Ultrastructure of the human enamel organ. I. External enamel epithelium, stellate reticulum, and stratum intermedium

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.

DENTAL ENAMEL FORMATION AND IMPLICATIONS FOR ORAL HEALTH AND DISEASE

Dental enamel is the hardest and most mineralized tissue in extinct and extant vertebrate species and provides maximum durability that allows teeth to function as weapons and/or tools as well as for food processing. Enamel development and mineralization is an intricate process tightly regulated by cells of the enamel organ called ameloblasts. These heavily polarized cells form a monolayer around the developing enamel tissue and move as a single forming front in specified directions as they lay down a proteinaceous matrix that serves as a template for crystal growth. Ameloblasts maintain intercellular connections creating a semi-permeable barrier that at one end (basal/proximal) receives nutrients and ions from blood vessels, and at the opposite end (secretory/apical/distal) forms extracellular crystals within specified pH conditions. In this unique environment, ameloblasts orchestrate crystal growth via multiple cellular activities including modulating the transport of minerals and ions, pH regulation, proteolysis, and endocytosis. In many vertebrates, the bulk of the enamel tissue volume is first formed and subsequently mineralized by these same cells as they retransform their morphology and function. Cell death by apoptosis and regression are the fates of many ameloblasts following enamel maturation, and what cells remain of the enamel organ are shed during tooth eruption, or are incorporated into the tooth's epithelial attachment to the oral gingiva. In this review, we examine key aspects of dental enamel formation, from its developmental genesis to the ever-increasing wealth of data on the mechanisms mediating ionic transport, as well as the clinical outcomes resulting from abnormal ameloblast function.

Compound heterozygous desmoplakin mutations result in a phenotype with a combination of myocardial, skin, hair, and enamel abnormalities

Desmoplakin (DP) anchors the intermediate filament cytoskeleton to the desmosomal cadherins and thereby confers structural stability to tissues. In this study, we present a patient with extensive mucocutaneous blisters, epidermolytic palmoplantar keratoderma, nail dystrophy, enamel dysplasia, and sparse woolly hair. The patient died at the age of 14 years from undiagnosed cardiomyopathy. The skin showed hyperplasia and acantholysis in the mid- and lower epidermal layers, whereas the heart showed extensive fibrosis and fibrofatty replacement in both ventricles. Immunofluorescence microscopy showed a reduction in the C-terminal domain of DP in the skin and oral mucosa. Sequencing of the DP gene showed undescribed mutations in the maternal and paternal alleles. Both mutations affected exon 24 encoding the C-terminal domain. The paternal mutation, c.6310delA, leads to a premature stop codon. The maternal mutation, c.7964 C to A, results in a substitution of an aspartic acid for a conserved alanine residue at amino acid 2655 (A2655D). Structural modeling indicated that this mutation changes the electrostatic potential of the mutated region of DP, possibly altering functions that depend on intermolecular interactions. To conclude, we describe a combination of DP mutation phenotypes affecting the skin, heart, hair, and teeth. This patient case emphasizes the importance of heart examination of patients with desmosomal genodermatoses.

Structural features of forming and developing blood capillaries of the enamel organ of rat molar tooth germs observed by light and electron microscopy

The process of vascularization of the enamel organ, a unique epithelial structure, occurs when the tooth germ is fully developed, i.e., at the onset of dentinogenesis. Although the three-dimensional organization of the capillaries has been previously investigated, the structural features underlying the formation of the new capillaries remains poorly understood. Thus, in the hope of better understanding the mechanism of formation of the stellate reticulum capillaries, upper first molar tooth germs of newborn and 3-day-old rats were fixed in glutaraldehyde-formaldehyde and processed for light and electron microscopy. Our results showed that blood capillaries are initially in close proximity to the outer enamel epithelium. Between and intercalated with the capillaries are round/ovoid clusters of cells, some of which are vacuolated, closely apposed to the outer enamel epithelium. The outer enamel epithelium is not a continuous layer, but exhibits gaps between the cells. This suggests that the capillaries penetrate the enamel organ through these gaps, since no invagination of the epithelium was observed. The presence of a cluster of cells containing vacuoles suggests that vasculogenesis is taking place. Images showing loss of the basal lamina, proliferation of endothelial cells, presence of filopodia and lateral sprouting suggests that angiogenesis is also occurring. Thus, neoformation of capillaries of the molar enamel organ of rat seems to occur simultaneously by mechanisms of vasculogenesis and angiogenesis.

Fine structure and Ca-ATPase activity of the stratum intermedium cells during odontogenesis in gars, Lepisosteus, Actinopterygii

This is the first report on the stratum intermedium in vertebrates other than mammals. The aim of this study is to elucidate the fine structure and cytochemical features of the stratum intermedium during the stages of enameloid formation in Lepisosteus. Inner dental epithelium, stratum intermedium, stellate reticulum, and outer dental epithelium are consistently present in the tooth germs of Lepisosteus. The stratum intermedium cells are oval in shape, contain elliptical nuclei, and extend many small processes. It is implied that the structure of the enamel organ is different among actinopterygians, and that constitution of the enamel organ in Lepisosteus resembles that in higher vertebrates. Marked Ca-ATPase activity is observed at the cell membrane of the stratum intermedium cells, suggesting that the cells are involved in calcium transport during the stages of enameloid formation.

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.

