Enamel rod formation in the monkey observed by scanning electron microscopy

Scanning electron microscopy of permanent tooth buds of the monkey confirmed that mineralizing interrod enamel surrounds Tomes' processes on three sides, forming pits that restrict enamel rod formation. The forming face of the enamel rod, which is the floor of the pit, angled toward the tooth surface at the apical edge of the pit, the side nearest the cervical region of the tooth. Consequently, the apical edge of each pit was the only site where both rod and interrod enamel were formed at the nascent tooth surface. The ameloblasts had two secretory surfaces. One was the microvillous surface of the short Tomes' process abutting the forming face of the enamel rod. The other surface, closer to the ameloblast, was between Tomes' processes, abutting the crests of interrod enamel which formed the pits. At each site forming enamel crystallites had specific orientations. Due to the angle of the forming face of the rod and the short Tomes' process, crystallites with both rod and interrod orientation form at the same time and the same plane within the apical (cervical) margin of each rod. It is hypothesized that indistinct boundaries between rod and interrod enamel at the apical margin of each rod are due to both secretory surfaces of ameloblasts secreting at the same time and at the same site.     

Enamel rod formation in the monkey observed by scanning electron microscopy.

Scanning electron microscopy of permanent tooth buds of the monkey confirmed that mineralizing interrod enamel surrounds Tomes' processes on three sides, forming pits that restrict enamel rod formation. The forming face of the enamel rod, which is the floor of the pit, angled toward the tooth surface at the apical edge of the pit, the side nearest the cervical region of the tooth. Consequently, the apical edge of each pit was the only site where both rod and interrod enamel were formed at the nascent tooth surface. The ameloblasts had two secretory surfaces. One was the microvillous surface of the short Tomes' process abutting the forming face of the enamel rod. The other surface, closer to the ameloblast, was between Tomes' processes, abutting the crests of interrod enamel which formed the pits. At each site forming enamel crystallites had specific orientations. Due to the angle of the forming face of the rod and the short Tomes' process, crystallites with both rod and interrod orientation form at the same time and the same plane within the apical (cervical) margin of each rod. It is hypothesized that indistinct boundaries between rod and interrod enamel at the apical margin of each rod are due to both secretory surfaces of ameloblasts secreting at the same time and at the same site.     

Knife chatter during thin sectioning of rat incisor enamel can cause periodicities resembling cross-striations

Cross-striations are traditionally associated with the enamel rods in many species including man. Although these striations are obvious with light microscopy, their exact nature has been difficult to determine with the transmission electron microscope on thin sections of enamel. Thin section microscopy either reveals no structures that can be called cross-striations, or shows periodic light and dark bands across the rods. Superficially, these bands resemble chatter artifact. To test this possibility, rat incisor enamel was used because cross-striations have not been demonstrated on these enamel rods. Thin sections were prepared of enamel blocks oriented in various ways with respect to the cutting edge of the diamond knife. The sections showed either uniform enamel or light and dark bands over rod profiles or interrod enamel. Since these bands could be produced artifactually it is concluded that similar bands seen on enamel rods of other species may also be artifacts.     

Expression,structure, and function of enamel proteinases

Proteinases serve two important functions during dental enamel formation: They (a) process and (b) degrade enamel proteins. Different enzymes carry out these functions. Enamelysin (MMP-20) is the foremost enamel matrix-processing enzyme. Its expression initiates prior to the onset of dentin mineralization and continues throughout the secretory stage of amelogenesis. In vitro, enamelysin catalyzes all of the amelogenin cleavages that are known to occur during the secretory stage in vivo, and it is probably the enzyme responsible for the processing of all enamel proteins. There is evidence suggesting that enamelysin activity is critical for proper enamel formation. Uncleaved and processed enamel proteins often segregate into different compartments within the developing enamel layer, suggesting that they may have different functions. Intact ameloblastin and its C-terminal cleavage products localize in the superficial rod and interrod enamel, while its N-terminal cleavage products congregate in the sheath space. Intact enamelin is only present at the mineralization front within a micrometer of the enamel surface, while its cleavage products concentrate in the rod and interrod enamel. Processed enamel proteins accumulate during the secretory stage, but disappear early in the maturation stage. Enamel matrix serine proteinase 1 (EMSP1), now officially designated kallikrein 4 (KLK4), is believed to be the predominant degradative enzyme that clears enamel proteins from the matrix during maturation. KLK4 expression initiates during the transition stage and continues throughout maturation. KLK4 concentrates at the enamel surface when the enamel matrix disappears, and aggressively degrades amelogenin in vitro. During tooth development, proteinases are secreted by ameloblasts into the extracellular space, where they cleave enamel proteins by catalyzing the hydrolysis of peptide bonds. Enamel proteinases are present in low abundance and are not likely to participate directly in the mineralization process. Two major enamel proteinases have been identified: enamelysin (MMP20) and kallikrein 4 (KLK4). These proteinases are expressed at different times and have different functions. Their roles are to modify and/or to eliminate enamel matrix proteins, which affects the way enamel proteins interact with each other and with the developing enamel crystallites. A brief review of dental enamel formation is presented, followed by a more detailed analysis of enamelysin and KLK4 expression, structure, and function.     

