the desmosomal junctions between stratum intermedium and ameloblasts were significantly reduced. These results suggest that heterophilic interaction between nectin-1 and nectin-3 recruits desmosomal junctions, and that these are required for proper enamel formation.

[Effect of intrauterine hypoxia upon newborn albino rat tooth germ cells anabolic activity]

The aim of this study was to examine the intrauterine hypoxia influence on dental hard tissue development. Pregnant rats were exposed in hypoxic environments between day 14 and 19 of pregnancy. The study was performed on 36 newborn albino rats. Analysis of nucleolar organizator parameters were performed in enameloblasts, odontoblasts and saliva gland epitheliocytes. Data obtained demonstrated that intrauterine hypoxia decreased nucleolar organizator quantity in enameloblasts of tooth germ.

The application of synchrotron radiation induced X-ray emission in the measurement of zinc and lead in Wistar rat ameloblasts

The development of analytical techniques for the measurement of trace elements in cellular compartments of developing teeth remains an important methodological issue in dental research. Recent advances in third generation synchrotron facilities have provided high brilliance X-ray sources that can be effectively used to study trace element distributions in small spatial regions with low detection limits. The present study describes for the first time the application of synchrotron radiation induced X-ray emission (SRIXE) in measuring the distribution of zinc and lead in the ameloblasts of developing Wistar rat teeth. Wistar rats were fed a standard rat diet, containing the normal dietary requirements of zinc, ad libitum and exposed to 100 ppm of lead in drinking water. Resin embedded sections of first mandibular molars were analysed using a 13.3 keV incident monochromatic X-ray beam focussed to a 0.2 microm spot. Characteristic X-rays arising from the entire thickness of the sample were measured using an energy dispersive detector for quantitative analysis of elemental concentrations. The results showed that intranuclear concentrations of zinc were greater than levels in the cytoplasm. Furthermore, nuclear and cytoplasmic concentrations of zinc in the maturation stage (742+/-27 and 424+/-25 ppm, respectively) were significantly higher than the zinc levels observed in the nucleus and cytoplasm of presecretory stage ameloblasts (132+/-10 and 109+/-10 ppm, respectively) (p<0.05). A clear lead signal above the background was not detected in the ameloblasts and lead concentrations could only be reliably measured in the developing enamel. Overall, SRIXE was an effective method of studying the spatial distribution of zinc in the cells of developing teeth and offered a unique combination of sub-micron spatial resolution and parts-per-million detection limits (0.8-1 and 0.6-1 ppm for zinc and lead, respectively).

Ectopic expression of dentin sialoprotein during amelogenesis hardens bulk enamel

Dentin sialophosphpoprotein (Dspp) is transiently expressed in the early stage of secretory ameloblasts. The secretion of ameloblast-derived Dspp is short-lived, correlates to the establishment of the dentinoenamel junction (DEJ), and is consistent with Dspp having a role in producing the specialized first-formed harder enamel adjacent to the DEJ. Crack diffusion by branching and dissipation within this specialized first-formed enamel close to the DEJ prevents catastrophic interfacial damage and tooth failure. Once Dspp is secreted, it is subjected to proteolytic cleavage that results in two distinct proteins referred to as dentin sialoprotein (Dsp) and dentin phosphoprotein (Dpp). The purpose of this study was to investigate the biological and mechanical contribution of Dsp and Dpp to enamel formation. Transgenic mice were engineered to overexpress either Dsp or Dpp in their enamel organs. The mechanical properties (hardness and toughness) of the mature enamel of transgenic mice were compared with genetically matched and age-matched nontransgenic animals. Dsp and Dpp contributions to enamel formation greatly differed. The inclusion of Dsp in bulk enamel significantly and uniformly increased enamel hardness (20%), whereas the inclusion of Dpp weakened the bulk enamel. Thus, Dsp appears to make a unique contribution to the physical properties of the DEJ. Dsp transgenic animals have been engineered with superior enamel mechanical properties.

Ultrastructural morphometric analysis of ameloblasts exposed to fluoride during tooth development

Since a considerable amount of the world population is exposed to high doses of fluoride, it is of special concern to investigate its action mechanisms during dental enamel development. In this study, the toxicity of fluoride in ameloblasts during enamel development was evaluated by means of ultrastructural morphometric analysis. A total of 18 male Wistar rats were distributed into three groups. In Group I, the animals received deionized drinking water ad libitum (negative control) and in Groups II and III, they received sodium fluorided (NaF) drinking water at doses of 7 and 100 ppm ad libitum, respectively, for 6 weeks. Morphometric data were expressed as volume density of the most significant organelles present in the secretory and maturation phases of amelogenesis such as RER, granules, lysosomes, phagic vacuoles, microfilaments and mitochondria. The results showed that the volume density of mitochondria in the 100 ppm experimental group was 29% (P < 0.05) higher than the control group in secretory ameloblasts. No remarkable differences were found in maturation ameloblasts for all organelles evaluated. Taken together, these data indicate that NaF at high doses is able to induce cellular damage in secretory ameloblasts, whereas no noxious effect was observed during maturation stage of amelogenesis as depicted by ultrastructural analysis.

Expression of HMGB1 during tooth development

High mobility group box 1 (HMGB1) is a nuclear and cytosolic protein that can act as a transcription factor, a growth factor, or a cytokine. To elucidate a possible role for HMGB1 in tooth development, we have studied the expression of HMGB1 and its receptor RAGE (receptor for advanced glycation end-products) during the late fetal and early postnatal period of rat by using light- and electron-microscopic immunohistochemistry. Low HMGB1 protein expression was observed during fetal and newborn stages of tooth development. However, from postnatal day 5 (P5) onward, a marked increase occurred in the levels of the protein in most dental cell types. Expression was particularly high in ameloblasts and odontoblasts at regions of ongoing mineralization. Although most HMGB1 immunoreactivity was confined to cell nuclei, it was also present in odontoblast cytoplasm. At P5, ameloblasts and odontoblasts also showed RAGE immunoreactivity, and reverse transcription-polymerase chain reaction demonstrated both HMGB1 and RAGE mRNA in human dental pulp cells in vitro. Immunoblots performed on extracts from bovine dentin demonstrated a principal band at approximately 27 kDa, indicating that HMGB1 participates in tooth mineralization. The expression of both ligand and receptor suggests an autocrine/paracrine HMGB1 signalling axis in odontoblasts.

