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S Risnes

Publications and source records attributed to S Risnes.

At least 37 records · Page 2Linked to original sources
PubMed1990

Shark tooth morphogenesis. An SEM and EDX analysis of enameloid and dentin development in various shark species.

The study provides a survey of shark tooth morphogenesis based on SEM and EDX analyses of whole tooth families in six shark species. The teeth, demonstrating different stages of development, were acid-etched and coated with palladium. Calcium content was determined semi-quantitatively by using the palladium coating as an internal standard. Due to the rapid development of the enameloid, all major events took place in the two or three youngest teeth of a tooth family. Enameloid appeared to develop as a transformation of the peripheral part of the dental papilla. Mineralization started immediately. Based on morphological criteria the middle zone of the enameloid was established at an early stage, excluding the possibility of an unambiguous centrifugal or centripetal direction of growth. Substantial mineral increase first occurred in the middle zone, spreading from the tooth tip toward the base. Dentin formed after the enameloid was completely established. Dentin formation started basally as a direct prolongation of the enameloid cap, then spreading toward the tooth tip, first along the edges. It is concluded that shark enameloid has a mesenchymal background, but a role played by the inner dental epithelium can not be excluded.

Amelogenesis↗
PubMed1989

Ectopic tooth enamel. An SEM study of the structure of enamel in enamel pearls.

Eighteen human molars with enamel pearls ranging in diameter from 0.8 to 2.7 mm were sectioned, acid-etched, and processed for SEM observation. In addition to pearl enamel, the specimens contained crown enamel for comparison. All pearls were of the composite type. The enamel layer reached maximal thicknesses of between 0.3 and 0.7 mm opposite the tip of the dentinal cone. The enamel structure was normal, but more variable and irregular than crown enamel. The prism course was often irregular throughout the whole thickness of enamel. Distinct Hunter-Schreger bands were absent. Prisms and interprism were occasionally difficult to identify as separate entities. The packing of crystals seemed to be less tight in pearl enamel. The scarcity of Retzius lines was attributed to the method of preparation. Prism cross-striations with a periodicity ranging between 1.3 and 4.5 microns were frequently encountered. A superficial prism-free zone with a regular 1.7-2.5-microns striation was present in localized regions of many pearls. The formation time of a medium-sized pearl was calculated to be about 11.5 months. A variety of cross-cut prism configurations was observed. Occasional hypoplastic lesions and hypomineralized areas were encountered. In conclusion, pearl enamel possesses most of the structural attributes of crown enamel, but in general its organization seems to be less orderly. Enamel pearls represent developmental disturbances in position and timing more than in structure.

Dental Enamel↗
PubMed1987

Multiplane sectioning and scanning electron microscopy as a method for studying the three-dimensional structure of mature dental enamel.

A method is described where teeth are sectioned/ground along at least two planes, etched, and viewed at various angles in the SEM in order to study the three-dimensional structure of enamel. This multiplane sectioning-scanning electron microscopy (MPS-SEM) method has been applied to the study of rat and human enamel. The method demonstrates in a direct way the complex three-dimensional structure of rat incisor enamel; the path of prisms and the distribution of interprismatic substance. The different appearance of alternate prism rows as seen in the longitudinal plane is seen to be due to prisms in alternate rows being inclined to different degrees in the transverse plane. In human enamel, the method reveals the three-dimensional nature of Retzius lines. In tangential planes cut at right angles to the prisms, Retzius lines are identified on the basis of an altered size, shape or etching pattern of prisms. Considerable variation in etching pattern within the same tangential plane was observed. Enamel tufts could be observed simultaneously in tangential and longitudinal planes, giving information about their three-dimensional extent and their relationship to Hunter-Schreger bands and to prisms and interprismatic substance. The main advantage of the method is that deduction of three-dimensional structure from the appearance of the structure in one plane is aided by the appearance of the structure in the adjoining plane.

Animals↗
PubMed1986

Enamel apposition rate and the prism periodicity in human teeth.