Expression,Structure, and Function of Enamel Proteinases

Proteinases serve two important functions during dental enamel formation: They (a) process and (b) degrade enamel proteins. Different enzymes carry out these functions. Enamelysin (MMP-20) is the foremost enamel matrix-processing enzyme. Its expression initiates prior to the onset of dentin mineralization and continues throughout the secretory stage of amelogenesis. In vitro, enamelysin catalyzes all of the amelogenin cleavages that are known to occur during the secretory stage in vivo, and it is probably the enzyme responsible for the processing of all enamel proteins. There is evidence suggesting that enamelysin activity is critical for proper enamel formation. Uncleaved and processed enamel proteins often segregate into different compartments within the developing enamel layer, suggesting that they may have different functions. Intact ameloblastin and its C-terminal cleavage products localize in the superficial rod and interrod enamel, while its N-terminal cleavage products congregate in the sheath space. Intact enamelin is only present at the mineralization front within a micrometer of the enamel surface, while its cleavage products concentrate in the rod and interrod enamel. Processed enamel proteins accumulate during the secretory stage, but disappear early in the maturation stage. Enamel matrix serine proteinase 1 (EMSP1), now officially designated kallikrein 4 (KLK4), is believed to be the predominant degradative enzyme that clears enamel proteins from the matrix during maturation. KLK4 expression initiates during the transition stage and continues throughout maturation. KLK4 concentrates at the enamel surface when the enamel matrix disappears, and aggressively degrades amelogenin in vitro. During tooth development, proteinases are secreted by ameloblasts into the extracellular space, where they cleave enamel proteins by catalyzing the hydrolysis of peptide bonds. Enamel proteinases are present in low abundance and are not likely to participate directly in the mineralization process. Two major enamel proteinases have been identified: enamelysin (MMP20) and kallikrein 4 (KLK4). These proteinases are expressed at different times and have different functions. Their roles are to modify and/or to eliminate enamel matrix proteins, which affects the way enamel proteins interact with each other and with the developing enamel crystallites. A brief review of dental enamel formation is presented, followed by a more detailed analysis of enamelysin and KLK4 expression, structure, and function.     

Earliest enamel deposits of the rat incisor examined by electron microscopy, electron diffraction, and electron probe microanalysis

In order to describe initial events in enamel mineralization and to help characterize inorganic-organic interactions in this tissue, the earliest rod and interrod enamel in mandibular incisors from normal young adult (100 gm) rats, perfused with 100% ethylene glycol, has been studied by transmission electron microscopy, selected area electron diffraction, and high-spatial-resolution electron probe microanalysis. Diffraction and probe data were correlated precisely from the same extracellular regions of the tissue. Sites were examined progressively as a function of location a) from the most recently deposited enamel adjacent to ameloblasts toward the dentin-enamel junction and b) from the apical portion of the tooth longitudinally toward its incisal end. Electron diffraction patterns consistent with that of a poorly crystalline hydroxyapatite were generated at all locations. Diffraction characteristics changed only slightly toward that of more crystalline hydroxyapatite at different locations. Earliest apical enamel generated molar Ca/P ratios in a range of 0.99-1.46 (average 1.24 +/- 0.15). Molar Ca/P ratios of the first enamel interrod elements increased from approximately 1.24 at ameloblast-enamel boundaries to approximately 1.40 at the dentin-enamel junction, small changes corresponding to those observed in electron diffraction characteristics.     

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.     