Overexpression of transforming growth factor-beta1 in teeth results in detachment of ameloblasts and enamel defects

Transforming growth factor-beta1 (TGF-beta1) is a key regulator of many cellular processes, including cell adhesion, the immune response and synthesis of extracellular matrix proteins. In the present study, we report the characterization of enamel defects in a transgenic mouse model overexpressing TGF-beta1 in odontoblasts and ameloblasts, its expression being driven by the promoter sequences of the dentin sialophosphoprotein gene. As reported earlier, these mice develop distinct dentin defects similar to those seen in human dentin dysplasia and dentinogenesis imperfecta. A further detailed examination of enamel in these mice revealed that from the early secretory stage, ameloblasts began to detach from dentin to form cyst-like structures. A soft X-ray analysis revealed that this cyst-like structure had a disorganized and partially mineralized matrix with an abnormal mineralization pattern and a globular appearance. In the molars, the enamel was not only pitted and hypoplastic, but enamel rods were completely lost. Thus, altered TGF-beta1 expression in the tooth seems to trigger detachment of ameloblasts and abnormal secretion and deposition of minerals in the cyst-like structures adjoining the dentin. We speculate that the altered expression of TGF-beta1 in teeth impacts the adhesion process of ameloblasts to dentin.

Developmental movements of the inner enamel epithelium as derived from micromorphological features

It was the purpose of this article to analyze the (micro) morphological structure of enamel at different stages of development in order to deduce movement patterns of ameloblasts during formation of the human dental primordium. Developing enamel and overlying ameloblasts were dried and fractured for scanning electron microscopy (SEM) and sectioned for transmission electron microscopy (TEM). Specimens of human permanent enamel were either fractured and/or ground and etched to visualize the enamel rods. All specimens were viewed by SEM. Moreover, three-dimensional reconstructions were made from serial ground sections of enamel blocks to follow the enamel rods for a longer distance. In addition, the outline of the dentino-enamel junction was analyzed under the SEM after removal (using nitric acid) of the enamel cap, and in serial histological sections. Two basic movements of the inner enamel epithelium can be derived from the micromorphological features: (i) the scalloped dentino-enamel junction may be a consequence of a bulged inner enamel epithelium owing to initial spatial impediment; and (ii) the undulating path of the enamel rods may be a consequence of unequal growth of the cells in the cervical loop.

A developmental comparison of matrix metalloproteinase-20 and amelogenin null mouse enamel

Mutations in both the human amelogenin and human matrix metalloproteinase-20 (MMP20, enamelysin) genes cause amelogenesis imperfecta. Both genes have also been individually deleted from the mouse and each deletion results in defective dental enamel. Here, we compare the stage-specific progression of enamel development in continuously erupting mouse incisors from amelogenin null and MMP-20 null mice. Our goal was to closely examine differences in enamel and enamel organ structure between these mice that would allow a better understanding of each protein's function. The predominant feature of the amelogenin null incisors was the late onset of mineral deposition, with little or no protein present within the forming mineral. Conversely, the developing MMP-20 null incisors had a layer of protein between the apical surface of the ameloblasts and the forming enamel. Furthermore, the protein present within the enamel matrix was disorganized. An analysis of crystal structure demonstrated that the thin amelogenin null enamel was plate-like, while the MMP-20 null enamel had a disrupted prism pattern. These results suggest that amelogenin is essential for appositional crystal growth during the early to mid-secretory stage and for the maintenance of the crystal ribbon structure. They also suggest that MMP-20 is responsible for enamel matrix organization and for subsequent efficient reabsorption of enamel matrix proteins. Both genes are essential for the generation of full-thickness enamel containing the characteristic decussating prism pattern.

Complex odontoma: confocal laser scanning microscopy analysis of a case

Confocal laser scanning microscopy (CLSM) represents a recent acquisition in the study of biological samples stained for fluorescence observation. Particularly, this technique allows a bidimensional investigation of tissues and cells with the possibility to elaborate a three-dimensional model. The aim of this study is the use of this technique, as a complementary and not substitutive application of the histological examination, for the morphological and histopathological analysis in a case of mixed complex-composed odontoma. The analyzed specimen has been surgically removed in the superior frontal region in a 12 year-old boy and submitted to conventional histopathological analysis. The specimen, hematoxylin-eosin stained, has been subsequently submitted to confocal laser scanning microscopic analysis in autofluorescence by using a Nikons C1 system. This analysis has underlined not visible aspects in traditional optical microscopy, such as the mineralization of hard tissues and the morpho-structural organization of the cellular component. The presence of enamel and dentin may be observed in the different phases of odontogenesis with clear fluorescence gradients determined by the different mineralization degrees. Thus, the odontogenetic components appear strongly autofluorescent in the classical follicular configuration. Three-dimensional reconstruction is made possible by the acquisition of serial bidimensional images that are subsequently analysed by using a specific software device. This study shows the confocal laser scanning microscopy versatility in the analysis of odontogenic neoplasms with production of mineralized tissues.

Rat enamel contains RP59: a new context for a protein from osteogenic and haematopoietic precursor cells

We have recently identified a protein, RP59, in bone marrow cells and young osteoblasts, in cells involved in bone repair and in young erythroblasts and megakaryocytes. Here, we report immunohistochemical data at the light- and electron-microscope level indicating that RP59 is also present in newly secreted tooth enamel of the rat and in ameloblasts, the formative cells. In enamel matrix, RP59 was located proximal to secretory ameloblasts only, i.e. in newly secreted material. Distal enamel and enamel in association with maturation stage ameloblasts were unlabelled. Secretory ameloblasts contained RP59 in the matrix-proximal region including Tomes' processes, post-secretory ameloblasts in the cell-matrix interface. Western blotting of proteins from tooth germs identified RP59 as a band at 90 kD, co-migrating with RP59 from bone marrow and spleen. Antisera versus a chemically synthesised RP59 peptide and versus a bacteria-synthesised protein fragment reacted in the same manner. In situ hybridisation of tooth tissue revealed RP59 RNA specifically in ameloblasts. The reverse transcription/polymerase chain reaction method identified tooth RNA coding for RP59. Sequence analysis indicated that RP59 RNA from tooth and marrow had the same sequence. An internal sequence motif was found in rat RP59 resembling a signal implicated in secretion of the chicken "engrailed" gene product. The findings indicate that RP59 is a genuine product of ameloblasts and that it is secreted in the course of enamel formation together with other matrix components.

Review: extracellular matrix regulates tooth morphogenesis

Mineralized tissues are unique in that they use proteins to attract and organize calcium and phosphate ions into a structured mineral phase, thus precise knowledge of the expression and extracellular distribution of matrix proteins is very important to understand their function. Tooth development is regulated by sequential and reciprocal interactions between neural crest-derived mesenchymal cells and the oral environment. However, the precise molecular mechanisms that mediate interactions between epithelium and mesenchymal cells are not clear, although basement membrane (BM) components have been shown to play important roles in these regulatory events. In addition, the extracellular matrix layer, whose main components are laminin, collagen IV, nidogen, and sulfated proteoglycan, and the BM layer are both considered to be involved with cell proliferation and differentiation. During tooth morphogenesis, extracellular matrices are dramatically changed. Further, the BM components, laminin and collagen IV support dental epithelium; however, in the late stage, they begin the processes of enamel matrix secretion and calcification, after which the BM structure between the dental epithelium and mesenchyme disappears. In addition, tooth abnormalities associated with several kinds of human diseases that cause mutations in the extracellular matrix, as well as the molecular mechanisms of the basement membrane and enamel matrix during tooth morphogenesis, are not clearly understood. In our review, we discuss the role of the extracellular matrix, with focus on the BM and enamel matrix during tooth morphogenesis.

