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 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.     

Hunter-Schreger Band patterns in human tooth enamel

Using light microscopy, we examined Hunter‐Schreger Band (HSB) patterns on the axial and occlusal/incisal surfaces of 160 human teeth, sectioned in both the buccolingual and mesiodistal planes. We found regional variations in HSB packing densities (number of HSBs per mm of amelodentinal junction length) and patterns throughout the crown of each class of tooth (maxillary and mandibular: incisor, canine, premolar, and molar) examined. HSB packing densities were greatest in areas where functional and occlusal loads are greatest, such as the occlusal surfaces of posterior teeth and the incisal regions of incisors and canines. From this it is possible to infer that the behaviour of ameloblasts forming enamel prisms during amelogenesis is guided by genetic/evolutionary controls that act to increase the fracture and wear resistance of human tooth enamel. It is suggested that HSB packing densities and patterns are important in modern clinical dental treatments, such as the bonding of adhesive restorations to enamel, and in the development of conditions, such as abfraction and cracked tooth syndrome.     

Movement of entire cell populations during renewal of the rat incisor as shown by radioautography after labeling with 3H-thymidine. The concept of a continuously differentiating cross-sectional segment

Renewal of the rat incisor was studied in three dimensions by employing a serial cross-sectioning technique to locate the boundary between labeled and unlabeled cells in the enamel organ and odontoblast layer at various times after a single injection of ³H-thymidine. This boundary, or leading edge of the front of labeling, was graphically illustrated through point-plotting reconstruction of the labial surface of the incisor. At one hour after the injection of ³H-thymidine the front of labeled ameloblasts was located within the presecretory zone related to early predentin secretion. This front formed a “C”-shaped curve stretching across the labial surface of the tooth from the lateral to the mesial cemento-enamel junction. The “C” was open anteriorly and the lateral arm extended almost twice as far incisally as the mesial arm. The edge of the front of labeled odontoblasts was positioned apical to and parallel with this “C”-shaped curve. The morphological appearance of all cells along each respective front was found to be similar. As the fronts of labeled ameloblasts and labeled odontoblasts moved forward with the erupting incisor, the cells along these fronts differentiated simulataneously and subsequently formed enamel and dentin. Throughout this movement the distance between fixed points along the leading edge of the front of labeled ameloblasts, and its positional relationship to the front of labeled odontoblasts, did not change appreciably. This indicated that cells of the tooth were being carried incisally at a uniform speed. It was concluded that renewal in the rat incisor consists of the generation by the bulbous part of the odontogenic organ of epithelial “U”-shaped cross-sectional segments which enclose a core of pulp. As this segment is transported towards the gingival margin, cellular differentiation and subsequent formation of hard tissue is seen to begin at the central labial side of the segment and to progress in a mesial and lateral direction towards the lingual side. In the process, the limits of the enamel organ at the mesial and lateral cemento-enamel junctions are established and the entire circumference of the segment is eventually enclosed by a rim of dentin.     

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.     

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.     

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.     

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.     

A method for sampling the stages of amelogenesis on mandibular rat incisors using the molars as a reference for dissection

A method for locating specific stages of amelogenesis on continuously erupting incisors was devised for rats weighing 101 +/- 5 g (n = 32). The technique is based on reflecting reference lines from the mandibular molars as perpendiculars to the labial surface of mandibular incisors. From these reference lines additional measurements are then made along the midline of the labial surface of the incisor in an apical or incisal direction to find the site desired for sampling. Histological studies on 24 decalcified incisors split into segments by using such reference lines and reconstructed by morphometry indicated that a reference line reflected from the contact point between the 2nd and 3rd molars crossed the enamel organ and adjacent enamel at 3,181 +/- 329 microns incisal to the start of the secretory zone of amelogenesis. A reference line from the 2nd and 1st molars crossed the enamel organ and enamel at 1,238 +/- 424 microns incisal to the start of the maturation zone of amelogenesis, while a reference line from the mesial side of the 1st molar crossed the enamel organ and enamel almost exactly where the enamel becomes completely soluble following prolonged decalcification in EDTA. Although reference lines were reproducible within a group of male rats having similar body weights, the linear distance between the apical end of the incisor and the point at which they crossed the tooth increased at a rate of 1 mm per 159 g for rats between 50 and 300 g body weight. This suggests that molars do not maintain a fixed relationship to incisors over time, and extreme care must be taken to standardize an experiment to a specific body weight when using this method.     

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.     

