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.     

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

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

The orientation of the enamel prisms at the enamel surface

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

Dimensional behavior of enamel prisms in various depths and regions of the enamel of human permanent teeth

Different regions of broken enamel of human permanent teeth are observed by means of a scanning electron microscope. The diameter of the prisms must be considered as constant from the dentino-enamel junction up to the enamel surface.     

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.     

Mechanics of longitudinal cracks in tooth enamel

A study is made of longitudinal "channel" cracking in tooth enamel from axial compressive loading. The cracks simulate those generated in the molar and premolar teeth of humans and animals by natural tooth function. Contact loading tests are made on extracted human molars with hard and soft indenting plates to determine the evolution of such cracks with increasing load. Fracture is largely stable, with initial slow growth followed by acceleration as the cracks approach completion around an enamel side wall. A simple power law relation expresses the critical load for full fracture in terms of characteristic tooth dimensions-base radius and enamel thickness-as well as enamel toughness. Extended three-dimensional finite element modeling with provision for growth of embedded cracks is used to validate this relation. The cracks leave "fingerprints" that offer valuable clues to dietary habits, and provide a basis for a priori prediction of bite forces for different animals from measured tooth dimensions.     

Properties of tooth enamel in great apes

A comparative study has been made of human and great ape molar tooth enamel. Nanoindentation techniques are used to map profiles of elastic modulus and hardness across sections from the enamel–dentin junction to the outer tooth surface. The measured data profiles overlap between species, suggesting a degree of commonality in material properties. Using established deformation and fracture relations, critical loads to produce function-threatening damage in the enamel of each species are calculated for characteristic tooth sizes and enamel thicknesses. The results suggest that differences in load-bearing capacity of molar teeth in primates are less a function of underlying material properties than of morphology.     

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.     

On the R-curve behavior of human tooth enamel

In this study the crack growth resistance behavior and fracture toughness of human tooth enamel were quantified using incremental crack growth measures and conventional fracture mechanics. Results showed that enamel undergoes an increase in crack growth resistance (i.e. rising R-curve) with crack extension from the outer to the inner enamel, and that the rise in toughness is a function of distance from the dentin enamel junction (DEJ). The outer enamel exhibited the lowest apparent toughness (0.67 ± 0.12 MPa m^0.5), and the inner enamel exhibited a rise in the growth toughness from 1.13 MPa m^0.5/mm to 3.93 MPa m^0.5/mm. The maximum crack growth resistance at fracture (i.e. fracture toughness (K_c)) ranged from 1.79 to 2.37 MPa m^0.5. Crack growth in the inner enamel was accompanied by a host of mechanisms operating from the micro- to the nano-scale. Decussation in the inner enamel promoted crack deflection and twist, resulting in a reduction of the local stress intensity at the crack tip. In addition, extrinsic mechanisms such as bridging by unbroken ligaments of the tissue and the organic matrix promoted crack closure. Microcracking due to loosening of prisms was also identified as an active source of energy dissipation. In summary, the unique microstructure of enamel in the decussated region promotes crack growth toughness that is approximately three times that of dentin and over ten times that of bone.     

Evolutionary and functional significance of hominoid tooth enamel

The purpose of this investigation is to evaluate enamel thickness in extant and extinct hominoids. The material used in this study spans the evolutionary history of this group, from 20 million years ago to the present. The objectives of this investigation are to test three hypotheses: (1) the Loading Hypothesis: loading areas of the crown have thicker enamel than non-loading areas; (2) the Phyletic Hypothesis: differences in enamel thickness provide a basis for determining evolutionary relationships; and (3) the Functional Hypothesis: differences among hominoids result from adaptations to differing dietary and ecological habitats, that is from folivory to frugivory to hard object feeding and from tropical to forest to savanna habitats. Thin sections were prepared and polished to approximately 100 microm in thickness. Each section was then enlarged and digitally captured to the computer. Image processing and analysis software, SigmaImage (was used to measure the sections. Subsequent statistical analysis was conducted with SigmaStat and SPSS statistical software programs. The data provides statistical support for all hypotheses. In particular, the data support the proposal that "thick" enamel is the ancestral condition for the great apes and human clade. Therefore, Pongo would have retained its enamel thickness from the common ancestor of the great apes and Gorilla and Pan would have secondarily reduced enamel thickness to "thin." The common ancestor of the hominids, the australopithecines, would have "thick" enamel. The "hyper-thick" enamel of the australopithecines would be a derived character for this clade due to increased crushing and grinding and adaptation to savanna habitat. Homo would have secondarily reduced enamel thickness to "thick." Evolutionary biology of enamel differs markedly in hominids from that found in other hominoids and primates. Increased enamel thickness involved both increases in absolute thickness of enamel and crown size in response to increase masticatory loading.     

