The Enamel Microstructure of the Early Eocene Pantodont Coryphodonand the Nature of the Zigzag Enamel

An additional method for the investigation of the microstructure of enamel is described using the teeth of Coryphodon, Uintatherium, Entelodon, and Crocuta. Under low magnification natural surfaces or sections of teeth display details of the enamel microstructure when the light guide effect of prisms is used. Under the same low magnification even more details were obtained from sputter-coated surfaces of sections. The method is of particular significance for the investigation of large teeth with thick enamels when structures are somewhat irregular. The new method provides a better general survey, where scanning electron microscope images often show confusing details. The enamel of Coryphodon shows oblique lines of nested chevrons that are similar, to some degree, to the zigzag enamel in Crocuta, but a distinct asymmetry between ascending and descending lineaments was observed. This specific Coryphodon -enamel was also found in Uintatherium and Entelodon. This enamel type, which evolved several times in parallel, cannot be attributed to a specific diet, but must be regarded as one of the several ways to strengthen the enamel against breakage.     

The microstructure of dentine in taxonomic and phylogenetic studies

Traditionally, physical anthropologists have focused their dental studies on features of teeth that can be observed with the unaided eye, measured with calipers, or observed with light microscopes. With the advent of the scanning electron microscope, there has been renewed interest in the use of the microstructure of enamel in phylogenetic reconstruction and taxonomic classification. The microstructure of dentine has received far less research attention than has enamel although several investigators have proposed that dentine has taxon specificity. We report the first results of a study in which we investigated the taxon specificity of dentine. In our study, we exposed the dentinal tubules on the mesial root surfaces of 12 Canis, 11 Papio, and 12 Homo mandibular first molars and compared tubule density and pattern among the three taxa. Based on these parameters, 91.43% of the teeth were correctly identified as being dog, baboon, or human, respectively. We conclude that dentine has taxon-specific structural characteristics, which can be used in anthropological investigations. To our knowledge, this is the first comparative study of the micro-structure of teeth utilizing statistical analysis.     

两种脱矿体系制备人工龋损的病理学比较

目的评价两种不同脱矿体系在人牙釉质与牛牙釉质形成人工龋样损害的情况,确定模拟早期龋实验研究的脱矿系统。方法两种脱矿体系：部分饱和酸缓冲溶液和酸性凝胶溶液,分别在人牙釉质与牛牙釉质制备人工早期釉质龋损,采用偏振光显微镜和扫描电子显微镜观察实验人工龋的组织病理学变化。结果部分饱和的酸缓冲体系制备的人工龋损都具有完整的表层和表层下脱矿,牛牙釉质除了脱矿深度大于人牙釉质外,其它病理学特点都类似于人牙釉质,与早期自然龋损相似。而凝胶液脱矿体系形成的人工龋损不管是人牙还是牛牙都没有明显的表层结构,实验牙表面多出现被腐蚀或溶解。结论部分饱和的酸缓冲液脱矿系统形成的人工龋损病理学变化更接近于自然龋;与人牙具有相似化学组成的牛牙很适合代替人牙来检测脱矿和再矿化的效果。     

Diagenetic alterations of <Emphasis Type="Italic">Meriones</Emphasis> incisors (Rodentia) of El Harhoura 2 cave,Morocco (late Pleistocene–middle Holocene)

Fossil teeth are used for palaeoenvironmental reconstructions. Among the criteria used, the geochemical composition is favoured, because it is said that the enamel is exempt from diagenetic changes. The combined microstructural and chemical analyses of rodent incisors from the cave of El Hahroura 2 (Morocco, Middle Palaeolithic–Neolithic) using scanning electron microscopy, an electron microprobe and FTIR spectroscopy show that all teeth were preserved in apatite. Nevertheless, the microstructure of both dentine and enamel is altered, as well as their chemical composition. Teeth excavated from the upper layers are already diagenetically modified despite being from the Neolithic. Teeth from three older stratigraphic layers are also altered but with a different pattern. Up to now, a direct correlation of the morphological and structural/compositional preservation has not been possible, showing that to use only one criterion to infer the quality of the preservation is not sufficient. Moreover, palaeoecological or palaeoenvironmental reconstructions based on geochemical data without control of the dental microstructure may be biased.     

