Procerberus Enamel: A Missing Link

In a fossil tooth of Procerberus (a Late Cretaceous palaeoryctid insectivore), there is a unique, clear and simultaneous expression of all three known forms of enamel: prismatic, pseudo-prismatic and aprismatic. The pseudoprismatic domain, generally regarded as the more primitive unit compared to the prismatic domain, may be interpreted in this material as that morphological territory in which the prism appears, or evolves, as an additional structural unit. It is possible to construct a three-dimensional, developmental scheme for Procerberus enamel on the basis of known principles and to use it to help build a conceptual bridge between synapsid and mammalian enamel

Chiropteran Enamel Structure

We have examined sectioned and whole teeth of Hipposideros diadema in order to provide a baseline description of bat enamel at electron microscope level. Intact and worn surfaces of whole teeth from representatives of all superfamilies of the suborder Microchiroptera, representatives of the suborder Megachiroptera, and the Ditjimanka fossil tooth have been surveyed. Whole chiropteran teeth lend themselves well to non-destructive SEM examination; all except Dobsonia sp. showing evidence of prismatic structure at the outer surface. The majority of prisms in chiropteran enamel are horse-shoe shaped becoming rounded and complete only towards the outer enamel surface. Prism packing is typically Pattern 3 near the enamel-dentine junction and Pattern 2 where the enamel is sufficiently thick. A major feature is the consistent presence of a minor boundary plane ( seam ) at the open end of horse-shoe shaped prisms. Enamel tubules were found at the enamel-dentine junction of a number of species and consistently at the outer enamel surface of one, Macroglossus minimus. Further examination of embedded material is planned in order to quantify prism shape, prism packing and the disposition of tubules in chiropteran enamel

Cleaning Procedures for Small Dental Instruments Prior to Sterilization

Observations were made in an institutional setting on the quality of cleaning of dental burs. Assessments were made by scanning electron microscopy of surface contamination and change to the surface quality of burs before and after use, and as a result of different cleaning procedures. The most significant finding was the corrosive action on carbon steel burs of a phosphoric acid based cleaning solution in routine use at the time. We show by comparison the effectiveness and non-corrosive nature of an alternative detergent based on sulfamic acid, and propose a suitable cleaning routine. Two other findings are presented: new burs as unpackaged are unacceptable for use without first being subjected to a cleaning process (we recommend a suitable procedure); and there is a need for distilled water rather than tap water rinsing after detergent use. It is an important part of an institution\u27s responsibility to monitor cleaning, sterilizing and supply services; the scanning electron microscope is a valuable adjunct in this aspect of quality assurance

Crystallite Orientation Discontinuities and the Evolution of Mammalian Enamel – Or, When is a Prism?

The nature and relationship of crystallite domains have been explored in fossil and extant enamels spanning an evolutionary period of 200 x 106 years. Minor crystallite orientation discontinuities, either linear or planar, were found to be consistent characteristics of all specimens examined. The earliest minor discontinuity is linear (convergence line), shown here in Oligokyphus and Eozostrodon. The convergence line would be the result of the occasional development of a conical Tomes\u27 process to the parent ameloblast. An increase in number and regularity of convergence lines, shown here in Haldanodon, marks the appearance of a regular pseudoprismatic enamel structure. The second minor discontinuity to appear is planar (seam), shown here in a dryolestid eupantothere. The seam has previously been deduced to relate developmentally to a central groove on the sloping floor-wall of the Tomes\u27 process pit. Coincident with the appearance of the seam is that of a rudimentary major planar discontinuity which does not enclose a domain to constitute what would normally be acknowledged as a prism. Its developmental basis would be the establishment of a steep wall and floor (however partial in circumference) to the Tomes\u27 process pit. The extent of the major planar discontinuities was found to increase subsequently to enclose a classically recognizable prismatic domain, shown here in Amphiperatherium, Hassianycteris, Smilodon and Felis. This would be consistent with the further development of a definitive floor and wall to the Tomes\u27 process pit. The sequential appearance of minor linear, minor planar and major planar discontinuities in crystallite orientation is seen as fundamental to the evolution of mammalian enamel structure

Ultrastructural Variation in Enamel of Australian Marsupials

This paper initiates a survey of the enamel of fossil and extant Australian marsupials by scanning electron microscopy. Enamel was examined from 17 extant and 11 extinct marsupials. Assessment was made of prism packing pattern, prism course, tubule presence, tubule size and distribution. Values calculated were: prism diameter; prism axis ratio; cross-sectional prism area; cross-sectional ameloblast area; and numerical prism density. Three different prism packing arrangements were found for extant and fossil marsupials within the classical Pattern 2. The Pattern 1 arrangement found in three extant species was relatively unexpected given the general acceptance of Marsupialia as having Pattern 2 enamel. Attention is drawn to the variable loss of prism demarcation towards the outer enamel surface. The majority of both extant and fossil marsupials exhibited a simple radial prism course. Prism diameters were small ranging from 1.4 μm to 3.9 μm and prism densities were high, compared to those for human and multituberculate enamel. A significant inter-species variation was noted in the presence and size of enamel tubules. The absence of enamel tubules in the incisors of D. optatum, N. tedfordi and T. rostratus and the molar of W. wakefieldi was confirmed. Large bulbous spaces were found either along or at the termination of enamel tubules in some teeth of five fossil species: these spaces may represent the resting place of an ameloblast. We have found: a greater variation in prism packing patterns; a greater difference in characteristics studied between incisor and molar teeth; and a greater variety of tubule morphology than anticipated. There are signs that useful enamel ultrastructural characters are emerging to help ultimately with taxonomic investigations of Australian marsupials