Resistance and volume changes caused by nitroprusside in the dog.

Changes in vascular volume caused by a pharmacologic agent are frequently inferred rather than directly measured. We investigated the effects of nitroprusside in 8 dogs divided into 2 groups: control and splenectomized. We anesthetized the dogs using pentobarbital, and surgically prepared a veno-right atrial bypass preparation whose controlled cardiac output and external reservoir allowed measurement of both changes in vascular resistance and changes in vascular volume. In both groups, blood pressure (mean +/- SD) decreased at each successive level of nitroprusside: 114 +/- 24 mmHg (base line), 101 +/- 19 mmHg (45 microgram/min), 90 +/- 16 mmHg (90 microgram/min), 81 +/- 17 mmHg (180 microgram/min), 68 +/- 18 mmHg (360 microgram/min). Nitroprusside caused a large and similar decrease in vascular resistance in both groups. In the control group, vascular volume increased above base line 5.5 +/- 2.7, 8.3 +/- 3.2, 11.6 +/- 2.9, and 14.7 +/- 3.5 ml/kg at each successive level of nitroprusside infusion, whereas in the splenectomized group vascular volume increased above base line 0.9 +/- 0.3, 2.5 +/- 1.0, 3.3 +/- 1.1, and 4.0 +/- 1.3 ml/kg at each successive level of nitroprusside infusion, but increased significantly less than the control group. We concluded that nitroprusside decreases vascular resistance and increases vascular volume and that the spleen is the major site of changes in vascular volume caused by nitroprusside.     

Apoptosis in the reduced enamel epithelium just after tooth emergence in rats

The reduced enamel epithelium transforms into a stratified squamous epithelium, i.e. a junctional epithelium, as the tooth erupts. In this study, we observed apoptosis in the reduced enamel epithelia of rats just after tooth eruption and before complete junctional epithelium formation, by the TUNEL method and electron microscopy. TUNEL-positive reactions were scattered in the reduced ameloblasts and in the external cells of the reduced enamel epithelium. Electron microscopic observation confirmed features of apoptosis, such as nuclei with chromatin condensation, cell shrinkage, and phagocytosis of apoptotic bodies by macro-phage-like cells and epithelial cells. These results suggest that apoptotic cell death is involved in the disappearance of reduced ameloblasts and the external cells of the reduced enamel epithelium during the formation of the junctional epithelium.     

On the mechanical properties of tooth enamel under spherical indentation

The mechanical properties of tooth enamel generally exhibit large variations, which reflect its structural and material complexity. Some key properties were evaluated under localized contact, simulating actual functioning conditions. Prominent cusps of extracted human molar teeth were polished down ∼0.7 mm below the cusp tip and indented by tungsten carbide balls. The internal damage was assessed after unloading from longitudinal or transverse sections. The ultimate tensile stress (UTS) was determined using a novel bilayer specimen. The damage is characterized by penny-like radial cracks driven by hoop stresses and cylindrical cracks driven along protein-rich interrod materials by shear stresses. Shallow cone cracks typical of homogeneous materials which may cause rapid tooth wear under repeat contact are thus avoided. The mean stress vs. indentation strain curve is highly nonlinear, attributable to plastic shearing of protein between and within enamel rods. This curve is also affected by damage, especially radial cracks, the onset of which depends on ball radius. Several material properties were extracted from the tests, including shear strain at the onset of ring cracks γ_F (= 0.14), UTS (= 119 MPa), toughness K_C (= 0.94 MPa m^1/2), a crack propagation law and a constitutive response determined by trial and error with the aid of a finite-element analysis. These quantities, which are only slightly sensitive to anatomical location within the enamel region tested, facilitate a quantitative assessment of crown failure. Causes for variations in published UTS and K_C values are discussed.     