A spontaneous mutation: amelogenesis imperfecta with cysts in rats

Amelogenesis imperfecta (AI) is an inherited dental disease of enamel formation in humans, and there are various phenotypes due to the combination of enamel quality and quantity. We encountered four female IGS rats with spontaneous AI including odontogenic cysts in the incisor teeth. Histopathologically, in the incisors of the rats, the enamel organ was disorganized with the remaining enamel matrix residing within the enamel space. The expanding cysts derived from the enamel organ were formed in the periosteal connective tissue on the labial side. At the bottom of the tooth germs, the precursor cells of the epithelial root sheath were arranged regularly and the enamel organs were preserved to the same degree as those of normal rats. In the molar teeth of the affected rats an enamel matrix remained on the neck and crown of the erupted teeth; however, no abnormality was observed at the tooth root. Although an animal model of AI has been developed from mutants of the SHR-SP rat strain, the present cases represent another potential model of the disease because of the differences in the way the enamel matured and the odontogenic cyst formation in the incisors.     

Murine enamelin: cDNA and derived protein sequences

Enamelin is the largest enamel protein. Recently we reported the characterization of a cDNA clone encoding porcine enamelin. The secreted protein has 1104 amino acids--over 6 times the length of amelogenin (173 amino acids) and almost 3 times the lengths of sheathlin (395 amino acids) and tuftelin (389 amino acids). Immunohistochemistry has shown that uncleaved porcine enamelin concentrates at the growing tips of the enamel crystallites while its cleavage products localize to rod and interrod enamel. Here we report the isolation and characterization of cDNA encoding murine amelogenin and demonstrate the tooth specificity of porcine enamelin. The murine clone is 4154 nucleotides in length and encodes a protein of 1274 amino acids. In the absence of post-translational modifications murine enamelin has an isotope averaged molecular mass of 137 kDa and an isoelectric point of 9.4. Multiple tissue Northern blot analyses detect porcine enamelin mRNA in developing teeth but not in liver, heart, brain, spleen, skeletal muscle and lung. Mouse and porcine enamelin share 61% amino acid identity and 75% DNA sequence identity. Mouse enamelin has 14 tandemly arranged copies of an 11 amino acid segment that is found only once in porcine enamelin.     

Fine structure of secretory ameloblasts in the kitten

Secretory ameloblasts in lower second molars of 1-week-old kittens were studied with the elctron microscope after perfusion-fixation with and without decalcification. Ameloblast height varied from 40–65 μm. Tomes' process presented a stepped profile with two type-1 faces (presumably enamel secreting) and one type-2 face (thought to be non-secreting). The type-1 face was associated with extensive membrane infoldings. The enamel facing the type-1 face was lined by a dense border, and stippled material was present between cell membrane and enamel. The type-2 face was characterized by minute membrane invaginations and, sometimes, stippled material between enamel and cell membrane. In cross section, the distal part of Tomes' process showed bilateral symmetry, with one dorsal, one ventral and two lateral aspects. It was suggested that enamel formation occurs in three stages. First, interrod enamel on the lateral aspects of Tomes' processes is formed by two adjacent amelblasts. Next, the interrod enamel on the dorso-ventral aspects of Tomes' processes is formed mainly by the dorsal extension of one Tomes' process. Finally, the enamel rod is formed by one ameloblast. Stippled material was consistently present in the extracellular spaces between Tomes' processes and in irregularly shaped vesicles in the ameloblast apex, and occasionally in large masses between the ameloblast cell bodies. Spherical vesicles within the ameloblast, thought to be lysosomes, also could contain material resembling stippled material. The possibility was considered that stippled material is partly derived from enamel, and finally phagocytosed by ameloblasts. There was no clear morphologic evidence for merocrine secretion playing a role in the formation of either enamel, stippled material or the rod sheath.     

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.     

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.     

