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 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.     

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. Anat. Rec. 252:355–368, 1998. © 1998 Wiley-Liss, Inc.     

Quantitative analysis of the calcium and phosphorus content of developing and permanent human teeth.

It was the aim of this study to investigate the distribution of Ca, P and C in predentin, dentin and enamel in human tooth buds and permanent teeth by EDX element analysis. The mandible of a 16-week-old human fetus containing eight mineralizing tooth buds and three human permanent molars were fixed in formaldehyde and embedded in Technovit 9100. Serial sections of 80 microm thickness of the mandible were cut in the frontal-dorsal direction, and polarized light micrographs were taken of these sections. The permanent teeth were cut in mesio-distal direction. The sections were investigated with scanning electron microscopy and EDX element analysis with a Philips XL 30 FEG scanning microscope and an EDAX energy-dispersive X-ray system using spot measurements, EDX line-scans and element mapping. Quantitative measurements were made in predentin, mineralizing dentin adjacent to predentin, mature dentin, mineralizing enamel and young enamel of developing teeth and mature enamel of permanent teeth. In developing teeth the Ca and P content increased rapidly from outer predentin towards mineralizing dentin. In enamel prisms of developing teeth the Ca and P content increased linearly from the surface towards the enamel-dentin junction. In permanent teeth only a small layer of predentin was found. The Ca and P content in enamel and circumpulpal dentin of permanent teeth was higher than in developing teeth. The Ca/P ratio differed between predentin and dentin areas reflecting different calcium phosphate compositions, but it was the same in mineralizing and young enamel. The differences in the distribution of Ca and P reflect different mineralizing patterns of the enamel and dentin matrices.     

Histochemical localization of cholinesterase activity in the dental epithelium of guinea pig teeth

Cholinesterase is known for its remarkable diversity in distribution and function. An association of this enzyme with proliferative and morpho-differentiating tissues has been reported in several species. Here we report on the first evidence of the presence of cholinesterase in the enamel organ of continuously erupting incisors and molars of the guinea pig. Frozen sections of the incisors and molars of the guinea pig were incubated for histochemical demonstration of cholinesterase activity by means of the thiocholine method as described by Karnovsky and Root. The cholinesterase activity was observed in several types of cells of the dental epithelium; cells forming the basal portion of the enamel organ, outer enamel epithelium and maturation stage ameloblasts of both the incisors and molars. In the crown analogue side, the outer enamel epithelial cells gained strong reactions for cholinesterase and maintained the reaction throughout the secretory and maturation stages of amelogenesis. In contrast, cholinesterase reactions were lacking in the inner enamel epithelium, pre-ameloblasts, and secretory ameloblasts. In the early stage of enamel maturation, ameloblasts began to show positive reactions for cholinesterase, which was upregulated in the incisal direction. Although both tooth types showed similar reactive patterns for cholinesterase at the growing ends, maturation ameloblasts depicted a different pattern of staining displaying the reactions only sporadically in molars. These data indicate the role of cholinesterase in the enamel organ in tooth morphogenesis and function of guinea pig teeth.     

A scanning electron microscope study of aberrations in the prism pattern of rat incisor inner enamel

The prism pattern in the inner enamel of adult rat incisors was studied with the SEM in unfixed tissues that had been sectioned, ground, polished, and etched. Six different types of aberrations in the prism pattern were encountered: 1. Prism lamellae may be shorter than the mesio-lateral width of enamel. 2. Prism lamellae may deviate from a transverse orientation. 3. Prism lamellae may “fuse” or “bifurcate.” 4. Prisms of two adjacent lamellae may pursue a common course. 5. Prisms may change direction. 6. Variations exist in the outline of transversely cut prism profiles. Aberrations were observed at any distance from the dentino-enamel junction. These observations were used as a basis for an analysis of the movement of ameloblasts during rat incisor amelogenesis. It was concluded that it is physically possible for the ameloblasts to create the observed aberrations as they move along the path of the prisms. However, the aberrations seem to make it more difficult to understand the factors controlling ameloblast movement. Occasionally crystallite bridges connecting adjacent prisms were observed. A configuration resembling a bifurcating prism is presented.     

