The initial process of enamel prism arrangement and its relation to the Hunter-Schreger bands in dog teeth

The three-dimensional architecture of enamel prisms at early stages of enamel formation and its spatial relationship to the Hunter-Schreger bands were examined in canine tooth germs by light and electron microscopy. In serial semithin sections of demineralized tooth germs tangential to the enamel-dentin junction, a straight row of enamel prisms was depicted along the longitudinal tooth axis at the level of the enamel-dentin junction and then their three-dimensional arrangement was reconstructed using computer software. The spatial arrangement of the groups of enamel rods oriented in specific sideward directions was also reconstructed in deep layers of the enamel. Initially, all enamel prisms were parallel to perpendicular toward the enamel-dentin junction, but at 10μm from the enamel-dentin junction, some small specks, or groups of enamel prisms--tilting to the right or the left--emerged as small islands. In each speck of enamel prism, the inclined prisms were uniformly oriented in a sideward direction and gradually expanded their boundary until merging with the neighboring specks inclined in the same direction. Consequently, at 50μm from the enamel-dentin junction, the group of enamel prisms oriented either to the right or the left formed alternately arranged horizontal belt-like zones, corresponding to the parazone or the diazone of the Hunter-Schreger bands. Reversed images of scanning electron-micrographs of the exposed surfaces of the developing enamel revealed round and bulb-like profiles of Tomes' processes at early amelogenesis and its changes into a characteristic structure combined with flat secretory and enclosing nonsecretory faces that dictated the orientation of corresponding enamel prisms. The results suggest that the groups of enamel prisms oriented in sideward directions first appear as small island-like specks near the enamel-dentin junction, which later merge and form alternating horizontal belt-like zones as a consequence of morphological changes of the Tomes' processes. However, the mechanisms whereby the functional grouping of secretory ameloblasts with similarly oriented Tomes' processes is induced are yet to be determined.     

Three-dimensional changes in direction and interrelationships among enamel prisms in the dog tooth.

This study describes the three-dimensional features of enamel prisms and their arrangement in dog teeth. Tangential semithin sections of demineralized tooth germ were serially cut from the enamel surface to the enamel-dentin junction. Straight rows of enamel prisms parallel or perpendicular to the meridian were selected at the enamel-dentin junction; these prisms were reconstructed from micrographs with a personal computer. Near the enamel-dentin junction, the arrangement of enamel prisms appeared regular. Viewed from the enamel surface, the cut-ends of the enamel prisms that were parallel to the meridian at the enamel-dentin junction appeared as a sine curve, with 16 enamel prisms forming one period. The enamel prisms in a row perpendicular to the meridian were parallel to each other and deflected to the left or right from the enamel-dentin junction. Away from the enamel-dentin junction, the periodicity of the prisms gradually disappeared. The sine curve formed by the cut-ends of prisms in a row parallel to the meridian became irregular, and prisms in rows perpendicular to the meridian crossed each other. The semithin sections showed belt-like zones arranged perpendicular to the meridian. Each belt-like zone consisted of enamel prisms oriented in the same direction, those in neighboring zones being oriented in opposite directions. The disappearance of the regular arrangement of prisms was related to changes in their location in the belt-like zones.     

The Enamel Microstructures of Bovine Mandibular Incisors

Bovine teeth have been considered as an excellent substitute for human teeth for dental research, however, the enamel microstructures of bovine incisors that include arrangements of prisms and interprisms, and their spatial relationships have not been well described. The aim of this study was to investigate the detail enamel microstructures of bovine incisors. Eight bovine mandibular incisors were cut into 77 pieces at eight equal intervals either in the longitudinal direction or in the horizontal direction before each piece had been tangentially cut (parallel to enamel–dentin junction) through the middle of the enamel thickness. All the sectioned surfaces were treated 1 M HCl for 10 sec to expose the prisms and interprisms before observation by scanning electron microscopy. The parallel enamel prisms were located in all the outer enamel, the cervical region and the incisal ridge of the bovine incisors. Most labial inner enamel and the cingulum of lingual inner enamel were composed of the Hunter–Schreger bands with the characteristics of decussating groups of prisms and decussating planes between interprisms and prisms. The interprisms were thicker in the inner enamel than in the outer enamel. Anat Rec, 2012. © 2012 Wiley Periodicals, Inc.     

Ultrastructural and immunocytochemical studies of enamel tufts in human permanent teeth.