Insights from the inside: histological analysis of abnormal enamel microstructure associated with hypoplastic enamel defects in human teeth

We studied the abnormalities in enamel microstructure associated with enamel hypoplasia in human teeth from the early medieval (5th-7th century AD) cemetery of Barbing, Germany, using light and scanning electron microscopy. The main aim of the study was to test the hypothesis that by analyzing the microstructure of fully formed enamel it is possible to reconstruct the reaction pattern of secretory ameloblasts to stress events leading to enamel hypoplasia. From the histological findings, a sequence of increasing impairment of secretory ameloblast function involving three thresholds was deduced. Surpassing of each of these thresholds is assumed to result in characteristic changes in enamel microstructure attributable to specific functional/morphological alterations of secretory ameloblasts. Based on our results we propose a model identifying the principal factors influencing the reaction of secretory ameloblasts to stress. The present study demonstrates that by including microscopic analysis in the study of enamel hypoplasia, it is possible to obtain a more complete picture of the formation of these developmental defects than is possible by inspection of crown surface features alone, and to draw more substantiated conclusions about the possible nature of developmental defects of enamel.     

Hunter-Schreger band orientation in interproximal enamel of modern human molars with complex morphological traits

A recent paper concerning characteristic groove-shaped wear in interproximal attritional facets of Neandertal posterior teeth correlated groove formation with deep enamel structural traits (Villa & Giacobini, 1995). Further studies were carried out on modern human teeth in order to evaluate a possible correlation between deep enamel structure and some crown complex morphological features. Seventy-five isolated molars from medieval and recent ossuaries as well as five molars recently extracted for therapeutic purpose were examined using scanning electron and optic microscope ground sections of interproximal enamel. The results showed possible correlations between deep enamel structural characteristics (i.e., Hunter-Schreger band verticalization) and some crown complex traits (i.e., occlusal groove pattern, mesial and distalfovea, mesial and distal accessory tubercules, interproximal marginal ridges). Observations reported in this study showed a significantly higher frequency of Hunter-Schreger band verticalization in lower molars with an occlusal complex pattern (i.e. primitive fissure pattern “Y” or “+”). This observation suggests a positive correlation among crown complex traits (which occur frequently in Neandertal teeth) with deep enamel structural characteristics. In addition, analysis of teeth grouped according to individual showed crown morphological traits and deep enamel structural characteristics corresponding to the same individual. This observation supports a genetic basis in occlusal shape and microstructural dental traits.     

Individual variation in enamel structure of human mandibular first premolars

Nine human mandibular first premolars were examined to assess variation in external morphology and enamel structural organization within a tooth type. The relationship of enamel ultrastructure to gross dental morphology was also studied. The teeth were cut in the mesiodistal direction just lingual to the buccal cusp, and etched. Montages were constructed of the cut enamel surface photographed in the scanning electron microscope at 100 X magnification. Parameters were measured and correlation coefficients were calculated for the comparison of various odontometric features. The mesiodistal and buccolingual dimensions were highly correlated and the occlusal thickness of enamel was significantly correlated to crown height but not crown width. Hunter-Schreger bands were less pronounced in fossa areas than at lateral aspects, cusps, or ridges; these bands were directly related to the geometry of the tooth. It was concluded that within this tooth type, there is a large amount of individual variation not only in gross morphology but also in enamel ultrastructure. This result underscores the fact that interspecific comparisons must be made with care.     

The dentition and dental tissues of the agamid lizard, Uromastyx

The dentition of Uromastyx hardwicki was examined in a series of carefully prepared dry skulls and was found to be very different from that of other agamid lizards. The anatomy of the dentition undergoes great changes from the time of hatching to advanced age, but no evidence of tooth replacement could be found. Extension of the tooth rows by addition of larger teeth posteriorly, together with elongation of the premaxilla, and a characteristic pattern of wear are responsible for the condition seen in aged specimens.
The structure of the dental tissues was investigated by means of a variety of histological techniques including scanning electron microscopy and it is established that the enamel has prismatic structure like that of mammalian enamel. The mode of formation of enamel with and without prisms is described and the occurrence and significance of prismatic structure in reptilian dental enamel discussed.     

