Lysosomes and removal of the basal lamina of ameloblasts in early stages of odontogenesis

Ultrastructural studies of developing egg tooth in grass snake Natrix natrix (Squamata, Serpentes) embryos, supported by X-ray microtomography analysis

The egg tooth development is similar to the development of all the other vertebrate teeth except earliest developmental stages because the egg tooth develops directly from the oral epithelium instead of the dental lamina similarly to null generation teeth. The developing egg tooth of Natrix natrix changes its curvature differently than the egg tooth of the other investigated unidentates due to the presence of the rostral groove. The developing grass snake egg tooth comprises dental pulp and the enamel organ. The fully differentiated enamel organ consists of outer enamel epithelium, stellate reticulum, and ameloblasts in its inner layer. The enamel organ directly in contact with the oral cavity is covered with periderm instead of outer enamel epithelium. Stellate reticulum cells in the grass snake egg tooth share intercellular spaces with the basal part of ameloblasts and are responsible for their nutrition. Ameloblasts during egg tooth differentiation pass through the following stages: presecretory, secretory, and mature. The ameloblasts from the grass snake egg tooth show the same cellular changes as reported during mammalian amelogenesis but are devoid of Tomes' processes. Odontoblasts of the developing grass snake egg tooth pass through the following classes: pre-odontoblasts, secretory odontoblasts, and ageing odontoblasts. They have highly differentiated secretory apparatus and in the course of their activity accumulate lipofuscin. Grass snake odontoblasts possess processes which are poor in organelles. In developing egg tooth cilia have been identified in odontoblasts, ameloblasts and cells of the stellate reticulum. Dental pulp cells remodel collagen matrix during growth of the grass snake egg tooth. They degenerate in a way previously not described in other teeth.

Endocytosis and Enamel Formation

Enamel formation requires consecutive stages of development to achieve its characteristic extreme mineral hardness. Mineralization depends on the initial presence then removal of degraded enamel proteins from the matrix via endocytosis. The ameloblast membrane resides at the interface between matrix and cell. Enamel formation is controlled by ameloblasts that produce enamel in stages to build the enamel layer (secretory stage) and to reach final mineralization (maturation stage). Each stage has specific functional requirements for the ameloblasts. Ameloblasts adopt different cell morphologies during each stage. Protein trafficking including the secretion and endocytosis of enamel proteins is a fundamental task in ameloblasts. The sites of internalization of enamel proteins on the ameloblast membrane are specific for every stage. In this review, an overview of endocytosis and trafficking of vesicles in ameloblasts is presented. The pathways for internalization and routing of vesicles are described. Endocytosis is proposed as a mechanism to remove debris of degraded enamel protein and to obtain feedback from the matrix on the status of the maturing enamel.

Variability of systemic and oro-dental phenotype in two families with non-lethal Raine syndrome with FAM20C mutations

Background

Raine syndrome (RS) is a rare autosomal recessive bone dysplasia typified by osteosclerosis and dysmorphic facies due to FAM20C mutations. Initially reported as lethal in infancy, survival is possible into adulthood. We describe the molecular analysis and clinical phenotypes of five individuals from two consanguineous Brazilian families with attenuated Raine Syndrome with previously unreported features.

Methods

The medical and dental clinical records were reviewed. Extracted deciduous and permanent teeth as well as oral soft tissues were analysed. Whole exome sequencing was undertaken and FAM20C cDNA sequenced in family 1.

Results

Family 1 included 3 siblings with hypoplastic Amelogenesis Imperfecta (AI) (inherited abnormal dental enamel formation). Mild facial dysmorphism was noted in the absence of other obvious skeletal or growth abnormalities. A mild hypophosphataemia and soft tissue ectopic mineralization were present. A homozygous FAM20C donor splice site mutation (c.784 + 5 g > c) was identified which led to abnormal cDNA sequence. Family 2 included 2 siblings with hypoplastic AI and tooth dentine abnormalities as part of a more obvious syndrome with facial dysmorphism. There was hypophosphataemia, soft tissue ectopic mineralization, but no osteosclerosis. A homozygous missense mutation in FAM20C (c.1487C > T; p.P496L) was identified.