Enamel tufts were exposed after decalcification of the enamel matrix and their fine structures and immunocytochemical characteristics were examined. Under the binocular microscope and the scanning electron microscope (SEM), enamel tufts appeared as corrugated ribbon-like structures located on the dentine parallel to the tooth axis. SEM observation disclosed enamel tufts as bundles of well extended tubular structures with cross striations attributable to hypocalcified enamel sheaths. Plate-like structures were observed at the center of enamel tufts, where they ran parallel to the enamel tufts. Under the transmission electron microscope (TEM), the plates of tufts revealed their origin in the superficial layer of the dentine, penetrating the hypercalcified zone adjacent to the dentine-enamel (D-E) junction, and then reaching the tuft region. The plates of tufts ran mainly along the enamel sheaths and partially across the prisms in the tuft region. THe protein-A-gold technique revealed an intense immunoreactivity for amelogenin over the superficial layer of the dentine, but over the enamel prisms in the tufts nor over the plates of tufts. The immunoreactivity for 13-17 kd protein was detected over the filamentous structures closely associated with the enamel sheaths in the enamel tuft. Thus our study disclosed that enamel tufts consist of both well extended hypocalcified enamel prisms and plates of tufts. The major organic component of the enamel tufts is suggested to be 13-17 kd protein rather than amelogenin.     

Origin of Enamel Prisms and Hunter-Schreger Bands in Reptilian Enamel

Enamel prisms in mammalian enamel structure including human are considered to have originated in the reptilian enamel. The author has reported the original pattern of enamel prisms and of Hunter-Schreger bands in fossil reptiles of Mosasaurus sp. and Phytosaurus sp. The origin of the patterns of crystallite groups appear to arise in the fossil reptiles. Mosasaurus sp. had an island pattern and Phytosaurus sp. had a slender pattern. These two types are of various sizes, and are formed by variable numbers of ameloblasts. The number of ameloblasts involved in the formation of the crystallite group may change during amelogenesis. The present study observed the initial crystallite groups as enamel islands, in the surface of the early secretory stage of amelogenesis of living Alligator mississippiensis. These crystallite groups become bigger and more uniform in size during amelogenesis, being formed by several ameloblasts in the maturation stage. The border of crystallite groups observed appear as the lamella in human enamel. These crystallite groups may develop into the enamel prisms and Hunter-Schreger bands.     

Safe Casting and Reliable Cusp Reconstruction Assisted by Micro-Computed Tomographic Scans of Fossil Teeth

Dental replicas are frequently utilized in paleoanthropological studies of perikymata and enamel hypoplasia. However, fossil teeth are often fragile and worn, causing two problems: (1) the risk of damage by removing enamel fragments when impression-making material is separated from the fossil tooth surface, and (2) the need to reconstruct worn portions of the crown to assess perikymata number, distribution, and hypoplasia timing. This study presents the advantages of μCT data of canines and lateral incisors for (1) detecting cracks along the enamel-dentine junction (EDJ) which could cause damage when casting, and (2) reliably and non-destructively reconstructing worn or broken cusps. Fragile teeth of Homo naledi, Miocene, and Pleistocene specimens were μCT-scanned: 2D virtual sections and 3D models allowed for inspecting crack pattern beyond the external surface and 2D virtual sections were used to digitally reconstruct cusp tips (only Homo naledi). Micro-CT scans allowed cracks running along the EDJ and communicating with radial cracks in the enamel to be identified prior to casting. Cusp reconstructions using μCT data were conducted as precisely as when using thin-sections or photographs, and with high intra- and inter-observer agreement, while preserving the original specimen and affording numerous planes of virtual section. When available, μCT data should be inspected prior to tooth casting to exclude teeth that show a pattern of cracks that could lead to damage. Virtual sections allow for accessible, reliable, and non-destructive cusp reconstructions that may be used for developmental (e.g., perikymata and enamel hypoplasia) or enamel thickness studies. Anat Rec, 302:1516–1535, 2019. © 2018 American Association for Anatomy     

Expression of E- and P-cadherin during tooth morphogenesis and cytodifferentiation of ameloblasts