Hertwig's epithelial root sheath, enamel matrix proteins, and initiation of cementogenesis in porcine teeth

OBJECTIVES

The aim of this study was to analyze the association between Hertwig's epithelial root sheath (HERS) cells, enamel matrix proteins (EMPs), and cementogenesis.

MATERIAL AND METHODS

Porcine teeth were examined at the beginning of root formation by light and transmission electron microscopy. Colloidal gold immunocytochemistry was used to analyze the protein expression of amelogenin and ameloblastin.

RESULTS

Before and during disintegration of HERS, its cells displayed the cytologic features of protein synthesis and secretion. While some cells assumed an ameloblast-like phenotype, others extended their territory away from the root surface. A collagenous matrix filled the widening intercellular spaces, and tonofilaments and desmosomes were still present in cells featuring the morphologic characteristics of cementoblasts. Labeling for amelogenin was observed but ameloblastin was not immunodetected. Labeling was associated with organic matrix deposits that were sporadically and randomly distributed both along the root surface and away from it among the dissipated epithelial cells.

CONCLUSIONS

These findings suggest that HERS' cells occasionally assume a lingering ameloblastic activity at the beginning of root formation in the pig. While the results do not support the hypothesis of a causal relationship between EMPs and cementogenesis, they lend support to the concept of an epithelial origin of cementoblasts.

TEM observations on the ameloblast/enamel interface in the rat incisor

The structure of rat incisor enamel is established at the topographically complex interface between secretory ameloblasts and forming enamel. The aim of this study was to gain additional information on this interface by sectioning parallel with the rows and the long axis of Tomes' processes and prisms. Rats were sacrificed and fixed by glutaraldehyde/paraformaldehyde perfusion. After dissection, demineralization and embedding transverse jaw/incisor segments were cut, reembedded, and reoriented. Sections were prepared for and observed in the transmission electron microscopy (TEM). The intraenamel part of Tomes' process was about 18 microns long. The forming prism occupied a longitudinally grooved invagination on its apical aspect. The parts of Tomes' process forming the side walls of the groove were attenuated and showed variation in extent and outline. Prism growth occurred over the whole grooved area. An estimation of Tomes' process secretory area in rat compared with data from humans suggests that there may be a relationship between secretory area and rate of prism formation. Prism crystals were oriented obliquely or parallel to the secretory surface of Tomes' process. At interprism growth sites matrix deposition was irregular and required some redistribution to conform to the pattern of interprism sheets.

Immunohistochemical localization of MMP-2, MMP-9, TIMP-1, and TIMP-2 in the forming rat incisor

Western blots analyses and gelatin zymography established the presence of matrix metalloproteinase (MMP)-2 and -9 in the forming zone of rat incisor. Light microscope immunohistochemistry carried out on undemineralized material provided evidence for strong MMP-2 staining in the secretory ameloblasts, odontoblasts, in the enamel organ, and in the pulp. A weaker staining was observed in predentin and in the outer part of the forming enamel. Using MMP-9 antibodies, the staining was generally weak, except for the secretory ameloblasts that were positively stained. Electron microscopic immunohistochemistry of undemineralized sections revealed a close association between gold-antibodies complexes and cytoskeletal microfilaments in the cytosol of secretory ameloblasts and odontoblasts, within the rough endoplasmic reticulum and along the plasma membrane. The striking feature of MMP-2 and -9 electron immunostaining was the particularly high labeling in the mantle dentin. By contrast, staining of tissue inhibitors of metalloproteinases (TIMP-1 and -2) was lowest in this region. We suggest that this uneven distribution may have some functional implications.

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.

Colchicine-induced apoptosis and anti-Fas localization in rat-incisor ameloblasts

Apoptosis of ameloblasts were examined by the TdT-mediated dUTP-biotin nick end-labelling method and electron microscopy 8 h after injection of colchicine. The results showed that extensive apoptosis occurred in ameloblasts of secretion to maturation zones. To determine the possible involvement of stimulators in ameloblast apoptosis, Fas, Fas ligand, tumor-necrosis-factor alpha, and tumor-necrosis-factor receptor 1 were examined utilizing immunohistochemistry and Western blotting analysis. Only Fas was consistently detected in the secretion, transition and maturation ameloblasts and overlying enamel organ epithelia. These results suggest that ameloblasts could undergo apoptosis by colchicine and that one of the ameloblast apoptosis mediators would be the Fas receptor.

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.

Development of larval and transformed teeth in Ambystoma mexicanum (Urodela, Amphibia): an ultrastructural study

Odontogenesis of early larval non-pedicellate teeth, late larval teeth with a more or less distinct dividing zone and fully transformed pedicellate teeth in Ambystoma mexicanum (Urodela) was studied to obtain insights into the development of differently structured teeth in lower vertebrates. Using transmission electron microscopy we investigated five developmental stages: (1) papilla; (2) bell stage (secretion of the matrix begins); (3) primordium (mineralization and activity of ameloblasts starts); (4) replacement tooth (young, old); and (5) established, functional tooth. Development of the differently structured teeth is largely identical in the first three stages. Mineralization takes place in apico-basal direction up to the (prospective) pedicel (early and some late larvae) or up to the zone that divides the late larval and transformed tooth in pedicel and dentine shaft (pedicellate condition). Mineralization starts directly at the collagen and by means of matrix vesicles. First odontoblasts develop small processes that extend to the basal lamina of the inner epithelial layer of the enamel organ. The processes are small and lack organelles in early larval teeth, but become larger, arborescent, and contain some organelles in late larval and transformed teeth. The processes are surrounded by unmineralized matrix (predentine). Odontoblasts at the basis of the teeth, at the pedicel, and in the zone of division do not develop significant cytoplasmic processes that extend into the matrix. Cells of the inner enamel epithelium differentiate to ameloblasts that secrete the enamel. In the early larval tooth they show an extensive basal labyrinth that becomes regressive when the enamel layer is completed. In late larval and transformed teeth, however, a large cavity arises between the basal ruffled border of ameloblasts and their basal lamina. This cavity appears to mediate amelogenesis. A small apical zone in early, but not in late larval teeth directly below the thin enamel layer consists of enameloid and is free of dentine channels.

Ultrastructural immunolocalization of enamel matrix proteins during early stages of ameloblast differentiation

Enamel matrix proteins (EMP) represent specific molecular markers of ameloblast secretion. In order to study early differentiation stages of the cells of the inner enamel epithelium, we have investigated the ultrastructural localization of EMP-immunoreactivity in rat tooth germ. Pre-secretory stages of ameloblast differentiation were identified by the absence of EMP-immunoreactivity within epithelial cells as well as adjoining extra-cellular matrix. During subsequent secretory stages EMP-like immunoreactive material could be detected both within epithelial cells as well as within the adjoining extra-cellular matrix. The intensity of the immunoreactivity increased while advancing with the differentiation of epithelial cells. Intracellularly, EMP-immunoreactivity was detectable in cytoplasmic compartments involved in exocrine secretion pathway. During the early secretory stage, EMP-immunoreactive material was also detectable in the basement membrane of the epithelial-mesenchymal interface and within the pre-dentine, close to odontoblast plasma membranes and processes. It is thus suggested that EMP may cross the basement membrane between epithelial and mesenchymal cells. Our study suggests that this aspect might be important in molecular mechanisms that regulate epithelial-mesenchymal interactions during odontogenesis.