Adult Phyllostomid (Bat) Enamel by Scanning Electron Microscopy - With a Note on Dermopteran Enamel

This study assesses the enamel of five phyllostomids of differing feeding habits; only one example of the microchiropteran super-family Phyllostomoidae having previously been studied by SEM. A dermopteran was also examined to ascertain whether the enamel might reveal insectivore, chiropteran or primate characteristics. The five phyllostomids were found to display the additional crystallite discontinuity feature (minor boundary plane or seam) which is a major characteristic of all the bats we have so far examined with the exception of two megachiropterans. The enamel of the four fruit and nectar feeders (Phyllostomus, Carollia, Glossophaga and Artibeus) is essentially similar and different to that of the blood feeder (Desmodus). The differentiating factor for the two groups is the poor degree of prism development in Desmodus; the prisms being restricted to the inner two thirds of the enamel over the cusps or sectorial ridge, and lacking in the greater part of the axial and the sulcular enamel. The poor prism development in the vampire bat raises interesting questions from both an ontological and a phylogenetic point of view. The dermopteran (Cynocephalus sp.) displays horse-shoe shaped prisms with associated minor boundary planes (seams); an appearance entirely similar to those microchiroptera we have examined. This finding could be advanced as evidence for a close phylogenetic relationship between the Dermoptera and Chiroptera as these features are not found to the same extent in insectivores or in primates; the other two orders to which dermopterans are assigned. The evolutionary significance of the seam feature is being studied further; it is very likely to be of importance in unravelling the history of mammalian enamel

Marsupial and Monotreme Enamel Structure

We present some recent developments in our understanding of two basic questions: the origin, extent, nature and course of marsupial enamel tubules; and the characterisation of monotreme enamel, more particularly, the prismatic nature of platypus enamel. Methods used included SEM of methacrylate casts of marsupial enamel tubules, worn and cut surfaces of whole marsupial teeth, developing and erupted platypus teeth, and a well-developed molar of the newly discovered Miocene ornithorhynchid Obdurodon sp., and tandem scanning reflected light microscopy of intact marsupial teeth. We conclude that there are significant species differences with respect to prism shape, row formation and tubule disposition in marsupials and, moreover, that these features change in a consistent way through the thickness of the enamel. Consideration of enamel prism course in incisor and molar enamel of Macropus eugenii, together with the tubule casts, enables us to conclude that there is a fundamental relationship of tubule to prism in the body of marsupial enamel. This and previously reported data put beyond dispute the essential relationship of the marsupial tubule to the formative ameloblast. The enamel of Ornithorhynchus anatinus is shown to be prismatic only in part, with well-formed regular prisms not being a primary structural feature. The enamel of the fossil monotreme is prismatic and tubular and displays large areas of Pattern 2 prism packing. Monotreme enamel has been interpreted as representing a structural stage intermediate between that of known multituberculates and extant tribosphenid mammals

Basis of the Structure and Development of Mammalian Enamel as Seen by Scanning Electron Microscopy

Mature enamel is the most mineralized of mammalian tissues, contains the least water and therefore does not present problems of shrinkage on preparation for SEM. However, the developing enamel is highly hydrated and presents severe problems in preparation. The structure of enamel is determined by the activity of its individual formative cells and their group behaviour. The peculiar, unequal secretion of matrix at the distal pole of the ameloblast leads to the presence of characteristically shaped pits in the surface of the formative tissue. Crystals grow in a special relationship to this surface. Sharp changes in orientation of the surface are reflected in abrupt changes in orientation of neighbouring crystals beneath it, leading to the formation of structural discontinuities at prism boundaries or junctions. Several different patterns of prism cross section have arisen in mammalian enamel. Inequalities in the rate of production of the tissue lead to the formation of features known as varicosities or cross striations. Exaggerations of this presumed daily incremental rhythm lead to the formation of the more major incremental lines which can also be visualized by scanning electron microscopy. Differences in the course of the ameloblasts throughout their life history, in the nature of a translatory motion over the surface which they are secreting, lead to the development of prism decussation, which shows characteristic patterns in different mammalian groups of probable functional significance. One largely ignored area in the study of comparative histology concerns the enamel-dentine junction. Particularly in the marsupial mammals, dentine tubules cross the junction and are continuous with enamel tubules. Methacrylate casting of these features has given new insights into these structures

Enamel of Yalkaparidon Coheni: Representative of a Distinctive Order of Tertiary Zalambdodont Marsupials

The enamel of an incisor and a premolar of Yalkaparidon coheni was examined by scanning electron microscopy in fractured and in sectioned, polished surfaces. The enamel of both teeth demonstrated: complete, ovoid and horse-shoe shaped prisms in a Pattern 2 arrangement; a simple parallel prism course; and, enamel tubules in abundance in the premolar but restricted to the innermost enamel in the incisor. Overall, the enamel ultrastructure supports the marsupial affiliation proposed for Yalkaparidon coheni but does not unambiguously ally it with any other order of marsupials. The observation of a significant ultrastructural difference between the anterior and posterior teeth of a marsupial emphasizes the need to sample both if available. In pursuing this, we report here also the lack of tubules in the anterior teeth of the extant Tarsipes rostratus. This together with a similar absence of typical marsupial tubules from the incisor of the extinct Yalkaparidon coheni, would suggest that the wombat is not the only surviving marsupial to have experimented so extensively with this particular structural feature. It is likely that further study will demonstrate an unexpected and relative lack of tubules in the incisor enamel of other fossil Australian marsupials