 Cell-cell adhesion is fundamental in morphogenesis and is known to be mediated by several groups of cell adhesion molecules. Cadherins are a group of such molecules involved in the Ca²⁺-dependent cell-cell adhesion mechanism and are found in most kinds of tissue. In this study using indirect immunofluorescence microscopy, we analyzed the distribution of two kinds of cadherins, E- and P-cadherin, in developing tooth germs. In the molar tooth germs at the early bud stage, marginal cells of the epithelial tooth bud expressed both E- and P-cadherin, whereas central cells expressed only E-cadherin. At the cap stage, in addition to the cells of the inner and outer enamel epithelium, which outline the enamal organ, cells of the enamel knot, which is thought to control tooth morphogenesis, strongly expressed P-cadherin. The expression of P-cadherin was prominent in the inner enamel epithelium during the early to mid bell stage, and was also evident in the non-dividing cell masses at future cusp tips, which are the so-called secondary enamel knots. In the tooth germ at the late bell stage when the cells of the inner enamel epithelium began to polarize to differentiate into ameloblasts, the polarizing ameloblasts lost P-cadherin and strongly expressed E-cadherin. However, E-cadherin was also lost from polarized ameloblasts at later stages. The stratum intermedium and the stellate reticulum were E-cadherin positive from the bell stage onward even at the stages when the ameloblasts became E-cadherin negative again. These results suggest that the differential expression of E- and P-cadherin during morphogenetic stages plays a role in the regulation of tooth morphogenesis, whereas alteration of E-cadherin expression during later stages of tooth development is related to differentiation and function of the ameloblasts and other cells supporting amelogenesis. Accepted: 29 January 1998     

Structure of retzius lines in partially demineralized human enamel

Human third molars were partially demineralized in an acid-alcohol solution and embedded in Epon 812. Six-micron sections were cut from regions of the cervical enamel exhibiting prominent Retzius lines. The plastic was removed from the specimens by microincineration and were examined with the scanning electron microscope. The most prominent structural feature seen along the Retzius lines was the cervical translocation of some of the prisms. The scanning electron microscopic images also suggested that prisms were translocating in the transverse plane of the tooth. A series of pores, which appeared to be empty, were observed in association with the translocations occurring along the Retzius lines.     

Structure of Retzius lines in partially demineralized human enamel.

Human third molars were partially demineralized in an acid-alcohol solution and embedded in Epon 812. Six-micron sections were cut from regions of the cervical enamel exhibiting prominent Retzius lines. The plastic was removed from the specimens by microincineration and were examined with the scanning electron microscope. The most prominent structural feature seen along the Retzius lines was the cervical translocation of some of the prisms. The scanning electron microscopic images also suggested that prisms were translocating in the transverse plane of the tooth. A series of pores, which appeared to be empty, were observed in association with the translocations occurring along the Retzius lines.     

Role of crystal arrangement on the mechanical performance of enamel

The superior mechanical properties of enamel, such as excellent penetration and crack resistance, are believed to be related to the unique microscopic structure. In this study, the effects of hydroxyapatite (HAP) crystallite orientation on the mechanical behavior of enamel have been investigated through a series of multiscale numerical simulations. A micromechanical model, which considers the HAP crystal arrangement in enamel prisms, the hierarchical structure of HAP crystals and the inelastic mechanical behavior of protein, has been developed. Numerical simulations revealed that, under compressive loading, plastic deformation progression took place in enamel prisms, which is responsible for the experimentally observed post-yield strain hardening. By comparing the mechanical responses for the uniform and non-uniform arrangement of HAP crystals within enamel prisms, it was found that the stiffness for the two cases was identical, while much greater energy dissipation was observed in the enamel with the non-uniform arrangement. Based on these results, we propose an important mechanism whereby the non-uniform arrangement of crystals in enamel rods enhances energy dissipation while maintaining sufficient stiffness to promote fracture toughness, mitigation of fracture and resistance to penetration deformation. Further simulations indicated that the non-uniform arrangement of the HAP crystals is a key factor responsible for the unique mechanical behavior of enamel, while the change in the nanostructure of nanocomposites could dictate the Young's modulus and yield strength of the biocomposite.     

The Function and Structure of the Marsupial Enamel

The aims of this study are to clarify the structure of tubular enamel and the function of enamel tubules on the marsupial of opossum (Monodelphis domestica). Almost all enamel prisms, surrounded by interprismatic enamel, ran obliquely from the dentinoenamel junction (DEJ), and bent near the enamel surface. The enamel tubules are distributed in both enamel prisms and the interprismatic enamel near the DEJ. From the middle to the surface of the enamel, one enamel tubule ran within a single enamel prism. Most of enamel tubules continued from the DEJ to near the enamel surface. It is suggested that each enamel tubule developed in relation to one ameloblast. The fibers of odontoblastic process penetrated the DEJ from the dentinal tubules into the enamel tubules, and some branched across the enamel prisms. The odontoblastic process may be actively cross into the ameloblastic layer and may be involved in the formation of enamel tubules. After in vivo injection of tetracycline, tetracycline labeling showed that the odontoblastic tubules continued to enamel tubules. And strontium was detected in enamel tubules from the DEJ to the enamel surface, as was the dentinal tubules. In conclusion, there was active transport by the odontoblast and it's process through the enamel tubules.     