Inositol hexasulphate, a casein kinase inhibitor, alters the distribution of dentin matrix protein 1 in cultured embryonic mouse tooth germs

Immunohistochemical studies using a polyclonal antibody, raised against the recombinant form of dentin matrix protein 1 (DMP1), show that DMP1 was detected mainly in odontoblasts in cultured mouse embryonic tooth germs. However, in restricted areas, DMP1 staining was also observed in secretory ameloblasts, in the stratum intermedium and stellate reticulum, but only when the odontoblasts located in front of them were unstained. When the embryonic tooth germs were cultured in the presence of inositol hexasulfate, a casein kinase I and II inhibitor, staining of odontoblasts was weak or nil, whereas, in contrast, ameloblasts and enamel organ were strongly immunolabelled, suggesting an enhanced translocation of DMP1 after secretion to the secretory ameloblasts and/or stratum intermedium and stellate reticulum. Moreover, DMP1--was shown to be a good substrate for gelatinase A (MMP-2), but not to gelatinase B (MMP- 9). We hypothesized that DMP1--or the sub-fractions cleaved by the MMP--could behave as diffusible signaling molecule (s) rather than as a true dentin extracellular matrix component.

Localized infusion of tunicamycin in rat hemimandibles: alteration of the basal lamina associated with maturation stage ameloblasts

At the beginning of the maturation stage of amelogenesis, ameloblasts deposit a basal lamina (BL) at the interface between their apical surface and maturing enamel. This structure is rich in glycoconjugates and is proposed to exhibit adhesive and/or filtering functions. To clarify its role, we have applied a recently developed surgical window model to locally administer tunicamycin (TM), an antibiotic that interferes with N-glycosylation, in the rat hemimandible using an osmotic minipump. Male Wistar rats were infused with either TM or saline as a control. Lectin-gold cytochemistry was performed to reveal glycoconjugates in the BL. Immunogold labeling of enamel proteins and albumin was carried out to verify whether depletion of N-linked sugars in the BL affects the content and distribution of endogenous and exogenous proteins in the enamel layer. Under the influence of the drug, the BL became irregular and exhibited alterations in structural organization and composition. The number of Helix pomatia agglutinin binding sites was not significantly affected but their distribution was altered. The labeling density of wheat germ agglutinin over the BL was slightly reduced. Immunoreactivity for enamel proteins showed only a small decrease, but that of albumin, both between ameloblasts and within the enamel layer, increased significantly. No structural alterations were observed in the contralateral incisor and in other sampled tissues and organs. These results demonstrate that it is possible to achieve a localized administration of TM without systemic side effects and lend support to the proposal that the BL represents a specialized structure with filtering functions.(J Histochem Cytochem 49:165-176, 2001)

Intercusp differences in enamel prism patterns in early and late stages of human tooth development

Enamel prism-packing patterns reflect the past history of ameloblasts, providing information about growth patterns in tooth development. Here, the area and density of enamel prisms on the cuspal surface of molar teeth were measured to examine if the onset and rate of enamel apposition differ according to stage of development and/or cusp type. Scanning electron-microscopic images were taken from the mesiobuccal and distal cusp tips of 30 mandibular first permanent molars at different stages of development recovered from archaeological sites in Israel dating to the past 10000 years. Selected enamel microstructural characters were measured for each cusp. The mean area of prisms on the mesiobuccal (MB) cusp was significantly larger than that of the distal (D) cusp at all stages of development and the differences in prism area between cusps were significant for each stage of development. Prism density was significantly smaller on the MB cusp than the D cusp at all stages of development but no significant differences were found between early and later stages in each cusp. This was interpreted as indicating that enamel formation in the MB cusp was almost complete, even in the earliest tooth germs studied, whereas in the D cusp it was less advanced. The differences between MB and D cusps are proposed to result from asynchrony of enamel formation between the different cusps of molar teeth in recent populations. The method provides a non-destructive approach to the study of growth patterns in teeth and provides baseline data for comparison with fossil teeth.

Localization of transcription factor AP-1 family proteins in ameloblast nuclei of the rat incisor

We examined by immunocytochemistry the localization of the AP-1 family proteins c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, and Fra-2 in rat incisor ameloblasts. Most of the antibodies against AP-1 family proteins, except for c-Fos-specific antibody, labeled ameloblast nuclei. The labeling intensity of the c-Jun, JunD, and Fra-2 antibodies was stronger than that of JunB, FosB, and Fra-1. Antibody reactivities of c-Jun, JunD, and Fra-2 were greatly enhanced during or after the transition zone. Furthermore, c-Jun antibodies labeled maturation ameloblasts in a cyclic pattern, which was correlated with ameloblast modulation. Disruption of ameloblast modulation by colchicine injection resulted in greatly decreased reactivity of the c-Jun antibody in the ameloblast nuclei of the maturation zone. Phospho-specific antibodies to c-Jun labeled ameloblast nuclei only weakly throughout the secretion, transition, and maturation zones. These results suggest that the stage-specific localization of AP-1 in ameloblasts is closely related to tooth enamel formation.

Transient expression of heat shock protein (Hsp)25 in the dental pulp and enamel organ during odontogenesis in the rat incisor

The expression of heat shock protein (Hsp) 25 during odontogenesis in the dental pulp and enamel organ of rat incisors was investigated by immunocytochemistry and confocal microscopy. In the process of dentin formation, immature odontoblasts first exhibited Hsp 25-immunoreactivity, and increased in immunointensity with the advance of their differentiation. In the dental pulp, in contrast, intense immunoreaction in the mesenchymal cells became weak or negative in parallel with the progress of cell differentiation. The immunoreaction for Hsp 25 in the enamel organ revealed a characteristic stage-related alteration during amelogenesis. In secretory ameloblasts, the immunoreaction for Hsp 25 was found throughout their cell bodies, intense reactivity being located near the proximal and distal terminal webs. At the maturation stage, ruffle-ended ameloblasts (RA) consistently showed Hsp 25-immunoreactivity throughout the cell bodies, whereas smooth-ended ameloblasts (SA) lacking a ruffled border were weak in immunoreaction at the distal cytoplasm. Other cellular elements of the enamel organ were negative. The subcellular localization of Hsp 25-immunoreactivity in this study appeared essentially identical to that of actin filaments as demonstrated by confocal microscopy using rhodamine-labeled phalloidin. These immunocytochemical data suggest that the Hsp 25 molecule is involved in reinforcement of the cell layer following cell movement during odontogenesis and in the formation and maintenance of the ruffled border of RA.