Enamel microstructure of the hominid KB 5223 from Kromdraai, South Africa

The Plio-Pleistocene site of Kromdraai, South Africa, is well known for the recovery of the holotype of Paranthropus robustus, one of nine individual hominids recovered from this site to date. Among the Kromdraai sample, the specimen KB 5223 comprises several isolated deciduous and permanent lower teeth assigned to Paranthropus, the only recognized genus at this site. However, a more recent analysis of this specimen suggested that it should be classified as Homo. The lower right first permanent molar of KB 5223 had been previously sectioned along the tips of the mesial cusps, exposing its enamel microstructure. Previous studies had indicated differences between Homo and Paranthropus at the microstructural level. A portable confocal scanning microscope was used to describe details of the enamel microstructure of the M1 and I1 of this specimen. Angles formed between the striae of Retzius and the enamel dentine junction (EDJ), daily secretion rates in cuspal enamel of the protoconid and metaconid and crown formation time of the RM1 are provided. The number of perikymata on the right I1 was counted. Results indicate that some features recorded in the KB 5223 molar differ from those of Paranthropus. However, the number of perikymata on the I1 is lower than values so far reported for early Homo but similar to Paranthropus. Crown formation time of KB 5223 M1 was markedly lower than mean values of M1 in H. sapiens, but similar to other early hominids. Daily secretion rates in the cuspal enamel of KB 5223 M1 were higher than in modern humans.     

Postcanine microstructure in Cricodon metabolus,a Middle Triassic gomphodont cynodont from south‐eastern Africa

Cricodon metabolus is a trirachodontid cynodont from the Anisian (Middle Triassic) of eastern and southern Africa. It has labiolingually expanded (gomphodont) postcanines but also a sectorial tooth in the last postcanine locus. In this paper, we examine the crown microstructure of isolated sectorial and gomphodont postcanines belonging to the holotype specimen of this taxon using scanning electron microscopy. The enamel of both teeth is prismless and composed of discontinuous columnar divergence units, supporting the consistent presence of synapsid columnar enamel in cynognathians. Abundant tubules and numerous irregularly spaced incremental lines are also visible in the enamel and dentine layers in each tooth. This study reveals that the enamel thickness varies along the tooth row in Cricodon as the enamel layer of the gomphodont postcanines is 11.5 times thicker than that of the sectorial crown. It is likely that this difference reflects occlusal stresses and fewer replacements in gomphodont postcanines relative to sectorial teeth. Approximately 100 incremental growth lines of von Ebner are present in the dentine layer, indicating that the deposition of the dentine by odontoblasts occurred for three months before the animal's death.     

Electron probe analysis of maturation ameloblasts of the rat incisor and calf molar

Rapidly frozen upper incisor teeth of rats and molar teeth of calves were freeze fractured, freeze dried and dry dissected in preparation for energy dispersive x-ray emission microanalysis in the scanning electron microscope. Successive zones of ameloblasts adjacent to maturing rat incisor enamel were examined, beginning with cells adjacent to the least mature enamel and progressing to cells over increasingly more mature enamel. Pronounced Kalpha1,2 x-ray peaks were obtained for P, S, Cl, K and Fe but not for Ca. Ca levels were also very low compared with P, S, Cl and K in calf molar maturation ameloblasts, whereas they were high in the distal poles of the secretory odontoblasts in the same specimens. The findings indicate that both intra- and extracellular Ca levels are extremely low in maturation ameloblasts. It is concluded that Ca is neither stored nor concentrated in large amounts by the maturation ameloblasts prior to its entry into the enamel. The suggestion is made that the maturation ameloblasts might regulate entry of calcium into enamel by serving as a selective barrier.     

许家窑人幼年个体的釉质发育缺陷

上世纪70年代在许家窑-侯家窑地点发现的人类化石对了解东亚中晚更新世过渡时期古人类的形态演化起到了重要作用。除形态特征外,针对许家窑人病理表现也开展过相关研究,其中许家窑人幼年个体前部牙齿上出现的黄色小坑被认为是患有氟斑牙病的证据,且可能代表世界范围内该牙病的最早实例。然而,经电子显微镜和显微断层扫描显示,这些黄色小坑或凹陷是釉质发育不良表现,而不是代表个体患有氟斑牙病后牙齿受咀嚼压力而产生的物理破损。同步辐射扫描显示,许家窑幼年个体前部牙齿釉质密度均匀,没有出现浅层釉质矿化减小的现象,不支持该个体因釉质矿化过程受影响而患有典型氟斑牙的结论。尽管如此,后部牙齿上大量坑状的釉质缺陷和坑状缺陷底部的加重生长线特征不排除是个体氟摄入过量而影响釉质形成的分泌期而导致的。除表面坑状釉质缺陷外,许家窑幼年个体恒齿不同部位的釉质内部出现缺失（空间结构一般为圆球状）。缺失主要集中在浅层区域,沿齿尖-齿颈方向的密集程度变化与釉质发育不良位置具有一定相关性。釉质内部缺失有时互连并与釉质表面垂直。以上特征显示釉质内部缺失是釉质形成过程受影响所致,且影响因素和发生机理可能与釉质发育不良的类似。许家窑幼年个体不同牙齿在外部釉质缺陷和内部釉质缺失密集程度上的差别可能反映的是个体在发育过程中所需应对的外界扰动因素程度不同。未来研究可对包含许家窑人化石的堆积物中以及许家窑人牙齿中的氟含量进行测定,以进一步研究许家窑人个体的生活环境中是否有过量的氟以及许家窑人是否摄入了过量氟元素,从而对许家窑人的牙齿发育缺陷机理有一个进一步的了解。     