Conclusions

The clinical phenotype of non-lethal Raine Syndrome is more variable, including between affected siblings, than previously described and an adverse impact on bone growth and health may not be a prominent feature. By contrast, a profound failure of dental enamel formation leading to a distinctive hypoplastic AI in all teeth should alert clinicians to the possibility of FAM20C mutations.

Electronic supplementary material

The online version of this article (doi:10.1186/s12881-015-0154-5) contains supplementary material, which is available to authorized users.

Transmission and scanning electron microscopic observations of epithelial-mesenchymal interface of rat tooth germs using dithiothreitol separation

Aperiodic fibrils (AF) project from the interstitial side of the lamina densa of the basement membrane (BM) of the inner enamel epithelium (IE), and show remarkable changes in their morphology during development. The three-dimensional morphology of aperiodic fibrils during development has not been observed, because of the difficulty of exposing the interstitial surface of the BM of the inner enamel epithelium. In the present study, the dithiothreitol separation method was applied to expose the interstitial side of the inner enamel epithelial BM of rat tooth germs for the purpose of observing the exposed aperiodic fibrils by transmission and scanning electron microscopy (TEM and SEM, respectively). After dithiothreitol treatment, the enamel organ (EO) was mechanically separated from the dental papilla (DP). In the region with poorly-developed aperiodic fibrils, the separation occurred at the junction between the inner enamel epithelial BM and the dental papilla, and the aperiodic fibrils were exposed, showing the typical picture of dithiothreitol separation. SEM observation of this region revealed that the aperiodic fibrils were connected to each other and they formed networks. These networks resembled those formed by the anchoring fibrils of epidermal and mucosal epithelial BMs. TEM and SEM observations revealed that there were sidechain-like structures on the surface of the aperiodic fibrils. In the region with well-developed aperiodic fibrils, dithiothreitol treatment was not entirely effective, and some mesenchymal tissues remained on the BM. In this region, TEM observation revealed that the aperiodic fibrils were arranged in parallel with each other, and were connected by the sidechains. Several thin collagen fibrils, which were thought to be immature collagen fibrils (CF) of the predentine, were also connected to the aperiodic fibrils with these sidechains and arranged in parallel with them. Based on SEM and TEM observations, the aperiodic fibrils may be regarded as a kind of anchoring fibrils and they may play a role in connecting the BM with the mesenchymal tissue below. They are also thought to guide the arrangement of collagen fibrils in the surface layer of the predentin.

Dipeptidyl-peptidase II and cathepsin B activities in amelogenesis of the rat incisor

A body of published evidence suggests that a significant portion of enamel matrix protein synthesized by ameloblasts localises in the lysosomal-endosomal organelles of these enamel organ cells. Little is known regarding the lysosomal proteolytic activities during amelogenesis. The aims of this study were to detect and measure the activities of lysosomal peptidases cathepsin B (E.C. 3.4.22.1) and dipeptidyl-peptidase II (E.C. 3.4.14.2) in the enamel organ of the rat incisor and to ascertain whether rat enamel matrix proteins are degraded by these peptidases in vitro. Whole enamel organs were dissected from rat mandibular incisors. Enamel protein was also collected from the rat teeth. Analysis indicated that the rat incisor enamel organs contained specific activities of both dipeptidyl-peptidase II and cathepsin B at levels comparable with those of kidney which is rich in both these lysosomal peptidases. Gel electrophoresis and immunoblotting demonstrated that both cathepsin B and dipeptidyl-peptidase II were able to substantially degrade the rat enamel proteins in vitro. Based on these observations, we propose that lysosomal proteases have roles in amelogenesis in the intracellular degradation of amelogenins.

Ultrastructure of early mineral deposition during hyaline layer formation in rat molars

There is no consensus on whether the first mineralized layer, the hyaline layer, that is juxtaposed to root dentine is a variety of dentine or cementum or even a tissue of epithelial origin. Some suggest that there is no intermediate tissue between the acellular extrinsic fibre cementum (AEFC) and the root dentine. Here, to study hyaline layer formation and mineralization we examined by transmission electron microscopy the early stages of root development in upper molars from 10 to 13 day old Wistar rats. In addition to conventionally processed material, undemineralized and unstained sections were examined, which showed the deposition of fine mineral crystals in contact with the mineralized surface of root dentine. Early mineralization of the hyaline layer occurred in the region of the inner basement membrane, which persisted between the inner cellular layer of Hertwig's epithelial root sheath and the outer mineralized root dentine. When the root sheath began its fragment, collagen fibrils from the developing periodontal ligament began to insert into the mineralising hyaline layer, which was 0.5-0.8 micron wide. As the fragmentation of the root sheath HERS increased, more collagen fibrils appeared intermingled with the mineralising hyaline layer. In more advanced stages, when the hyaline layer had become fully mineralized and the formation of the AEFC began, the hyaline layer could no longer be identified. Thus, the hyaline layer is clearly discernible at early stages of periodontal development. Subsequently, it is masked by intermingling of cementum and dentine and therefore it is not possible to detect it in the formed roots of rat molars.