Inositol hexasulphate, a casein kinase inhibitor, alters enamel formation in cultured embryonic mouse tooth germs

Post-translational modification of enamel proteins is regulated by casein kinases (CK) and results in binding sites for calcium ions that subsequently play a key role during the initial stages of mineralization. Phosphorylation may also influence the secretion and extracellular organization of enamel proteins. Previous studies indicated that inositol hexasulphate inhibited the activity of CK-I and/or CK-II in mouse tooth germs (Torres-Quintana et al., 1998). We hypothesized that inositol hexasulphate would also inhibit the activity of the specific casein kinase(s) identified in secretory ameloblasts, and would prove useful for determination of the extent to which phosphorylation might influence the organization of enamel proteins at early stages of enamel formation. To test this hypothesis, we dissected mandibular first molars from 18-day-old mouse embryos and cultured them for 11 days in the presence of 0-0.1 mM inositol hexasulphate. Ultastructural analysis revealed that the formation of enamel was largely impaired at an inhibitor concentration > or = 0.08 mM. Quantitative radioautographic analysis of [33P]phosphate incorporation indicated that radiolabeled phosphate normally secreted into forming enamel was retained within ameloblasts. In contrast, no significant difference was observed between control and inositol-hexasulphate-treated tooth germs when cultures were labeled with [3H]serine and [3H]proline. SDS-PAGE and Western blot analysis confirmed that while inositol hexasulphate inhibited CK-mediated phosphorylation, it did not significantly alter protein synthesis. We conclude that impairment of phosphorylation leads to intracellular accumulation of [3H]phosphate-containing material by ameloblasts. We also conclude that when non-phosphorylated enamel matrix proteins are secreted, they are either unable to form an enamel matrix that supports mineralization, or they diffuse throughout a poorly mineralized dentin.

A tuftelin-interacting protein (TIP39) localizes to the apical secretory pole of mouse ameloblasts

Enamel biomineralization is a complex process that involves interactions between extracellular matrix proteins. To identify proteins interacting with tuftelin, a potential nucleator of enamel crystallites, the yeast two-hybrid system was applied to a mouse tooth expression library and a tuftelin-interacting protein (TIP) was isolated for further characterization. Polyclonal antibodies were prepared against two recombinant variants of this protein. Both antibodies identified a major protein product in tooth organs at 39 kDa, and this protein has been called TIP39. Northern analysis showed TIP39 messenger RNA in multiple organs, a pattern similar to that of tuftelin messenger RNA. In situ hybridization of mandibles of 1-day-old mice detected TIP39 RNA in secretory ameloblasts and odontoblasts. Immunolocalization of TIP39 and tuftelin in cultured ameloblast-like cells showed that these two proteins colocalize. Within the developing tooth organ, TIP39 and tuftelin immunolocalized to the apical pole of secretory ameloblasts (Tomes' processes) and to the newly secreted extracellular enamel matrix. TIP39 amino acid sequence appears to be highly conserved with similarities to proteins in species as diverse as yeast and primates. Available sequence data and the findings reported here suggest a role for TIP39 in the secretory pathway of extracellular proteins.

The ameloblast movement in rat incisor. L.M., S.E.M. and C.L.S.M. study

The internal epithelium of enamel organ and the below enamel surface during growth of the lower incisor, were examinated in ten Wistar rat 12-27 weeks old and weighing between 150/200 gr, by means of immuno histochemical, light and scanning electron microscopy techniques. Our specimens indicate that during the outer enamel secretion the anti-actin positivity goes from distal terminal web to infra nuclear region of cell body. The results of the present study do not support the active movement hypothesis, conversely they support the Warshawsky (1992) hypothesis, i.e. the distal terminal web permits the maintenance and the assembling of ameloblasts during enamel growth. Hence we do agree with Osborn (1970) who reported that, during secretion, ameloblasts move passively in response to secretory forces.

Specialized basement membrane of monkey maturation stage ameloblasts mediates firm ameloblast-enamel association by its partial calcification

A basement membrane-like structure associated with the maturation stage ameloblasts of the monkey (Macaca fuscata) tooth germ was examined with high resolution electron microscopy. The tissue was prepared either with or without demineralization. This structure was composed of a lamina lucida-like (lamina lucida) and lamina densa-like (lamina densa) structure. The latter was made up of a fine "cord" network, the major constituent of the basement membrane. It was closely associated with the third layer of a 200 nm wide looser cord network. In specimens without demineralization the third layer and a part of the lamina densa were calcified, and it formed the edge of the enamel. This particular area had a higher electron density, and the size, shape, and arrangement of mineral crystals were different from those of the rest of the enamel. Also, mineralization appeared to be proceeding along the cords. These observations indicate that this dense layer is a highly specialized basement membrane which mediates the firm association of maturation stage ameloblasts with the enamel by means of the mineralization of a part of this basement membrane itself which becomes integrated as a part of the enamel. Also, this highly specialized manner of association is favorable with the reported control of the loss of organic substances in the maturing enamel by maturation stage ameloblasts.

Ultrastructural preservation of rat embryonic dental tissues after rapid fixation and dehydration under microwave irradiation

Adequate preservation of the cells and matrix of mineralising tissues remains difficult, as organic components and initial mineral deposits may be lost during conventional processing for electron microscopy. In this study, we have reduced significantly the processing time using microwave irradiation. Rat molar tooth germs were fixed in 4% glutaraldehyde + 4% formaldehyde with 0.1 M sodium cacodylate in a laboratory microwave oven for two periods of 20 s with a maximal temperature of 37 degrees C. After conventional washing and post-fixation, specimens were dehydrated in graded ethanols under microwave irradiation for a total of 7 min 20 s. For comparison, some specimens were processed by conventional methods. After embedding, ultrathin sections were examined by electron microscopy. In differentiating ameloblasts and odontoblasts, plasma membranes, mitochondria, rough endoplasmic reticulum, the Golgi complex, together with all other cytoplasmic organelles exhibited excellent preservation. Microtubules, microfilaments and coated vesicles were particularly evident. Crystal-like mineral deposits were conspicuously present in relation to dentine matrix vesicles and collagen fibrils as well as in enamel matrix. The matrix of forming enamel had a globular electron-lucent appearance. It is concluded that this is a rapid method which provides a preserved or even improved morphology.

Ameloblast morphogenesis during amelogenesis. S.E.M. study

A study has been carried out on the S.E.M. on the enamel of an albino rat's inferior incisor. The observations concern almost the entire ameloblastic cycle, specifically, from the end of the cellular differentiation phase (pre-ameloblasts III) to the end of the modulation phase. The authors have pointed out some morphogenetical variations which during the depositing phase, are manifested on the distal extremity of the cells and are concentrated at this level in the differentiation of the Tomes' processes. Successively, during the transitional post-secretory and cellular modulation phases, the morphogenetical differences at the distal extremities also involve the lateral walls of the ameloblasts and their spatial relationships. Some morphological differences are correlated with different functional moments and revealed by an examination of the corresponding superficial areas of the enamel in the course of its formation. In this study, the constant orientation of the perpendicular is evident at the secretion plane in opposition with other studies that propose a 'pendulum movement' theory of these cells during the depositing phase of the prismatic layers. In addition, a morphological classification is proposed consisting of four types of modulative ameloblasts.