Interproximal contact hypoplasia in primary teeth: A new enamel defect with anthropological and clinical relevance

This study reports the prevalence, distribution, and expression of enamel defects in a sample of primary teeth (n = 225) from a prehistoric site in western India (1400–700 BC). Five enamel surfaces of individual, isolated primary teeth were observed for surface defects using a binocular stereomicroscope with variable power of magnification (8–20×). Standards for evaluating dental enamel defects (DDE) recommended by the Fédération Dentaire International (FDI) were employed. Details of defect expression were also recorded, including size, shape, and surface of tooth crown affected. Hypoplastic enamel defects were observed in 28% of teeth, but the distribution and expression of defects was not random. More than 50% of canine teeth had hypoplastic defects (HD); incisors and molar teeth exhibited far fewer HD. The buccal surface of canines was the most commonly affected crown surface. Areas of missing enamel were also common on the mesial and distal surfaces of canines and incisors and on the mesial surface of molar teeth. The high frequency of enamel defects found on interproximal crown surfaces warrants a label, and the name interproximal contact hypoplasia (IPCH) is proposed. Linear enamel hypoplasia (LEH) was absent from this primary dental sample. IPCH is more frequent in mandibular than in maxillary teeth, but no side preference was detected. In canine teeth, buccal hypoplasias (localized hypoplasia of primary canines; LHPC) were not positively correlated with interproximal hypoplastic defects. The etiology of IPCH may involve mesial compaction of developing teeth due to slow longitudinal growth of the jaws. Episodic bone remodeling results in ephemeral fenestrae in the mesial and distal walls of the dental crypt permitting tooth–tooth contact and disruption of amelogenesis. IPCH prevalence decreases across the subsistence transition from sedentary Early Jorwe agriculturalists to seminomadic Late Jorwe hunters and foragers, but the difference is not statistically significant. This may be due to underrepresentation of mandibular teeth in the sample. Am. J. Hum. Biol. 11:718–734, 1999. © 1999 Wiley-Liss, 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-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.     

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.     

A scanning electron microscopic study on the distribution of peritubular dentine in cheek teeth of Cervidae and Suidae (Mammalia, Artiodactyla)

Summary Distribution of peritubular dentine was studied in cheek teeth of fallow deer (Dama dama), roe deer (Capreolus capreolus) and wild boar (Sus scrofa). In the two cervid species, especially intense peritubular dentine formation was found in the outer regions of mid and cuspal coronal dentine. In these areas a marked asymmetry occurred, peritubular dentine being predominantly secreted onto the side of the dentinal tubule walls nearest to the dentinoenamel junction. Intensity and asymmetry of peritubular dentine formation decreased cervically. In root dentine, the walls of the dentinal tubules were covered with only a thin peritubular dentine layer of even thickness. Here, in contrast to peripheral coronal dentine, the volume of intertubular dentine far exceeded that of peritubular dentine. In porcine coronal dentine, PTD asymmetry, being of lesser extent than in cervids, was observed only in peripheral areas of cuspal and flank regions of the cheek teeth. Because peritubular is more highly mineralized than intertubular dentine, the relative volume of dentine made up from the two components has an important influence on dentinal wear resistance. The significance of variations in volume and distribution of peritubular dentine between different dentinal regions for achieving and maintaining a functional occlusal surface is shown for cervid cheek teeth. Our results suggest that dentinal structure (in addition to enamel structure) should be taken more into consideration when discussing occlusal surface morphology in herbi- and omnivores from a functional point of view.Abbreviations DEJ dentinoenamel junction - DPI dentine-pulp interface - ITD intertubular dentine - PTD peritubular dentine     

Overexpression of TRAP in the enamel matrix does not alter the enamel structural hierarchy

The secreted, full-length amelogenin is the dominant protein of the forming enamel organ. As enamel mineralization progresses, amelogenin is quickly subjected to proteolytic activity, and eliminated from the enamel environment. Mature enamel contains only traces of structural proteins, including enamelin and the sheath protein ameloblastin. In addition, a proteolytic fragment of amelogenin, known as the tyrosine-rich amelogenin peptide or TRAP, is present in low but isolatable quantities. By overexpressing TRAP during enamel development we sought to determine if such overexpression would result in structural alterations to the mature enamel. We reasoned that overexpressing a protein associated with enamel maturation, at an inappropriate developmental stage, would result in alterations to the enamel protein assembly and hence, alterations in enamel structure and morphology. As judged by transmission and scanning electron microscopy, the enamel formed by overexpressing TRAP showed little morphological differences when compared to the enamel of normal nontransgenic animals. Based on scanning electron-microscopic images, there was modest hypomineralization evident in the interrod enamel of the TRAP-overexpressing animals. However, this finding was inconsistent and inconsequential from a structural and functional perspective. From these results it appears that additional amounts of TRAP protein in the immature enamel matrix are not sufficient to alter the properties of the enamel extracellular matrix to an extent that the hierarchical structure of mature enamel is altered.     

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.