Presence of enamel on the incisors of the llama (Lama glama) and alpaca (Lama pacos)

Attempts have been made to define the relationships among the South American camelids, the guanaco, llama, alpaca, and vicuna, by comparing the morphology of their incisors. The alpaca has been reported to have an incisor morphology similar to the vicuna, lacking enamel on the lingual surface. The llama and guanaco are said to have enamel on both the labial and lingual surface of their incisor teeth. These comparisons have been based on gross morphological observations and not on histologic analysis. This study was undertaken to determine whether or not alpaca teeth have enamel on the lingual surface. The cross-sectional histologic anatomy of the incisor teeth was compared in two closely related South American camelid species, the llama (Lama glama), and the alpaca (Lama pacos). Thick sections (300 μm) and scanning electron microscopy were the techniques utilized. The mandibular first, second, and third incisors were examined in four llamas and five alpacas. A substantial layer of enamel was present on all surfaces of all llama incisors. The enamel layer on the labial surface of the alpaca incisors closely resembled that found in the llama. The enamel layer on the lingual surface of the alpaca incisors, although greatly reduced, was distinctly present. Alpacas may be more closely related to guanacos and llamas than to vicunas. A histologic study of vicuna incisors would help to better define the relationships of the four camelids. Anat Rec. 249:441–449, 1997. © 1997 Wiley-Liss, Inc.     

Characteristics of the transverse 2D uniserial arrangement of rows of decussating enamel rods in the inner enamel layer of mouse mandibular incisors

The 2D arrangement of rows of enamel rods with alternating (decussating) tilt angles across the thickness of the inner layer in rat and mouse incisor enamel is well known and assumed to occur in a uniform and repetitive pattern. Some irregularities in the arrangement of rows have been reported, but no detailed investigation of row structure across the entire inner enamel layer currently exists. This investigation was undertaken to determine if the global row pattern in mouse mandibular incisor enamel is predominately regular in nature with only occasional anomalies or if rows of enamel rods have more spatial complexity than previously suspected. The data from this investigation indicate that rows of enamel rods are highly variable in length and have complex transverse arrangements across the width and thickness of the inner enamel layer. The majority of rows are short or medium in length, with 87% having < 100 rods per row. The remaining 13% are long rows (with 100–233 rods per row) that contain 46% of all enamel rods seen in transverse sections. Variable numbers of rows were associated with the lateral, central and mesial regions of the enamel layer. Each region contained different ratios of short, medium and long rows. A variety of relationships was found along the transverse length of rows in each region, including uniform associations of alternating rod tilts between neighboring rows, and instances where two rows having the same rod tilt were paired for variable distances then moved apart to accommodate rows of opposite tilt. Sometimes a row appeared to branch into two rows with the same tilt, or conversely where two rows merged into one row depending upon the mesial‐to‐lateral direction in which the row was viewed. Some rows showed both pairing and branching/merging along their length. These tended to be among the longest rows identified, and they often crossed the central region with extensions into the lateral and mesial regions. The most frequent row arrangement was a row of petite length nestled at the side of another row having the same rod tilt (30% of all rows). These were termed ‘focal stacks’ and may relate to the evolution of uniserial rat and mouse incisor enamel from a multilayered ancestor. The mesial and lateral endpoints of rows also showed complex arrangements with the dentinoenamel junction (DEJ), the inner enamel layer itself, and the boundary area to the outer enamel layer. It was concluded that the diversity in row lengths and various spatial arrangements both within and between rows across the transverse plane provides a method to interlock the enamel layer across each region and keep the enamel layer compact relative to the curving DEJ surface. The uniserial pattern for rows in mouse mandibular incisors is not uniform, but diverse and very complex.     

The orientation of prisms in the dental enamel of human permanent teeth

Enamel of human permanent teeth was sectioned and ground with 2 planes perpendicular to each other extending centrifugally from the dentino-enamel-junction to the crown surface. Prisms were made visible by acid etching before evaluation under the SEM. In the vicinity of the dentino-enamel-junction more prisms were found to be cut longitudinally, while close to the crown surface more prisms were cut transversely. In the perpendicularly ground plane the corresponding prisms were seen to deviate the more from the centrifugal orientation the more close they came to the crown surface. In a geometric model the angle under which the prisms deviate from the centrifugal orientation was calculated in dependence from the distance to the dentino-enamel-junction. The results correspond with the SEM-findings. We conclude that--since the prism diameter is today known to be constant--the form of the enamel mantle is created by a specific orientation of the prisms: They stand perpendicular at the dentino-enamel-junction, and the farther away they run towards the periphery the more they deviate from the perpendicular path. By this increasingly oblique orientation of the prisms the volume increment of the enamel mantle is created. The maximal angle of deviation is found morphologically and mathematically between 60 degrees-70 degrees at the crown surface. This arrangement of the prisms demonstrated by us is now seen to be the reason for erroneous assumptions about an increase of the prism diameter when ground sections were used. Because the prisms are oriented more and more oblique towards the periphery, correspondingly larger effective diameters must be produced while making ground sections tangential to the crown surface.     