Stromelysin-1 (MMP-3) in forming enamel and predentine in rat incisor-coordinated distribution with proteoglycans suggests a functional role

Stromelysin-1 (matrix metalloproteinase-3) or proteoglycanase was visualized by light and electron microscopy immunolabelling in the forming zone of rat incisors. In predentine, labelling was more dense at the transition zone between the inner proximal third and the two outer thirds. Odontoblast processes were also positively stained, mostly in predentine and to a lesser degree in dentine. The dentine-enamel junction was intensely labelled, whereas dentine and forming enamel were only faintly stained. Gold-antibodies complexes were seen inside secretory ameloblasts and odontoblasts in cytosolic locations. The distribution of stromelysin-1 was compared with the distribution of 2-B-6 epitope, an antibody recognizing chondroitin-4-sulphate/dermatan sulphate and which showed a decreasing gradient from the proximal zone to the distal part of predentine. In contrast, both 5-D-4, an anti-keratan sulphate antibody and an anti-lumican antibody displayed a reversed distribution, with an increase seen from the proximal and central thirds to the distal part of predentine. This coordinated distribution suggests that stromelysin-1 may have a functional role, being implicated in predentine in the degradation of chondroitin-4-sulphate/dermatan sulphate-containing proteoglycans, and consequently allowing keratan sulphate proteoglycan concentration to increase near the border where mineralization is initiated.

Developmentally regulated changes in phospholipid composition in murine molar tooth

In order to explore the possibility that phospholipids are differently expressed during the cascade of events leading to tooth formation, we decided to carry out simultaneous biochemical, histological and electron histochemical studies. High performance thin-layer chromatography and gas-liquid chromatography were used to compare the composition of embryonic mouse first molar tooth germs at day 18 of gestation (E18) and at birth (D1), erupting teeth at day 7 (D7) and erupted molars at day 21 (D21). For the latter, non-demineralized and EDTA-demineralized lipid extracts were analysed separately. Moreover, an ultrahistochemical study was carried out using the iodoplatinate reaction which retains and visualizes phospholipids. Developmentally regulated changes occurred and were closely correlated with an increase in cell membrane phospholipids. Gradual accumulation of phospholipids was identified in the extracellular matrix, at an early stage of tooth germ development within the basement membrane and later, as predentine/dentine and enamel components participating in mineralization processes. Matrix vesicles transiently present in dentine were partly responsible for the lipids that were detected. A first group of phospholipids including phosphatidylcholine as the major membrane-associated phospholipid and phosphatidylinositol as the intracellular second messenger increased by a factor of 2.3 between E18 and D21. This increase is probably associated with cell lengthening and was relatively modest compared with the higher increase detected for a second group of phospholipids, namely phosphatidylethanolamine (x4.8), phosphatidylserine (x 5.9) and sphingomyelin (x5.4). This second group of extracellular matrix-associated phospholipids constituted 68% of the demineralized lipid extract and, therefore, contributes to the mineralization of dental tissues.

Three-dimensional direction and interrelationship of prisms in cuspal and cervical enamel of dog tooth

The three-dimensional architecture of enamel prisms was examined in cuspal enamel and compared with that in cervical enamel by light and electron microscopy as well as computer-assisted reconstruction using the developing enamel of several dog teeth. Dog tooth enamel consists of two groups of alternately arranged enamel prisms oriented in opposite sideward directions basically forming thick horizontal rings, partly branching off from the stem. Along a 8-10 enamel prism-wide group, the enamel prisms emerge in parallel tilting uniformly to the same sideward direction. In cervices, groups of enamel prisms are arranged nearly in parallel displaying a regular arrangement of prisms. Approaching the cusp of tooth, the groups of enamel prisms fuse to a concentric cusp-centered arrangement and the prisms exhibit no periodic arrangement as shown in the cervical enamel. It is suggested that the three-dimensional structure of enamel becomes complicated close to the cusp, contributing to the chewing stress of tooth.

Lectin binding patterns of odontogenic epithelium in the rat during various phases of molar tooth development

Light and electron microscopic investigations of lectin binding patterns in rat tooth germs were undertaken in order to elucidate glycoconjugate localization in cells of the reduced enamel epithelia and their derivatives. It was found that Ulex europaeus agglutinin (UEA-1), peanut agglutinin (PNA), soybean agglutinin (SBA), and wheat germ agglutinin (WGA) exhibited variable reactivity patterns with different epithelia. UEA-1 was reactive with cells of the stratum intermedium and stellate reticulum in the tooth germ but unreactive with ameloblasts, outer enamel epithelial cells, and junctional epithelium at later stages. Reaction patterns of PNA in these cells differed from those of UEA-1. Results indicated that inner and outer cells of the reduced enamel epithelium are heterogeneous with regard to lectin binding patterns.

A quantitative backscattered electron imaging study of hypomineralization and hypoplasia in fluorosed dental enamel of deer

Mineral content and distribution of fluorosed and unfluorosed (control) dental enamel of roe deer and red deer cheek teeth were analyzed using digital backscattered electron (BSE) imaging of PMMA-embedded specimens. Compared to the controls, the fluorosed enamel exhibited various aberrations resulting from a fluoride-induced disturbance of the processes involved in enamel formation. Thus, the presence of surface hypoplasias and an enhancement of the incremental pattern in the fluorosed enamel are evidence of a fluoride impact on the secretory ameloblasts, whereas a (subsurface) hypomineralization of different depth and extent is indicative of a fluoride effect on the maturation stage of amelogenesis. The marked variation in the severity of enamel hypomineralization seen along the coronocervical axis of a specimen pointed to a fluoride impact of varying intensity during this period of tooth development. Our observations further indicated that, in some locations, ameloblasts severely affected by fluoride during enamel matrix formation were able to recover from this insult and to function quite normally during the maturation stage of amelogenesis. A major advantage of the BSE imaging technique used in the present study over other methods is that it allows for a combination of micromorphological information with quantitative data on the mineralization of the analyzed tissue, which proved to be very useful for the characterization of fluoride-induced changes in dental enamel.