Immunohistochemical localization of connexin 43 in the developing tooth germ of rat

Distribution of gap junction protein in maxillary tooth germs of 1-day-old rats was examined by immunohistochemistry, using an affinity-purified antibody specific to residues 360–376 of rat connexin (CX) 43. In 1-day-old rats, the maxillary second molar formed the shape of the cusp, but neither dentine nor enamel was formed between the cells of the dental papilla and the inner enamel epithelium. In the tooth germ, CX 43 was expressed in the cells of the stratum intermedium and the inner enamel epithelium. Labelling in the stratum inter-medium was extensive and showed an increasing gradient from peripheral to cuspal regions. CX 43 detected in the inner enamel epithelium was at cell surfaces facing the interface between the dental papilla and the inner enamel epithelium. The cells of the dental papilla and the inner enamel epithelium began differentiation as odontoblasts and secretory ameloblasts respectively, in the cusps of the first molars, where predentine and dentine were formed but enamel matrix was not secreted. CX 43 was present in the stratum intermedium, inner enamel epithelium, preodontoblasts, odontoblasts and subodontoblasts. The incisors showed the most advanced stage of development, where the enamel matrix and calcified dentine were formed in the labial part of the teeth. The CX 43 epitope was seen in the stratum intermedium, inner enamel epithelium, preameloblasts, preodontoblasts, odontoblasts, and subodontoblasts. Immunolabelling was more extensive in the stratum intermedium and subodontoblasts than in preameloblasts, preodontoblasts, and odontoblasts. The immunolabelling in preameloblasts and preodontoblasts was accumulated at cell surfaces facing the predentine. Further, the labelling in preameloblasts and preodontoblasts disappeared or was reduced at the initiation of enamel matrix secretion and calcification of dentine matrix.The present results suggest that gap junctional cell communication has important roles in tooth development. Further, the extensive CX 43 expression in the stratum intermedium and the subodontoblast layer suggests that gap junctions have an important role in amelogenesis and dentinogenesis.     

A freeze-fracture study of the topographic relationship between inner enamel-secretory ameloblasts in the rat incisor

A correlated study using freeze-fracture replicas and routine thin sectioning was done on the ameloblasts which secrete inner enamel in the rat incisor. These ameloblasts are columnar cells aligned in rows parallel to the cross-sectional plane of the incisor. Each cell has a Tomes' process at its distal end which is contained within a cavity of interrod enamel. Tomes' processes of a particular row of cells are inclined towards the mesial side of the tooth, while those of the next row are inclined towards the lateral side, so that the processes cross each other at 90°. Freeze-fracture replicas were used to examine the surfaces of ameloblasts between adjacent rows and the surfaces of the cells within the row. Replicas of the surfaces between rows showed that ameloblasts are curved so that their proximal and distal ends (Tomes' processes) are both directed towards the same side of the tooth. The surface of each row bears the impression left upon it by the cells in the adjacent row which have been fractured away. This cell-impression is also curved, but in the opposite direction to the cells which it crosses. It was thus shown that not only the Tomes' processes of adjacent rows but the entire cell bodies cross one another. This crossing is slight at the proximal ends of the cells, but marked at the level of the distal junctional complex. At this level a cell from a particular row curves across several cells of the neighbouring row and forms large tight junctions with at least two cells in that row. The proximal and distal junctional complexes form zones of tight junctions around the cell. The distal complex is more extensive than the proximal and the tight junctions are better developed between adjacent rows. Gap junctions are, however, larger between cells within the rows. The study demonstrated the effectiveness of freeze-fracture replicas in obtaining 3-dimensional details about ameloblast shape, and this information was used to evaluate the supposed movement of ameloblasts during inner enamel formation.     

人上颌切牙邻面牙釉厚度的X光片比较

通过对牙釉牙厚度的测量比较,分析不同性别、不同牙位及牙齿不同部位间牙釉质厚度的差异,探讨其原因及意义。方法：利用临床Ｘ光检查,对１４至１９岁青少年的上颌切牙进行摄片测量,并进行统计学分析。结果：人上颌切牙邻面的牙釉质厚度在不同性别和不同牙位之间均无显著性差异,但位于近中面和远中面的牙釉质厚度比较则差异显著。结论：牙釉质的厚度与所行使的功能以及牙胚发育期的造釉细胞功能相关     

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.     