A method is presented which allows the average daily rate of enamel apposition to be estimated based on measurements of the length of prisms in longitudinal ground sections, and on available data for the time of formation of the tooth crown. The method was applied on 21 sections of human maxillary premolars. The reproducibility of the method was satisfactory, the error of measurement being in the order of 3.8%. After corrections having been made for the deviation of prisms from a radial direction, the average daily rate of enamel apposition along the prisms for any maxillary premolar was estimated to be within a range of from 2.0 to 6.1 microns, while a more probable rate would be 2.7-4.6 microns. These results are in agreement with experimental data, and are also compatible with the observed cross-striation prism periodicity. The present study supports the theory that there exists a relationship between the prism periodicity and a 24-h rhythm in enamel apposition in human teeth.

Ameloblasts↗
PubMed1985

A scanning electron microscope study of the three-dimensional extent of Retzius lines in human dental enamel.

A method involving carefully planned sectioning and grinding of human maxillary first premolars, allowing enamel structure to be studied with the SEM in more than one plane at the same time, was employed to demonstrate the extent of Retzius lines within the confines of the three-dimensional enamel cap. It was found that the Retzius lines behaved as could be predicted by the theory stating that the striae represent one-dimensional cuts of two-dimensional, curved growth planes. Retzius lines in adjoining planes were continuous across the edge where the two planes met. In a supernumerary tooth, the surface of its hypoplastic tooth tip was continuous with distinct Retzius lines. The present observations provide for the first time direct morphologic verification of the supposed nature of Retzius lines, with reference to extent and continuity in three-dimensional space.

Bicuspid↗
PubMed1985

Circumferential continuity of perikymata in human dental enamel investigated by scanning electron microscopy.

Theoretically the perikymata may represent one of two possible configurations: closed circles or continuous spirals. In the present study one randomly selected perikyma groove in one randomly selected tooth (the mandibular first premolar of a 12-yr-old girl) was tracked around the circumference of the crown in the scanning electron microscope. It was found that the perikymata geometrically represented closed circles. The possibility of methodologic error was excluded.

Bicuspid↗
PubMed1984

Rationale for consistency in the use of enamel surface terms: perikymata and imbrications.

The use of the enamel surface terms perikymata and imbrications/imbrication lines in the dental literature is inconsistent and therefore confusing. Based on an analysis of these terms and their original use, the following system for their future use is recommended: the whole pattern of ridges and grooves on the enamel surface is termed perikymata or imbrications. Distinction between the two elements of the pattern is attained by adding ridge/crest or groove/furrow. In such combination terms perikymata should be used in its singular form, perikyma, e.g. perikyma ridge.

Dental Enamel↗
PubMed1982

Multiangular viewing of dental enamel in the SEM: a simple specimen holder system.

The specimen holder system described here has been found very useful when studying the three-dimensional organization of dental enamel in the SEM. The system includes small brass cylinders on which the specimens are mounted. The cylinders can be placed both vertically and horizontally in a specially designed holder. With this holder and a tilting capacity of 45 degrees all specimen aspects deviating as much as 135 degrees from the top surface can be studied.

Dental Enamel↗
PubMed1982

The lingual angle of incisors.

A morphologic trait, not previously registered, on the incisor lingual surface: "The lingual angle of incisors", was studied in Norwegians. The lingual angle is the angle between the floor of the lingual fossa and the dental tubercle. Measurements of the lingual angle could be performed with sufficient accuracy. Deviations from normal distribution in skewness and kurtosis values did not affect statistical testing. The general similarity of the frequency polygons for all incisors in the way they were peaked and stepped is possibly due to participation of the same factors on the genetic level in the determination of the trait. The mean lingual angles were (in degrees): I1sup = 142, I2sup equal 156, I1inf = 153 and I2inf = 154. No significant bilateral asymmetry or sex differences were found. Correlations were high for both central to central and lateral to lateral incisors in both jaws, highest for centrals in the maxilla and laterals in the mandibula. Central to lateral correlations were relatively low, especially in the upper jaw. Interjaw correlations were low, but I1sup showed a tendency to be better correlated than I2sup to lower incisors. I2sup and I1inf were the most variable incisors in each jaw. The dental tubercle probably plays an important role in determining the lingual angle. Measurements of the lingual angle give information on the profile of the lingual surface in inciso-cervical direction. Statistical analysis of a phenotypic trait may give information on factors which determine the trait.