Ultrastructure and composition of basement membranes in the tooth

The tooth, the hardest organ in the body, is known to be formed through highly elaborate, unique processes of differentiation and development. Basement membranes play critical roles in fundamentally important biological processes such as growth and differentiation, and for better understanding of the mechanism of development and maintenance of the tooth, specializations of tooth basement membranes are reviewed in detail in relation to their roles. The basement membrane at such diverse locations in the tooth as the inner enamel epithelium, maturation-stage ameloblasts, and junctional epithelium at the dentogingival border are specialized in their own highly unique ways for anchoring, firm binding, or mediation in the transport of substances. Thus, the role of basement membranes in the developing and mature tooth is manifold and for these roles individual basement membranes are specialized in their own specific ways which are rare or not seen in nondental tissues, and these specializations are essential for successful development and maintenance of the tooth.

Immunohistochemistry of the intercellular matrix components and the epithelio-mesenchymal junction of the human tooth germ

The immunohistochemical localization of heparan sulphate, collagen type I, III and IV, laminin, tenascin, plasma- and cellular fibronectin was studied in tooth germs from human fetuses. The lamina basalis ameloblastica or membrana preformativa, which separates the pre-ameloblasts from the pre-dentin and dentin, contained heparan sulphate, collagen type IV, laminin and fibronectin. Enamel reacted with antifibronectin, but the reaction varied depending on the type of fibronectin and the source of antibody. In early pre-dentin, collagen type I, laminin, tenascin and fibronectin were present. In late pre-dentin and dentin collagen type I was found in intertubular dentin and in the zone between enamel and dentin. The close relationship between collagen type I in dentin and fibronectin in immature enamel is interesting, as it may contribute to the stabilization of the amelodentinal interface. In dental pulp, collagen type IV and laminin were found in the endothelial basement membranes. Collagen type I and III, tenascin and fibronectin were localized to the mesenchymal intercellular matrix. The results of this study have supported the assumption that the lamina basalis ameloblastica is a basement membrane, and have lead to the suggestion that ameloblasts are producers of fibronectin or a fibronectin-like substance.

Association between the expression of murine 72 kDa type IV collagenase by odontoblasts and basement membrane degradation during mouse tooth development

In situ hybridization was used to study the expression of the 72 kDa type IV collagenase gene and its association with morphogenesis and cell differentiation during advancing mouse tooth development. The epithelia were completely negative during all developmental stages. The dental mesenchyme was uniformly positive during the early stages of tooth morphogenesis, and no association of type IV collagenase with morphogenetic events was observed. However, at the bell stage the expression increased in differentiating preodontoblasts. Expression was intense in the odontoblasts during secretion of the first predentine matrix. The expression was, however, transient; it decreased around the time when mineralization of dentine started until it completely ceased. Transcripts for 72 kDa type IV collagenase also gradually disappeared from the dental pulp. The expression of 72 kDa type IV collagenase was also strong in the osteoblastic cell lineage. The preosteoblasts at the beginning of the formation of mandibular bone as well as the osteoblasts of the alveolar bone expressed more 72 kDa type IV collagenase than did other mesenchymal cells. The increased gene expression in the odontoblasts correlates with the disappearance of the dental basement membrane as shown by immunolabelling with antibodies against type IV collagen. The onset of increased expression in the odontoblasts preceded the disappearance of the basement membrane and at the time when type IV collagenase transcripts were lost from all odontoblasts the basement membrane was completely removed. It can be speculated that during early stages of tooth development the 72 kDa type IV collagenase acts as a gelatinase whereas during later stages, when odontoblasts and ameloblasts differentiate and the deposition of predentine and enamel matrix is initiated, the enzyme may act as a type IV collagenase and contribute to the degradation of the dental basement membrane.