The spatial distribution of focal stacks within the inner enamel layer of mandibular mouse incisors

Focal stacks are an alternative spatial arrangement of enamel rods within the inner enamel of mandibular mouse incisors where short rows comprised of 2–45 enamel rods are nestled at the side of much longer rows, both sharing the same rod tilt directed mesially or laterally. The significance of focal stacks to enamel function is unknown, but their high frequency in transverse sections (30% of all rows) suggests that they serve some purpose beyond representing an oddity of enamel development. In this study, we characterized the spatial distribution of focal stacks in random transverse sections relative to different regions of the inner enamel and to different locations across enamel thickness. The curving dentinoenamel junction (DEJ) in transverse sections complicated spatial distribution analyses, and a technique was developed to “unbend” the curving DEJ allowing for more linear quantitative analyses to be carried out. The data indicated that on average there were 36 ± 7 focal stacks located variably within the inner enamel in any given transverse section. Consistent with area distributions, focal stacks were four times more frequent in the lateral region (53%) and twice as frequent in the mesial region (33%) compared to the central region (14%). Focal stacks were equally split by tilt (52% mesial vs. 48% lateral, not significant), but those having a mesial tilt were more frequently encountered in the lateral and central regions (2:1) and those having a lateral tilt were more numerous in the mesial region (1:3). Focal stacks having a mesial tilt were longer on average compared to those having a lateral tilt (7.5 ± 5.6 vs. 5.9 ± 4.0 rods per row, p < 0.01). There was no relationship between the length of a focal stack and its location within the inner enamel. All results were consistent with the notion that focal stacks travel from the DEJ to the outer enamel the same as the longer and decussating companion rows to which they are paired. The spatial distribution of focal stacks within the inner enamel was not spatially random but best fit a null model based on a heterogenous Poisson point process dependent on regional location within the transverse plane of the enamel layer.     

Effect of Down syndrome on the dimensions of dental crowns and tissues

Abnormal growth in Down syndrome (DS) is reflected by variable reduction in size and simplification in form of many physical traits. This study aimed to compare the thickness of enamel and dentine in deciduous and permanent mandibular incisor teeth between DS and non‐DS individuals and to clarify how these tissues contribute to altered tooth size in DS. Sample groups comprised 61 mandibular incisors (29 permanent and 32 deciduous) from DS individuals and 55 mandibular incisors (29 permanent and 26 deciduous) from non‐DS individuals. Maximum mesiodistal and labiolingual crown dimensions were measured initially, then the crowns were sectioned midsagittally and photographed using a stereomicroscope. Linear measurements of enamel and dentine thickness were obtained on the labial and lingual surfaces of the crowns, together with enamel and dentine–pulp areas and lengths of the dentino‐enamel junction. Reduced permanent crown size in DS was associated with a reduction in both enamel and dentine thickness. After adjustments were made for tooth size, DS permanent incisors had significantly thinner enamel than non‐DS permanent teeth. The DS permanent teeth also exhibited significant differences in shape and greater variability in dimensions than the non‐DS permanent teeth. Crown dimensions of deciduous incisors were similar in size or larger in DS compared with non‐DS deciduous teeth. Enamel and dentine thicknesses of the deciduous teeth were similar in DS and non‐DS individuals. The findings indicate that growth retardation in DS reduces both enamel and dentine deposition in the permanent incisors but not in the earlier‐forming deciduous predecessors. The results are also consistent with the concept of amplified developmental instability for dental traits in DS. Am. J. Hum. Biol. 13:690–698, 2001. © 2001 Wiley‐Liss, Inc.     