Synthesis, secretion, degradation, and fate of ameloblastin during the matrix formation stage of the rat incisor as shown by immunocytochemistry and immunochemistry using region-specific antibodies

Rat ameloblastin is a recently cloned tooth-specific enamel matrix protein containing 422 amino acid residues. We investigated the expression of this protein during the matrix formation stage of the rat incisor immunohistochemically and immunochemically, using anti-synthetic peptide antibodies that recognize residues 27-47 (Nt), 98-107 (M-1), 224-232 (M-2), 386-399 (M-3), and 406-419 (Ct) of ameloblastin. Immunohistochemical preparations using antibodies Nt and M-1 stained the Golgi apparatus and secretory granules of the secretory ameloblast and the entire thickness of the enamel matrix. Only M-1 intensely stained the peripheral region of the enamel rods. Immunostained protein bands were observed near 65, 55, and below 22 kD. Immunohistochemical preparations using antibodies M-2 and Ct stained the Golgi apparatus and secretory granules of the ameloblast and the immature enamel adjacent to the secretion sites, but not deeper enamel layers. Immunostaining using M-2 and Ct revealed protein bands near 65 and 40-56 kD, and 65, 55, 48, 36, and 25 kD, respectively. M-3 stained the cis side of the Golgi apparatus but not the enamel matrix. This antibody recognized a protein band near 55 kD, but none larger. After brefeldin A treatment, immunoreaction of the 55-kD protein band intensified, and dilated cisternae of rER of the secretory ameloblast contained immunoreactive material irrespective of the antibodies used. These data indicate that ameloblastin is synthesized as a 55-kD core protein and then is post-translationally modified with O-linked oligosaccharides to become the 65-kD secretory form. Initial cleavages of the 65-kD protein generate N-terminal polypeptides, some of which concentrate in the prism sheath, and C-terminal polypeptides, which are rapidly degraded and lost from the enamel matrix soon after secretion.

Immuno-histochemical demonstration of alpha-tubulin distribution in rat incisor ameloblasts after vinblastine administration

This study used the immunoperoxidase method to demonstrate post-vinblastine-administration distribution of alpha-tubulin, a structural component of the microtubule, in rat incisor secretory-stage and maturation-stage ameloblasts. Immunostaining for alpha-tubulin in secretory-stage ameloblasts and maturation-stage ameloblasts decreased from 1 hr to 6 hrs after vinblastine administration. At 12 to 24 hrs after administration, it appeared to begin recovering from the distal region of the cytoplasm toward the proximal side of the cytoplasm and, by 48 hrs after administration, had almost reached control levels. In both stages, ameloblast cell shapes and organelle polarities changed concurrently after the decrease in alpha-tubulin staining.

Immunochemical and immunohistochemical study of the 27- and 29-kDa calcium-binding proteins and related proteins in the porcine tooth germ

Our previous report identified 27- and 29-kDa calcium-binding proteins in porcine immature dental enamel. In this study we revealed that the N-terminal amino acid sequences of the two proteins were identical: LLANPXGXIPNLARGPAGRSRGPPG. The sequence matches a portion of the amino acid sequence of the porcine sheath protein, sheathlin. Porcine tooth germs were investigated immunochemically and immunohistochemically using specific antibodies raised against synthetic peptide that included residues 13-25 of this sequence. The affinity-purified antibodies reacted with several proteins extracted from newly formed immature enamel in immunochemical analyses, especially protein bands migrating at 62, 35-45, 29, and 27 kDa in SDS-polyacrylamide gels. The largest protein detected was a weak band near 70 kDa. In immunochemical analyses of proteins extracted from the inner (old) immature enamel, the antibody reacted faintly with the 27- and 29-kDa proteins. In immunohistochemical preparations, the Golgi apparatus and secretory granules of the secretory ameloblast, and the surface layer of immature enamel showed immunoreactivity. The immunoreactivity of immature enamel just beneath the secretory face of the Tomes' process was intense. No immunoreactivity was found in the Golgi apparatus of the maturation ameloblast. These results suggest that the 70-kDa protein, whose degradation might be very fast, is the parent protein of the 27- and 29-kDa proteins.

Structure and function of secretory ameloblasts in enamel formation

Secretory ameloblasts have multiple functions including the synthesis and resorption of enamel matrix proteins and calcium transport during enamel formation. We have examined these functions by means of cytochemistry and immunocytochemistry. Enamel proteins, amelogenins and enamelins are localized in the biosynthetic pathways of ameloblasts and in the forming enamel. Sulfated glycoconjugates are present in secretory ameloblasts. The distal junctional complex of ameloblasts may act as a permeability barrier to enamel proteins, thereby confining the secreted proteins to the growing enamel front. Secretory ameloblasts contain lysosomal enzymes in the Golgi lysosome endoplasmic reticulum system and also exhibit absorptive capacity, which might be associated with an early decrease in extracellularly degraded enamel proteins. Active calcium transport through the ameloblasts towards the growing enamel is indicated by the demonstration of Ca-ATPase activity along the plasma membranes. A calcium-dependent modulator protein, calmodulin, is localized in ameloblasts, suggesting that early enamel mineralization is dependent upon calmodulin-regulated Ca-ATPase in ameloblasts. These results suggest that the secretory ameloblast is a highly specialized multifunctional cell in the production, resorption and degradation of enamel matrix and in the active calcium transport essential for matrix mineralization during enamel formation.

Microstructure of enamel

Enamel is a composite material consisting of mineral and organic phases. The properties of the mineral phase are modulated dramatically by its division into microscopic crystals, cemented together by the organic matrix protein polymer. A good concept of the 3D orientations of the crystals derives from visualizing their growth perpendicular to the surface in which they develop, which is pitted by the secretory poles of the ameloblasts. The arrangement of the crystals is the cause of the discontinuities, known as the prism boundaries or junctions, in the otherwise continuous structure. These locations acquire a more concentrated organic matrix during maturation, and they are both crack stoppers and crack propagation tracks in the adult tissue. Any tendency of prisms to cleave may be reduced by their varicosities, which reflect daily variations in the rate of production; their cross-sectional shape; the non-parallelism of adjacent groups, which develops through translocation of groups of cells across the surface during development; and the support of any one microscopic tissue element by other tissue, including dentine, placed to resist an applied load. Incremental growth lines are preferential cleavage planes within the enamel. Failure patterns of enamel in normal and abnormal use can be explained by these parameters, with additional consideration of functional variation and fatigue.

Phagocytotic processing of apoptotic bodies of transitional ameloblasts by MHC Class II-expressing macrophages in rat incisor

We examined the processing of apoptotic ameloblast fragments at the transitional stage by immunocytochemistry and electron microscopy. Both macrophages and epithelial cells in the enamel organ ingested apoptotic bodies. MHC Class II antigen expression and the phagolysosomal nature of the macrophages were further examined by immunoelectron microscopy using OX6 and ED1 antibodies followed by HRP- or gold-conjugated secondary antibodies. The results showed that macrophages in the enamel organ at the transitional stage expressed MHC Class II antigens at their plasma membranes and that the same cells contained apoptotic fragments in ED1-positive vacuoles. We suggest that diverse immunological responses, such as the self-antigen presentation, may be invoked in the process of apoptosis.