Ultrastructural features of odontoclasts that resorb enamel in human deciduous teeth prior to shedding

Three dental hard tissues, i.e., cementum, dentin, and enamel, are resorbed by multinucleated cells referred to as “odontoclasts.” These cells have morphological and functional characteristics similar to those of bone-resorbing osteoclasts. However, concerning enamel resorption, which is a process that may occur during tooth eruption, satisfactory ultrastructural data on odontoclastic resorption are still lacking. Ultrastructural and histochemical characteristics of odontoclasts resorbing enamel of human deciduous teeth prior to shedding were examined by means of light microscopy and transmission and scanning electron microscopy. Odontoclasts that that resorbed enamel were tartrate-resistant acid phosphatase (TRAP)-positive multinucleated giant cells that were essentially the same as those that resorbed dentin and cementum. Ultrastructurally, they had numerous mitochondria, lysosomes, and free polysomes in their cytoplasm. In addition, they were characteristically rich in large cytoplasmic vacuoles containing enamel crystals in the cytoplasm opposite the ruffled border. Although they extended a well-developed, ruffled border against enamel surface, a clear zone—an area typically devoid of organelles—was rarely seen in these cells. In many cases, the cells were in very close contact with the enamel surface by the peripheral part of their cytoplasm. The enamel prisms at the resorption surface contained more loosely packed and electron-lucent enamel crystals compared with those of unresorbed, intact enamel. Furthermore, numerous thin needle- or plate-like enamel crystals that were liberated from the enamel matrix were found in the extracellular channels of the ruffled border and in various-sized cytoplasmic vacuoles in their cytoplasm. The superficial layer of the enamel matrix undergoing odontoclastic resorption stained positively with toluidine blue and for TRAP activity. The results of the present study suggest that odontoclasts resorbing enamel secrete acids as well as organic components, including hydrolytic enzymes, into the resorption zone underlying their ruffled border and that they phagocytose crystals that have been liberated from the partially demineralized enamel matrix by acids, subsequently dissolving them intracellularly. Anat. Rec. 252:215–228, 1998. © 1998 Wiley-Liss, Inc.     

Analysis of fracture and deformation modes in teeth subjected to occlusal loading

An analysis of fracture and deformation modes in tooth enamel subjected to occlusal loading is presented. Several competing modes are identified: deformation by yield beneath the indenter; median cracking from the ensuing plastic zone and analogous radial cracking from the dentin–enamel junction along the load axis; and margin cracking from the cervical enamel–cement junction. The analysis, based on a simple model of tooth geometry, presents relations for the critical loads to initiate these damage modes within the enamel, and to drive ensuing cracks longitudinally around the tooth walls to failure. The relations are explicit in their dependence on characteristic tooth dimensions – enamel thickness and cuspal radius – and on material properties – modulus, hardness, toughness and strength. Provision is made to incorporate properties of the occlusal contact, whether from opposing dentition or intervening food particles. All these features are demonstrated on critical-load master diagrams. A characteristic feature of the damage evolution is the gradual evolution of each mode with increasing load, so that failure is generally a prolonged rather than abrupt event. This accounts for the remarkable damage tolerance of natural teeth. The equations may enable basic predictions of tooth responses for humans and animals under a variety of specified dietary and functional conditions.     

Comparison between deciduous and permanent incisor teeth in morphology of bovine enamel.

Functional incisor teeth (deciduous and permanent teeth) from Bovidae (14 species) were prepared for scanning electron microscopic observation. Ultrastructural patterns of the enamel layer of deciduous and permanent incisor teeth varied (ex. prisms, arrangement pattern of matrices, and in thickness of enamel layer) in each species. The ultrastructures of prisms in longitudinal sections were classified into three types; A, radial, B, tangential, and C, mix of A and B arrangement enamel; modified Koenigswald's method (1982) in examined species. Type A was found in a large part of permanent and a small part of deciduous incisor teeth, while types B and C were mainly found in the deciduous teeth. These morphological features show the remarkable correlation between permanent and deciduous teeth.     