Three-dimensional analysis of mandibular growth and tooth eruption

Normal and abnormal jaw growth and tooth eruption are topics of great importance for several dental and medical disciplines. Thus far, clinical studies on these topics have used two‐dimensional (2D) radiographic techniques. The purpose of the present study was to analyse normal mandibular growth and tooth eruption in three dimensions based on computer tomography (CT) scans, extending the principles of mandibular growth analysis proposed by Björk in 1969 from two to three dimensions. As longitudinal CT data from normal children are not available (for ethical reasons), CT data from children with Apert syndrome were employed, because it has been shown that the mandible in Apert syndrome is unaffected by the malformation, and these children often have several craniofacial CT scans performed during childhood for planning of cranial and midface surgery and for follow‐up after surgery. A total of 49 datasets from ten children with Apert syndrome were available for study. The number of datasets from each individual ranged from three to seven. The first CT scan in each of the ten series was carried out before 1 year of age, and the ages for the 49 scans ranged from 1 week to 14.5 years. The mandible and the teeth were segmented and iso‐surfaces generated. Landmarks were placed on the surface of the mandible, along the mandibular canals, the inner contour of the cortical plate at the lower border of the symphysis menti, and on the teeth. Superimposition of the mandibles in the longitudinal series was performed using the symphysis menti and the mandibular canals as suggested by Björk. The study supported the findings of stability of the symphysis menti and the mandibular canals as seen in profile view previously reported by Björk & Skieller in 1983. However, the mandibular canals were, actually, relocated laterally during growth. Furthermore, the position of tooth buds remained relatively stable inside the jaw until root formation started. Eruption paths of canines and premolars were vertical, whereas molars erupted in a lingual direction. The 3D method would seem to offer new insight into jaw growth and tooth eruption, but further studies are needed.     

Correlations between AFM and SEM imaging of acid-etched tooth enamel

Enamel bond strength is an important factor in restorative dentistry and crucially depends on the enamel roughness. To increase roughness, different etching procedures are employed and profilometric estimations, with probe profilometers, including atomic force microscopy (AFM), have been made. However, no correlation between roughness and bond strength has been found. To search for a possible error source leading to the underestimation of enamel roughness when utilizing probe profilometers, the authors compared scanning electron microscopy and AFM images of acid-etched tooth enamel. The results showed that AFM imaging cannot correctly depict the acid-etched enamel surface, because of the high steepness of the enamel crystallites and the generation of convolute images. This leads to a large underestimation of the profilometric parameters measured with AFM, or other profilometers, on acid-etched tooth enamel surfaces.     

Ultrastructure of alveolar bone during tooth eruption in the dog

Previous work from our laboratories has established that eruption of the permanent mandibular premolars in dogs is dependent upon the presence of the dental follicle and that it involves resorption of alveolar bone and the roots of the deciduous predecessor above and formation of alveolar bone below the developing crown. This study illustrates the topography of the bone surfaces of the crypt by scanning electron microscopy and the ultrastructure of the cells on alveolar bone surfaces during tooth eruption. Above the developing crown where the eruption pathway forms, the bone surface is a pitted sheet, the characteristic topographic feature of bone resorption; between the crown and the mandibular canal, the bone surface has numerous interconnecting trabeculae. Transmission electron microscopy of bone cells lining the eruption pathway area of the crypt showed numerous osteoclasts with adjacent mononuclear cells. Both cell types contained specific, membrane-bound cytoplasmic vesicles shown by the work of others to be characteristic features of osteoclasts and their precursors. Basal trabeculated bone in the crypt was covered by plump osteoblasts. These data show that the metabolic events in alveolar bone associated with tooth eruption have the appropriate cellular and bony surface correlates and that the suspected control of alveolar bone resorption by the dental follicle may be mediated by its recruiting and directing to adjacent bone surfaces the mononuclear precursors of osteoclasts.     

Age-related changes in the ventricular system of the dog brain.

The cerebral ventricles represent cavities of the brain which are used for diagnostic purposes. Although a wide variety of age related changes have been described in the Central Nervous System (CNS) of many species, few studies about the effect of ageing on the canine brain have been published until now. However, to date there is no previous data concerning the ventricular system of dogs. The present study deals with the morphometry of the various parts of the cerebral cavities of the German Shepherd dog by means of Batson's casts and the possible changes which take place in the ventricular system with age and/or sex. In this study, two age groups were considered: young (2-5 years) and old (10-12 years). A total number of forty seven dogs (12 young males, 13 old males, 13 young females and 9 old females), weighing 34-42 kg, were used for experimentation. Our results describe the enlargement which takes place in the ventricular system with age, which is probably related to a general age-related atrophy of neural tissue. On the contrary, there are no remarkable changes related to sex. These age-related changes are similar to the best known changes which occur in the human species. Our data could corroborate the usefulness of the dog as a natural animal model for the study of normal ageing and neurodegenerative diseases.