The immunohistochemical localization of stat-2, -3, -4 and -5 during early enamel and dentine formation in rat molars

STATs (signal transduction and activators of transcription) are key components of the signal transduction pathways in the cytokine receptor superfamily-linked pathway. STATs are activated directly by members of the Jak (Janus kinase) family and, when activated, migrate to the nucleus to modify gene expression to produce a variety of cellular responses. Individual cytokines activate specific combinations of the Jak/STAT isoforms. A previous study localized the known Jak isoforms and STAT-1 in 5-day-old rat molars during the early stages of enamel and dentine formation. The present study was undertaken to localize immunohistochemically STAT isoforms STAT-2. -3, -4 and -5 in association with events involved in early dentine and enamel formation in 5-day-old rat molars. Each of the isoform localization patterns was different from the others. Combining the results of the previous study with the present findings, it appears that all of the known Jaks and STATs-1, -2, -3, -4 and -5 are located in the cells directly involved in early enamel or dentine formation. Using colocalization patterns of the individual Jaks and STATs, individual receptor locations may be predicted. In the proximal ends of differentiated ameloblasts, several cytokine receptors [interleukin (IL) -5, -6, -7, -9, -10, -12, growth hormone granulocyte colony-stimulating factor interferon-alpha/beta. -gamma] are predicted. In other areas of the early odontogenic cells, the proximal ends of differentiating ameloblasts are predicted to have IL-7 receptors, inner enamel epithelium IL-6 and IL-10 receptors, and stratum intermedium cells IL-6 receptors. In the early developing dentine, differentiating odontoblasts are predicted to have IL-6 and IL-10 receptors, and differentiated odontoblasts no cytokine receptors identified by known Jak/STAT combinations. Mapping of the Jak and STAT isoforms in the cells involved in early enamel and dentine formation indicates that a sizeable list of ligands and their respective cytokine receptor/pathway complexes are involved in the regulation of these processes.

Immunolocalization of the cation-independent mannose 6-phosphate receptor and cathepsin B in the enamel organ and alveolar bone of the rat incisor

In order to examine our hypothesis that maturation ameloblasts could degrade the enamel matrix in a manner analogous to bone resorption mediated by osteoclasts, we have assessed the distribution of lysosomal enzymes in the enamel organ by immunolocalizing the cation-in-independent mannose 6-phosphate receptor (MPR) and the lysosomal enzyme cathepsin B at all stages of amelogenesis. Secretory ameloblasts showed strong immunoreactivity for MPR in the supranuclear Golgi region and in the cytoplasm between the Golgi region and the distal junctional complexes. However, cathepsin B immunoreactivity was mainly seen in the distal portion of Tomes' process, which was unreactive for MPR immunogenicity. In maturation ameloblasts, the MPR was observed on the ruffled border of the ruffle-ended ameloblast (RA) but not on the distal cell membrane of the smooth-ended ameloblast (SA), although both cell types demonstrated strong immunoreactivity for MPR in the Golgi region. Immunoreactive cathepsin B was seen at the distal ends of both RA and SA. It is postulated that the nascent lysosomal enzymes bind to the mannose 6-phosphate receptors which target them not only to intracellular lysosomes, but also to the ruffled border of maturation ameloblasts where these enzymes are secreted into the enamel. Since MPR and lysosomal enzymes were also detected on the ruffled border of osteoclasts (Ocl) adjacent to alveolar bone, our immunocytochemical approach provides strong evidence for a similarity between the maturation process in enamel, as mediated by the ruffle-ended maturation ameloblasts, and bone resorption mediated by osteoclasts. This study has established that a common mechanism, based on MPR-targeted lysosomal secretion and matrix degradation, is basic to the maturation process involved in calcified tissues as different as bone and enamel.

Calbindin-D9k and calbindin-D28k expression in rat mineralized tissues in vivo

Following their terminal differentiation, highly specialized cells, ameloblasts, odontoblasts, and osteoblasts sequentially elaborate mineralized tissues. While the developmental expression pattern of matrix proteins has been studied extensively, less attention has been paid to the molecules involved in calcium handling, such as calcium-binding proteins. This shortcoming, as well as previous conflicting data, led us to conduct studies on calbindin-D9k and calbindin-D28k in rat mandibular bone and incisor based on several methods established on rat ameloblasts in vivo. Radioimmunoassays showed that calbindin-D28k accounts for approximately 0.1% of cytosolic proteins in the ectomesenchymal fraction and 1% in the epithelial fraction of the rat incisor and is 100-fold more concentrated than calbindin-D9k in both tissue types. Western blot analysis confirmed that the anticalbindin-D28k reactive species corresponded to the well characterized renal calbindin-D28k in the ectomesenchyme. In this tissue, calbindin-D28k was ultrastructurally immunolocalized in the odontoblasts. Quantitative immunocytochemistry showed that labeling was distributed throughout their nucleus and cytoplasm. The similar cytoplasmic distribution of both calbindin-D proteins and mRNAs suggests that their expression is regulated at the subcellular level. In particular, immunoreactive calbindin-D28k appeared to be associated with rough endoplasmic reticulum. Calbindin-D9k antisense probe showed negligible labeling in odontoblasts, in parallel with the protein quantities measured (approximately 10 ng/mg of total protein). Finally, in situ hybridization showed transcripts for both calbindins-D in ameloblasts and also in osteoblasts. In summary, the present results support the concept that an elevated expression of these vitamin D-dependent calcium-binding proteins may characterize the phenotype of cells directly involved in the elaboration of mineralized tissues, enamel, dentine, and bone.

Endocytotic functions of ameloblasts and odontoblasts: immunocytochemical and tracer studies on the uptake of plasma proteins

BACKGROUND

Biochemical, (immuno)cytochemical, and radioautographic data accumulated over several years have lead to the view that ameloblasts carry out both secretory and degradative functions throughout amelogenesis. Whereas it has been assumed that maturation stage ameloblasts endocytose aged enamel proteins from the enamel layer, the origin of the newly formed ones detected in the endosomal/lysosomal compartment of ameloblasts from all stages remains to be elucidated. One possible source is from secretory products released ectopically along basolateral surfaces.

METHODS

To test this hypothesis, we have investigated, using colloidal gold immunocytochemistry, whether plasma proteins (albumin and alpha 2HS-glycoprotein) found in the interstitial fluid are endocytosed by rat incisor ameloblasts and other cells from hard and soft tissues. Rat albumin, tagged with dinitrophenol, was injected intravenously to trace the movement of this protein.

RESULTS

Plasma proteins were immunodetected along the baso-lateral surfaces and in multivesicular bodies of ameloblasts where enamel proteins were also found. By 2 hours following intravenous administration of dinitrophenylated albumin, the tracer had left the blood and diffused into the enamel organ and between odontoblasts and osteoblasts. The tracer was also found in multivesicular bodies of all cells examined.

CONCLUSIONS

The uptake of albumin by many different cell types suggests that this process is not restricted to ameloblasts and likely occurs in a nonselective manner. Hence, baso-lateral uptake in ameloblasts may play a role not only in the continuous removal of plasma proteins leaking from the blood, but also of enamel proteins 'dumped' laterally between these cells. Likewise, odontoblasts may use the same mechanism to internalize some of the plasma proteins and any enamel protein that diffuse toward them.