Structural characteristics of staircase-type Retzius lines in human dental enamel analyzed by scanning electron microscopy

Based on a scanning electron microscopic (SEM) analysis of adjoining, acid-etched planes cut through human cervical enamel, the structural characteristics of staircase-type Retzius lines have been clarified. Structural features associated with this type of Retzius lines--such as cleftlike defects, decreased dimension of prisms, increased interprism, club-shaped appearance of prisms, oblique ridges, and triangular regions--have been incorporated into a unifying, three-dimensional model. A continuous discontinuity defect, involving both prisms and interprism, is a prominent feature of this type of Retzius lines. Prisms and interprism facing the deep aspect of the cleftlike defect show an enlarged, flat surface that encroaches on the cervically situated prisms being rebuilt from the same cleft. The initial part of a prism taking off from the cleft is of distorted shape. As the prism reaches the level of the interprism cleft, it abruptly regains its normal size and shape. The relationship between this type of Retzius lines and the carious process is discussed, and it is suggested that the discontinuity defect may retard the carious lesion due to a protective effect of its supposed organic content. The developmental events creating staircase-type Retzius lines are discussed, and it is suggested that Tomes' processes are constricted near their bases with a corresponding increase in interprismatic growth region. Tomes' processes will have to reshape plastically as they move out of the constricted pits, trailing the parent ameloblasts as they resume enamel production and move in the direction of the prisms.     

Enamel biorhythms of humans and great apes: the Havers‐Halberg Oscillation hypothesis reconsidered

The Havers‐Halberg Oscillation (HHO) hypothesis links evidence for the timing of a biorhythm retained in permanent tooth enamel (Retzius periodicity) to adult body mass and life history traits across mammals. Potentially, these links provide a way to access life history of fossil species from teeth. Recently we assessed intra‐specific predictions of the HHO on human children. We reported Retzius periodicity (RP) corresponded with enamel thickness, and cusp formation time, when calculated from isolated deciduous teeth. We proposed the biorhythm might not remain constant within an individual. Here, we test our findings. RP is compared between deciduous second and permanent first molars within the maxillae of four human children. Following this, we report the first RPs for deciduous teeth from modern great apes (n = 4), and compare these with new data for permanent teeth (n = 18) from these species, as well as with previously published values. We also explore RP in teeth that retain hypoplastic defects. Results show RP changed within the maxilla of each child, from thinner to thicker enameled molars, and from one side of a hypoplastic defect to the other. When considered alongside correlations between RP and cusp formation time, these observations provide further evidence that RP is associated with enamel growth processes and does not always remain constant within an individual. RP of 5 days for great ape deciduous teeth lay below the lowermost range of those from permanent teeth of modern orangutan and gorilla, and within the lowermost range of RPs from chimpanzee permanent teeth. Our data suggest associations between RP and enamel growth processes of humans might extend to great apes. These findings provide a new framework from which to develop the HHO hypothesis, which can incorporate enamel growth along with other physiological systems. Applications of the HHO to fossil teeth should avoid transferring RP between deciduous and permanent enamel, or including hypoplastic teeth.     

Origin of enamel prisms and Hunter-Schreger bands in reptilian enamel

Enamel prisms in mammalian enamel structure including human are considered to have originated in the reptilian enamel. The author has reported the original pattern of enamel prisms and of Hunter-Schreger bands in fossil reptiles of Mosasaurus sp. and Phytosaurus sp. The origin of the patterns of crystallite groups appear to arise in the fossil reptiles. Mosasaurus sp. had an island pattern and Phytosaurus sp. had a slender pattern. These two types are of various sizes, and are formed by variable numbers of ameloblasts. The number of ameloblasts involved in the formation of the crystallite group may change during amelogenesis. The present study observed the initial crystallite groups as enamel islands, in the surface of the early secretory stage of amelogenesis of living Alligator mississippiensis. These crystallite groups become bigger and more uniform in size during amelogenesis, being formed by several ameloblasts in the maturation stage. The border of crystallite groups observed appear as the lamella in human enamel. These crystallite groups may develop into the enamel prisms and Hunter-Schreger bands.     

The orientation of the enamel prisms at the enamel surface

The angulation that prisms obtain with the surface in human permanent teeth was analysed on broken enamel by means of the scanning electron microscope. In the cervical region the prisms end nearly perpendicular towards the surface, while the angle becomes more and more acute towards the occlusal region of the crown. In the cuspal area the deviation from the perpendicular direction of the prisms approaches 70 degrees. The size discrepancy between the inner and the outer surface of the enamel mantle can be explained by the more or less angulated position of the prisms.