Structure of the upper teeth of the filefish, Stephanolepis cirrhifer

The teeth of the filefish were investigated with light and scanning electron microscopy. Anterior and posterior rows of pleurodont teeth are present in the maxilla. The teeth in the anterior row possess a large labial and small lingual surface and the teeth in the posterior row possess a small labial and large lingual surface. The enameloid consists of three layers-the outer, middle, and inner layer. The outer layer consists of fine, parallel crystallites oriented perpendicular to the surface. They are similar to the crystallites in enamel derived from ectoderm. In the middle layer the crystallites form parallel bundles that are oriented in the same direction as that of the crystallites in the outer layer. In the inner layer bundles of crystallites interlace with each other. The tubular structures and the terminal branches of the dentinal tubules are located among the crystallites in the inner layer. In the dentine numerous dentinal tubules radiate from the pulp cavity toward the periphery and are seen to curve slightly in an S-shaped course. At the occlusal tip the alternating parabolic layers are seen and these are considered to be mineralized lines. The openings of the dentinal tubules are round or oval, surrounded by interlacing fibrils. The teeth are tightly fixed to the jawbone by bundles of fibrils. At the lower part of the lingual surface of the teeth in the anterior row and the labial surface of the teeth in the posterior row the bundles of fibrils start at the dentine and some fibrils run through connective tissue, while others terminate in projections of the jawbones.(ABSTRACT TRUNCATED AT 250 WORDS)     

Structure–function relations of primate lower incisors: a study of the deformation of Macaca mulatta dentition using electronic speckle pattern interferometry (ESPI)

Teeth adopt a variety of different morphologies, each of which is presumably optimized for performing specific functions during feeding. It is generally agreed that the enamel cap is a crucial element in controlling the mechanical behavior of mammalian teeth under load. Incisors are particularly interesting in terms of structure–function relations, as their role in feeding is that of the ‘first bite’. However, little is known how incisor cap morphology is related to tooth deformation. In the present paper we examine the mechanical behavior of mandibular central incisors in the cercopithecine primate Macaca mulatta under loads similar to those encountered during ingestion. We map three‐dimensional displacements on the labial surface of the crown as it is compressed, using electronic speckle pattern interferometry (ESPI), an optical metrology method. In addition, micro‐computed tomography is used to obtain data regarding the morphology of the enamel cap, which in the M. mulatta lower incisors exhibits missing or very little enamel on the lingual face. The results showed that although compressed along a longitudinal axis, deformation in the incisors mostly occurred in the lingual direction and orthogonal to the direction of the applied load. Both isolated, embedded teeth and teeth in the mandible showed considerable lingual deformation. Incisor deformation in the mandible was generally greater, reflecting the additional freedom of movement enabled by the supporting structures. We show that the association with adjacent teeth in the arch is significant for the behavior of the tooth under load. Finally, loading two teeth simultaneously in the mandible showed that they work as one functional unit. We suggest that these results demonstrate the importance of enamel cap morphology in directing deformation behavior; an ability stemming from the stiffness of the enamel cap overlying the more pliable dentin.     

Application of Image Compression Ratio Analysis as a Method for Quantifying Complexity of Dental Enamel Microstructure

It is well recognized that enamel microanatomy in mammals reflects biomechanical demands placed upon teeth, as determined by mechanical properties of species' diets, use of teeth as weapons, and so forth. However, there are limited options for researchers wishing to perform large-scale comparisons of enamel microstructure with adaptive questions in mind. This is because to date there has been no efficient method for quantification and statistical analysis of enamel complexity. Our study proposes to apply a method previously developed for quantification of 3D tooth cusp morphology to the problem of quantifying microstructural enamel complexity. Here, we use image compression ratio (ICR) as a proxy variable for enamel complexity in 2D enamel photomicrographs taken using circularly polarized transmitted light microscopy. ICR describes the relationship between a digital image captured in an uncompressed file format and the identical image that has had its file size compressed using computer algorithms; more complex images receive less compression. In our analyses, ICR analysis is able to distinguish between images of teeth with simple, radial enamel and teeth with complex decussating enamel. Moreover, our results show a significant correlation between ICR and enamel complexity ranks assigned via visual assessment. Therefore, our results demonstrate that ICR analysis provides a viable methodology for efficient comparison of overall enamel complexity among dental samples. Anat Rec, 302:2279–2286, 2019. © 2019 American Association for Anatomy     

Relationships Between Enamel Prism Decussation and Organization of the Ameloblast Layer in Rodent Incisors