Evidence for uptake of basement membrane by differentiating ameloblasts in the rat incisor enamel organ

Demonstration of type-IV collagen and acid phosphatase (ACPase) was carried out in the rat incisor enamel organ after the animals were fixed by perfusion with periodate-lysine-paraformaldehyde. Their incisors were dissected out, demineralized with EDTA, and prepared into 6-microns-thick frozen sections. The sections, which had been treated by means of antibody incubation for type-IV collagen, were washed with a Trismaleate buffer, incubated in Novikoff's medium for acid phosphatase (ACPase), and then incubated in a 3, 3'-diaminobenzidine solution. After osmification, the sections were embedded in epoxy resin for electron microscopy. The plasma membranes of the distal ends of the inner-enamel-epithelial cells were relatively even and were lined with a basement membrane. Type-IV collagen was localized both in the lamina densa and in the filaments attached to the lamina densa. In differentiating ameloblasts, the remarkably undulating distal plasma membranes formed irregular shallow and deep invaginations, and small cytoplasmic processes that penetrated the basement membrane. Coated pits occurred in various parts of these undulating plasma membranes. Positive reaction to type-IV collagen was observed in the invaginations and coated pits. ACPase-positive granules, present in inner-enamel-epithelial cells, increased in number and sometimes appeared close to both shallow and deep invaginations of differentiating ameloblasts. These results indicate that type-IV collagen in the basement membrane of the enamel organ is removed and degraded by differentiating ameloblasts by means of their engulfing system.

Epithelial-mesenchymal junctional area in an early stage of odontogenesis in Macaca fuscata

The epithelial-mesenchymal junctional area in tooth germs of one- and three-year-old Japanese monkeys was studied with electron microscopy.

The plasma membranes at the distal ends of inner enamel epithelium cells were relatively even, and were associated with basement membrane. A large number of filaments, which were 15 nm in diameter and up to 2 μm in length, were present, extending perpendicularly from the basement membrane toward the dental papilla, forming an unique fibrillar layer. The distal cytoplasm of the cell contained rather few vesicles and granules which were positive for the acid phosphatase reaction. The distal ends of differentiating ameloblasts showed irregular undulations and numerous small processes which penetrated through the basement membrane and fibrillar layer. Following an increase of the undulations, the fibrillar layer and the basement membrane were engulfed by the cells and removed from the surface of pre-dentin. Large irregular bodies, which were filled with the filaments of the disintegrating fibrillar layer, were observed frequently. The distal cytoplasm contained a large number of coated pits, coated vesicles, and acid-phosphatase-positive granules. The fibrillar layer then disappeared, being replaced by collagen fibrils in the pre-dentin, which was in the stage of early mineralization.

These results indicate that the fibrillar layer is removed by the differentiating ameloblasts and degraded through an engulfing system which is formed by the cell surfaces.

Early mouse molar root development: cellular changes and distribution of fibronectin, laminin and type-IV collagen

We analysed epithelial-mesenchymal interactions that occur during the early stages of the formation of mouse molar roots using light and electron microscopy. Morphological changes observed in the cells of Hertwig's epithelial sheath, the pulp and the follicular mesenchyme are described. The Hertwig's epithelial cells lose their cuboidal form and become flattened, apparently intermixing with the cells of the follicular mesenchyme. At the light- and electron-microscope levels, immunoperoxidase techniques were used to localize fibronectin, laminin and type-IV collagen. These appear to be closely associated with cell differentiation and matrix deposition in developing tooth roots. In addition, at the ultrastructural level, intracellular immunoreactivity was detected. The rough endoplasmic reticulum and nuclear envelope of some cells of the periodontal ligament facing the acellular cementum exhibited specific reactivity with laminin and type-IV collagen. Moreover, these periodontal ligament cells express keratin, but not vimentin, filaments. Our results demonstrate that Hertwig's epithelial cells maintain their capacity to synthesize laminin and type-IV collagen, as well as to express keratin filaments, despite basement membrane fragmentation and the disorganization of Hertwig's epithelial sheath. Thus, some Hertwig's epithelial cells remain in the periodontal ligament intermixed with follicular mesenchyme cells.