On thick and thin enamel in hominoids

Martin (1983, 1985) reviewed the significance of enamel thickness in hominoid evolution. He studied cut faces of hominoid teeth using the scanning electron microscope and related enamel prism packing patterns to both enamel formation rates and enamel thickness, although he did not present primary data on formation rates, which he summarised as being either “fast” or “slow.” Martin concluded that thick enamel formed at a fast rate represented the ancestral condition in the human and great ape clade. Thin enamel in African apes reflected a secondary reduction in secretion rates, with outer enamel being formed at a slow rate. The present study on ground sections of great ape and human teeth, using polarised light microscopy, was designed to measure the spacing between incremental growth lines in enamel, including striae of Retzius and prism cross striations, to determine rates of enamel formation in hominoids. Measurements on stria spacing showed that striae generally diverged as they passed outwards through enamel in all taxa. Cross-striation spacings also increased from inner to outer enamel. Secretion rates did not fall into two exclusive categories but varied, giving a spectrum of values generally increasing from within outwards at any one crown level and reducing in cervical enamel. There was no evidence for a reduction in enamel formation rates in outer enamel among African apes. These findings cast doubt on the proposition that the common ancestor of great apes and man had thick enamel formed at a fast rate. It is possible that thin enamel was the primitive condition, in which case thick enamel in humans and in Sivapithecus is derived, suggesting that thick enamel on low cusped teeth evolved on more than one occasion.     

Developmental and Post-Eruptive Defects in Molar Enamel of Free-Ranging Eastern Grey Kangaroos (Macropus giganteus) Exposed to High Environmental Levels of Fluoride

Dental fluorosis has recently been diagnosed in wild marsupials inhabiting a high-fluoride area in Victoria, Australia. Information on the histopathology of fluorotic marsupial enamel has thus far not been available. This study analyzed the developmental and post-eruptive defects in fluorotic molar enamel of eastern grey kangaroos (Macropus giganteus) from the same high-fluoride area using light microscopy and backscattered electron imaging in the scanning electron microscope. The fluorotic enamel exhibited a brownish to blackish discolouration due to post-eruptive infiltration of stains from the oral cavity and was less resistant to wear than normally mineralized enamel of kangaroos from low-fluoride areas. Developmental defects of enamel included enamel hypoplasia and a pronounced hypomineralization of the outer (sub-surface) enamel underneath a thin rim of well-mineralized surface enamel. While the hypoplastic defects denote a disturbance of ameloblast function during the secretory stage of amelogenesis, the hypomineralization is attributed to an impairment of enamel maturation. In addition to hypoplastic defects, the fluorotic molars also exhibited numerous post-eruptive enamel defects due to the flaking-off of portions of the outer, hypomineralized enamel layer during mastication. The macroscopic and histopathological lesions in fluorotic enamel of M. giganteus match those previously described for placental mammals. It is therefore concluded that there exist no principal differences in the pathogenic mechanisms of dental fluorosis between marsupial and placental mammals. The regular occurrence of hypomineralized, opaque outer enamel in the teeth of M. giganteus and other macropodids must be considered in the differential diagnosis of dental fluorosis in these species.     