Immunocytochemical and radioautographic evidence for secretion and intracellular degradation of enamel proteins by ameloblasts during the maturation stage of amelogenesis in rat incisors

In the continuously erupting rat incisor the ameloblasts progress through distinct stages associated with the secretion and maturation of enamel. We have examined the possibility that the so-called "postsecretory" ameloblasts of the maturation stage of amelogenesis remain biosynthetically active and are engaged in the synthesis, secretion, and degradation of enamel gene products. The ultrastructural distribution of antigenic sites for enamel proteins was studied within enamel organ cells during the early maturation stage of amelogenesis in rat incisors by using the protein A-gold immunocytochemical technique and rabbit polyclonal antibodies developed against mouse amelogenins. All regions of amelogenesis from late secretion through the first complete modulation from ruffle-ended to smooth-ended ameloblasts were examined. Specific immunolabelling was found within the rough endoplasmic reticulum, Golgi saccules, secretory granules, and lysosomes of ameloblasts throughout these regions. The heaviest intracellular immunolabelling was found within secretory granules and lysosomes (multivesicular type). Quantitative analyses showed that the Golgi saccules and the multivesicular lysosomes of modulating ameloblasts were generally less immunoreactive compared to similar organelles in ameloblasts secreting the inner enamel layer. Radioautographic studies confirmed that ameloblasts of the maturation stage incorporated 3H-leucine and 3H-methionine and secreted labelled proteins into the enamel layer. Grain counts indicated that ameloblasts from the first ruffle-ended band incorporated about two-fold less 3H-methionine and secreted about tenfold less labelled proteins into the enamel compared to ameloblasts secreting the inner enamel layer. The results of this study confirm that ameloblasts do not terminate biosynthesis and secretion of enamel proteins once the final layer has been deposited on the surface of the developing enamel. They continue to form and release new proteins during the maturation stage which intermix with older proteins laid down initially during the secretory stage of amelogenesis. Secretory activity for enamel proteins has been detected in ameloblasts up to at least the second ruffle-ended phase of maturation, at which point the enamel matrix is partially soluble in EDTA.     

Distribution of Amelotin in Mouse Tooth Development

Amelotin is expressed and secreted by ameloblasts in tooth development, but amelotin distribution during enamel development is not clear. In this report, we first investigated amelotin expression in developing teeth by immunohistochemistry. Amelotin was detected in the enamel matrix at the secretion and maturation stages of enamel development. Amelotin was also observed at Tomes' processes on the apical ends of secretory ameloblasts. We then compared amelotin gene expression with those of amelogenin, enamelin, and ameloblastin in the mandibles of postnatal mice by RT‐PCR. The expression of amelotin was detected as early as in postnatal day 0 mandibles and amelotin was coexpressed with amelogenin, ameloblastin, and enamelin during tooth development. These data strongly suggest that amelotin is an enamel matrix protein expressed at the secretion and maturation stages of enamel development. Anat Rec, 2010. © 2009 Wiley‐Liss, Inc.     

Effects of chronic fluoride exposure on morphometric parameters defining the stages of amelogenesis and ameloblast modulation in rat incisors

The response of ameloblasts to long-term (6 weeks) exposure to 100 ppm fluoride was examined in continuously erupting mandibular incisors of female Sprague-Dawley rats as compared to control rats receiving a similar diet (Teklad L-356) but no sodium fluoride in their drinking water. After treatment, animals from both groups were perfused intravascularly with glutaraldehyde, and the incisors were removed and processed for light microscope morphometric analyses directly from 1 μm thick Epon sections. Other animals were injected intravenously with calcein (green fluorescence) followed 4 hours later by xylenol orange (red fluorescence) in order to reveal smooth-ended ameloblast modulation bands and thereby allow quantification of parameters related to the creation and movement of modulation waves within the maturation zone of these teeth. The results indicated that rat incisors expressed four major changes in normal amelogenesis which could be attributed to the chronic fluoride treatment. First, ameloblasts produced a thinner than normal enamel layer by the time they completed the secretory stage and entered the maturation stage of amelogenesis. Second, enamel organ cells within the maturation zone, especially those from the papillary layer, were shorter in height than normal. Third, ameloblasts related to maturing enamel in areas where it was partially soluble and/or fully soluble in EDTA modulated at a rate that was much slower than normal. In some locations ameloblasts remained ruffle-ended for as much as 30% longer than normal per cycle. This upset the usual pattern such that fewer total modulation cycles were completed per unit time by these ameloblasts. Fourth, enamel proteins were lost from the maturing enamel layer at a rate that was about 40% slower than normal. The data suggested that ameloblasts detected the delay in the extracellular breakdown and/or loss of enamel proteins and they responded by remaining ruffle-ended for longer intervals than usual (positive feedback). © 1993 Wiley-Liss Inc.