Adolescent↗
PubMed1981

Uncoated specimen of human enamel observed in the scanning electron microscope.

A 1-mm-thick section of a human premolar was acid etched and observed uncoated in the SEM at accelerating voltages of 3, 5, 10 and 15 kV. Prisms and interprismatic substance were easily distinguishable. Low-voltage operation (3 and 5 kV) gave the best results. Specimen charging was detectable at 5 kV and caused reduced image quality at 10 and 15 kV. Application of silver paste did not reduce charging appreciably. Prolonged observation at high magnification (x 10 000) resulted in contamination of the specimen with consequent charging and reduced resolution. Dental enamel seems to be a material which is well suited when uncoated for observation in the SEM. This may be due both to the high content of the relatively heavy atom calcium, giving good secondary electron emission, and possibly to a certain degree of conductivity caused by diffusible ions.

Acid Etching, Dental↗
PubMed1979

A scanning electron microscope study of aberrations in the prism pattern of rat incisor inner enamel.

The prism pattern in the inner enamel of adult rat incisors was studied with the SEM in unfixed tissues that had been sectioned, ground, polished, and etched. Six different types of aberrations in the prism pattern were encountered: 1. Prism lamellae may be shorter than the mesio-lateral width of enamel. 2. Prism lamellae may deviate from a transverse orientation. 3. Prism lamellae may "fuse" or "bifurcate." 4. Prisms of two adjacent lamellae may pursue a common course. 5. Prisms may change direction. 6. Variations exist in the outline of transversely cut prism profiles. Aberrations were observed at any distance from the dentino-enamel junction. These observations were used as a basis for an analysis of the movement of ameloblasts during rat incisor amelogenesis. It was concluded that it is physically possible for the ameloblasts to create the observed aberrations as they move along the path of the prisms. However, the aberrations seem to make it more difficult to understand the factors controlling ameloblast movement. Occasionally crystallite bridges connecting adjacent prisms were observed. A configuration resembling a bifurcating prism is pesented.

Ameloblasts↗
PubMed1979

The prism pattern of rat molar enamel: a scanning electron microscope study.

Rat molar enamel has been studied by sectioning the enamel along various planes, and observing the etched surfaces in the SEM. It was found that the prism pattern was much more variable than in rat incisor enamel. Regions without prism decussation seemed to dominate in the occlusal half of the molars. Where present, prism decussation was of the uniserial lamellar type, but it varied considerably in distribution, extent, and distinctness. Prism decussation seemed to have a predilection for the cervical enamel, and was almost absent in the enamel on the occlusal surface. The interprismatic substance showed a characteristic configuration: In the inner enamel it appeared in the form of radially oriented sheets, which tended to delimit radially directed, single lines of prisms. In regions with prism decussation these single lines of prisms encompassed prisms belonging to different prism lamellae. In the outer part of the enamel the interprismatic substance exhibited a honeycomb appearance. The similarities and differences between the prism patterns of rat incisor and molar enamel may be of importance for understanding the mechanisms of amelogenesis, especially for the recognition of factors controlling the movement of ameloblasts.

Animals↗
PubMed1979

Serially etched shark enameloid observed by incident light microscopy.

Longitudinal and transverse tooth sections of Isurus oxyrinchus were serially etched in 2.6% nitric acid. The changing optical properties of the etched surfaces were observed during the serial etchings, and the descent of the enameloid surfaces was measured. Shark enameloid seems to be less effectively etched by acid than human enamel; this difference may be due to differences in solubility between fluorapatite and hydroxyapatite. Most of the information regarding the structure of the enameloid was gained during the first five of ten etchings. The reflection of light from the surface was influenced by the orientation of the crystallites, longitudinally sectioned crystallites reflecting the light better than transversely sectioned crystallites. The dentinal extensions were continuous with and of the same structure as the underlying dentine. The radial fibers originated from the dentinal extensions, and they both contained organic material and were accompanied by crystallites. When the specimens were imbibed with water the distinctness of the dentinal extensions and radial fibers was improved.

Acid Etching, Dental↗