蜥臀目霸王龙及有鳞目沧龙牙齿组织的微细结构

本文是利用扫描电子显微镜对陆栖恐龙tvrannosaurid与海栖渐增mosasaurid”牙齿结构进行的比较解剖学研究。化石采自加拿大RedDeerhiverValley上白里统Horse－shoeCanyon组。通过研究地层中出现的生物化石,特别是动物牙齿的组织结构,可以了解动物为了适应生活环境而发生的进化过程,也可以推测它们的系统发育关系。tyrannosaund与mosas。id都拥有锥状的同形齿,牙齿侧向扁平,且略向后弯曲。研究结果确认了tyrannosaurid的牙齿由于薄层的无柱釉质bPrismaticenamel）向齿质的侵人而造成许多的凹凸构造,此锯齿状构造沿着牙齿的前后缘,由牙齿的顶端分布至基部。因此tyrannosaurid的牙齿呈现着锐利的切缘;在这些凹凸状切缘的沟与小窝的深部可观察到有机物的沉积。但是类似的锯齿状构造只能在齿冠呈钝圆状的mosasaurid牙齿的基部附近观察到。我们以扫描电子显微镜（SEM）检索,确认两爬行类的齿质皆是属于中间型的真性齿质（intermediatetypeorthodentine）;所谓orthodentine即是细管齿质（tubulardentine）。tyrannosaurid的真性齿质的齿质小管只在齿质一釉质相接处Uentino—enamaljunction）附近放散出规则性的分歧与末枝。但mosasaurid的真性齿质的齿质J。管,在齿质的中间层与表层中,呈现着由复杂     

On the nature of the opaque and translucent enamel regions of some macropodinae (Macropus giganteus,Wallabia bicolor and Peradorcas concinna)

Summary Teeth of three macropod species, M. giganteus, W. bicolor and P. concinna, have been studied using the techniques of light microscopy, scanning- and transmission-electron microscopy and hardness measurement. Light microscope observations showed that the teeth of these species had a translucent enamel region close to the dentine and an outer opaque enamel region at the tooth's surface. These regions were not related to the presence or absence of tubules which are a characteristic feature of marsupial enamel. Hardness tests showed that the opaque enamel was softer than the translucent enamel. Scanning electron microscope observations revealed that there was no correlation between any particular prism packing or orientation and the opaque and translucent enamel regions. Transmission electron microscope observations showed that the translucent enamel region consisted of well defined prisms and well packed, lath-like crystals, whereas the opaque enamel was disrupted by voids (which ranged in size from enlarged micropores to about 2 m in diameter in extreme cases) between crystals and some randomly oriented, loosely packed crystals. This disruption within the opaque enamel region was more common at prism boundaries but pockets of disrupted enamel were also found within prisms and interprismatic regions. The opacity of the enamel was caused by scattering of light from the voids. The ultrastructure of the opaque enamel region indicated that this region was hypomineralized; hardness tests and polarized light microscope observations were consistent with these results.     

Scanning electron microscopy and calcification in amelogenesis imperfecta in anterior and posterior human teeth

Teeth fragments from members of a family clinically and genetically diagnosed as having amelogenesis imperfecta were studied by scanning electron microscopy and X-ray microprobe analysis to establish the morphological patterns and the quantitative concentration of calcium in the enamel of anterior (canine, incisor) and posterior (premolar and molar) teeth. The prism patterns in the enamel of teeth from both regions were parallel or irregularly decussate, with occasional filamentous prisms accompanied by small, irregularly rounded formations. Prismless enamel showed the R- and P-type patterns. Calcium levels in enamel of amelogenesis imperfecta and control teeth differed significantly between anterior and posterior teeth, indicating that the factors that influence normal mineralization in different regions of the dental arch are not altered in the process of amelogenesis imperfecta.     

Enamel structure and microwear: an experimental study of the response of enamel to shearing force.

The anisotropic fracturing and differential wear properties of enamel microstructure represent factors that can obscure the predictive relationship between dental microwear and diet. To assess the impact of enamel structure on microwear, this in vitro experimental study examines the relative contributions to wear of three factors: 1) species differences in microstructure, 2) direction of shearing force relative to enamel prisms and crystallites, and 3) size of abrasive particles. Teeth of Lemur, Ovis, Homo, and Crocodylus, representing, respectively, the structural categories of prismatic patterns 1, 2, and 3 and nonprismatic enamel, were abraded by shearing forces (forces having a component directed parallel to abraded surfaces) and examined by scanning electron microscopy. Striation width increased with particle size for nonprismatic, but not for prismatic, specimens. Direction of shear relative to prism and crystallite orientation had a significant influence on striation width in only some prismatic enamels. The different responses of prismatic and nonprismatic enamels to abrasion reflect the influence of structure, but at the level of organization of crystallites rather than prisms per se. Such interactions explain in part the inability of striation width to discriminate among animals with different dietary habits. Heteroscedasticity and deviations from normality also may confound parametric analyses of microwear variables. Variation in crystallite orientation in prismatic enamels may contribute to optimal dental function through the property of differential wear in functionally distinct regions of teeth.