Intercellular junctions in the cells of the human enamel organ as revealed by freeze-fracture

Examined by thin sections and freeze-fracture replication techniques, secretory ameloblasts possessed two sets of the junctional complexes at both proximal and distal ends of the cell bodies, which consisted of tight junctions and occasional gap junctions and desmosomes. The proximal tight junction was fascia occludens, whereas the distal tight junction was zonula occludens. Between adjacent ameloblasts, mature gap junctions were frequent. The stratum-intermedium cells were connected to each other and to the stellate-reticulum cells and ameloblasts by well-developed desmosomes, gap junctions and fascia or macula-type tight junctions. Stellate-reticulum cells were inter-connected by many extensive cytoplasmic processes, in which well-developed desmosomes, small gap junctions and occasional macula-type tight junctions appeared. Thus fascia or macula-type tight junctions as well as many desmosomes seem to serve in mechanical, cell-to-cell adhesion during tooth formation. Frequent and large gap junctions between adjacent stratum-intermedium cells and between the stratum intermedium and the base of the ameloblast suggest that, in relation to enamel formation, these two cell layers form a functional unit.

An electron-microscopic and cytochemical study of follicular ameloblastoma

The ultrastructural and enzyme cytochemical features of two follicular ameloblastomas were investigated. The peripheral cells of the follicular areas in both lesions had several types of tall columnar cells which were highly polarized and showed varying degrees of cellular differentiation. These polarized cells had their nuclei situated away from the basal lamina, and often contained dilated strands of endoplasmic reticulum in the subnuclear cytoplasm. Some of these cells also contained dense-cored secretory granules, condensing granules and coated vesicles in the cytoplasm adjacent to the basal plasma membrane. These cells bore a striking resemblance to pre-ameloblasts and early secretory ameloblasts. Alkaline phosphatase and ATPase cytochemistry supported these morphologic observations. Interestingly, the central cells of the follicular areas were consistently negative for alkaline phosphatase activity as were the peripheral cells, while both cell types had ATPase activity demonstrable at their cell surface.

Freeze-fracture studies of neonatal mouse incisors

Differentiation of preodontoblasts to odontoblasts and preameloblasts to ameloblasts during development of the mouse mandibular incisor proceeds in a gradient from the area of the odontogenic organ, where undifferentiated ectomesenchymal and epithelial cells proliferate, toward the incisal tip where mature tooth tissues, dentin and enamel, are present. The freeze-fracture technique has been used in the work presented here to study cell membrane ultrastructure of preodontoblasts and preameloblasts at several stages of differentiation. At early stages of differentiation, cuboidal preameloblasts are joined together distally by numerous gap junctions. Relatively fewer junctions occur elsewhere on the lateral plasma membranes, but gap junctions frequently occur proximally between preameloblasts and stratum intermedium cells. As differentiation proceeds and the cells become columnar, distal and proximal junctions persist. Tight junctions, however, were not observed at any of the stages studied. Intramembrane particle concentration of the lateral preameloblast plasmalemma appears to increase as differentiation proceeds. Odontoblasts are also joined distally by numerous gap junctions which persists through later stages of differentiation. Although odontoblast cell processes were observed to project toward the preameloblast layer, no clear points of cell to cell contact or defined intercellular junctions between the two cell types were observed.

Ameloblastoma. Light and electron microscopic study

Light and electron microscopic studies of ameloblastoma were reviewed. The 21 cases of ameloblastoma examined were classified into 12 cases of the plexiform type and nine of the follicular type. The average age of the patients with the plexiform type was 25.3 years, while that of those with the follicular type was 54.4 years. Histologically, in the follicular type, the tumor cells consisted of two cell types, central polyhedral and star-shaped cells resembling the stellate reticulum and peripheral cuboidal and columnar cells similar to the inner enamel epithelium. The resemblance between the tumor follicle and enamel organ was confirmed electron microscopically. In the plexiform type, however the tumor cells did not show two cell types, but resembled squamous epithelium. Electron microscopically, all cells of the tumor strands had relatively numerous bundles of tonofilaments and were joined together by desmosomes. Differentiation of tumor cells to squamous epithelium less evident than in normal surface epithelium. We speculate that these histological differences between plexiform and follicular types represent the differentiation tendency of the remnant of the dental lamina at the time of neoplastic transformation. What decides the histological pattern is unknown but age may be a factor. Central epidermization with keratinization or microcytic changes was frequently seen in the follicular type. Keratinization and microcystic changes rarely occurred in the plexiform type. We do not believe that these changes are a form of involution or result from multipotentiality of the tumor cells.

Morphology of a simple ameloblastoma related to the human enamel organ

The ultrastructure of a simple ameloblastoma was investigated. The epithelial cells of the tumor could be divided into peripheral cells of varying height, and dark and light central cells. The morphology was correlated to that of the human enamel organ. The low peripheral cells were very similar to the external enamel epithelium cells. The central cells had a certain resemblance to the stellate reticulum and stratum intermedium cells. The high peripheral cells had no counterparts in the enamel organ. Unlike the enamel organ the ameloblastoma showed extremely few and small gap junctions.

Ultrastructure of the human enamel organ. II. Internal enamel epithelium, preameloblasts, and secretory ameloblasts

The fine structure of internal enamel epithelium, preameloblasts and secretory ameloblasts in primary tooth germs (bell stage) from four human foetuses was investigated. The characteristics of the differentiation of internal enamel epithelium via preameloblasts to secretory ameloblasts are described. The internal enamel epithelium consists of a row of low differentiated prismatic cells separated from the dental papilla by a distinct even basal lamina. In the preameloblasts the rough endoplasmic reticulum cisterns and mitochondria increase in number, the Golgi complexes become extensive and take up a distal position, and secretory granules are formed. Furthermore, the basal lamina is removed by coated vesicles, and proximally and distally in the cells a complex of zonulae adhaerentes, terminal webs and gap junctions is formed. The secretory ameloblasts make up a layer of highly differentiated cells demonstrating typical merocrine secretion.