Rodent enamel microstructure has been extensively investigated, primarily on the basis of 2D electronic microscopy data. The nature and dynamics of the ameloblasts (the enamel-secreting cells) have also been well studied. However, critical issues still remain surrounding exactly how the ameloblasts produce the astonishing microstructural complexity of enamel, and how this subtle architecture evolved through time. In this article, we used a new methodology based on confocal laser microscopy to reconstruct the enamel microstructure of rodent incisors in three dimensions (3D) with the ameloblasts in situ. We proposed interpretations regarding the possible relationships between the workings of the ameloblasts and the resulting enamel prisms, especially how the phenomenon of decussation is generated. Finally, we were able to represent the two main types of modern rodent incisor microstructures (uniserial and multiserial decussations), as a set of parameters that have been entered into the 3D enamel simulation software Simulenam to generate 3D models that can be digitally manipulated. Associating 2D data of incisor enamel microstructure of fossil rodents and Simulenam, it was then possible to better understand how the various decussation parameters evolved through time and gave rise to the two modern microstructure types from the same ancestral type (pauciserial). This study also confirmed that rodent and artiodactyl enamel do not share the same mechanism of decussation formation. Anat Rec, 302:1195–1209, 2019. © 2018 Wiley Periodicals, Inc.     

Correlation of the arrangement pattern of enamel rods and secretory ameloblasts in pig and monkey teeth: a possible role of the terminal webs in ameloblast movement during secretion.

Enamel rod architecture and ameloblast arrangement were examined in pig and monkey teeth using light microscopy and scanning and transmission electron microscopy. Enamel rods in the pig teeth were arranged in longitudinal straight rows in the initial enamel layer, in longitudinal wavy rows in the inner enamel layer, and in a staggered pattern in the outer enamel layer. Rod decussation was seen only in the inner layer. Cross-sectioned enamel rods in the pig were arcade-shaped in the initial and inner layers, and mostly round in shape with circular boundaries in the outer layer. Arrangement of secretory ameloblasts at the level of the distal terminal web and Tomes' processes, and shape of Tomes' processes, corresponded to those of the enamel rod in the enamel layers. Distal terminal webs were well developed between straight rows of the ameloblasts forming the initial layer and between wavy rows of the ameloblasts forming the inner layer, and less developed within a row. The filament bundles in the distal terminal webs were also oriented along the rows. However, in the ameloblasts forming the outer layer, which lost their row pattern, distal terminal web filaments were distributed uniformly at the cell periphery. A similar arrangement of wavy rows of ameloblasts at the level of distal terminal web and Tomes' processes was also seen in monkey teeth.     

Correlation of the arrangement pattern of enamel rods and secretory ameloblasts in pig and monkey teeth: A possible role of the terminal webs in ameloblast movement during secretion

Enamel rod architecture and ameloblast arrangement were examined in pig and monkey teeth using light microscopy and scanning and transmission electron microscopy. Enamel rods in the pig teeth were arranged in longitudinal straight rows in the initial enamel layer, in longitudinal wavy rows in the inner enamel layer, and in a staggered pattern in the outer enamel layer. Rod decussation was seen only in the inner layer. Cross-sectined enamel rods in the pig were arcade-shaped in the initial and inner layers, and mostly round in shape with circular boundaries in the outer layer. Arrangement of secretory ameloblasts at the level of the distal terminal web and Tomes' processes, and shape of Tomes' processes, corresponded to those of the enamel rod in the enamel layers. Distal terminal webs were well developed between straight rows of the ameloblasts forming the initial layer and between wavy rows of the ameloblasts forming the inner layer, and less developed within a row. The filament bundles in the distal terminal webs were also oriented along the rows. However, in the ameloblasts forming the outer layer, which lost their row pattern, distal terminal web filaments were distributed uniformly at the cell periphery. A similar arrangement of wavy rows of ameloblasts at the level of distal terminal web and Tomes' processes was also seen in monkey teeth.     

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.     

Hidden contributions of the enamel rods on the fracture resistance of human teeth

The enamel of human teeth is generally regarded as a brittle material with low fracture toughness. Consequently, the contributions of this tissue in resisting tooth fracture and the importance of its complex microstructure have been largely overlooked. In this study an experimental evaluation of the crack growth resistance of human enamel was conducted to characterize the role of rod (i.e. prism) orientation and degree of decussation on the fracture behavior of this tissue. Incremental crack growth was achieved in-plane, with the rods in directions longitudinal or transverse to their axes. Results showed that the fracture resistance of enamel is both inhomogeneous and spatially anisotropic. Cracks extending transverse to the rods in the outer enamel undergo a lower rise in toughness with extension, and achieve significantly lower fracture resistance than in the longitudinal direction. Though cracks initiating at the surface of teeth may begin extension towards the dentin–enamel junction, they are deflected by the decussated rods and continue growth about the tooth’s periphery, transverse to the rods in the outer enamel. This process facilitates dissipation of fracture energy and averts cracks from extending towards the dentin and vital pulp.