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.

The ultrastructure of the dental tissues in dentinogenesis imperfecta in man.

Eight primary incisors obtained from a child with dentinogenesis imperfecta (DI) type II were examined histologically using light microscopy and scanning electron microscopy. In both the DI and control teeth, large dentinal canals were observed along the midline of the crown distributed mesial-distally and coursing towards the pulp. Variably-structured mantle dentine was seen in the DI teeth ranging from tubular to virtually atubular. Enamel separation occurred at the dentine-enamel junction despite apparently normal scalloping. In the enamel, fractures occurred along accentuated striae of Retzius where ultrastructurally there was prism bending and discontinuity. The structure of DI teeth probably results from a structural or regulatory protein abnormality and irregular epithelial-mesenchymal interaction. The combined influence of these factors appears to cause variable histologic appearances and rates of tissue deposition.

Effect of various root surface treatments on the attachment and growth of human gingival fibroblasts: histologic and scanning electron microscopic evaluation.

Human teeth extracted because of advanced periodontal disease were obtained. The portions of the roots which had been exposed in periodontal pockets were either untreated or were treated with root planing or citric acid, or root planing followed by citric acid. Human gingival fibroblasts were then added to the roots so treated and were allowed to incubate for 72 h. The ability of cells to attach to and grow onto these roots was assessed by means of gross evaluation of staining intensity and by histologic and scanning electron microscopic observation. The results of multiple experiments in each root-treatment category indicated that only roots which had been planed, whether or not citric acid demineralization was used, promoted cell attachment and growth. In addition, there were no discernible morphologic differences in the cells which were plated onto roots which were root planed only, compared to those which were root planed and citric-acid treated. In both situations too, the cells displayed morphology typical of human gingival fibroblasts in culture.

Dentin Apposition rates as markers of primate growth.

The incremental lines of von Ebner frequently have been described as ultradian markers of dentin calcification. To determine the relationship between these lines and the quantity of dentin formed, reference was made to an in vivo marker of calcification, tetracycline. These markers were produced by injecting four juvenile monkeys periodically over a period of 175 days. These animals had been subjects in the heat stress study and were exposed to a series of heat stresses and cortisone injections. At the end of the study, undecalcified thin sections of premolars and second molars were prepared by standard histological techniques. We found that linear apposition rates increased in a gradient toward the pulp chamber. These rates varied within each tooth but not in corresponding parts of different teeth. Also, we determined that dentin apposition can be temporarily depressed by certain metabolic stresses. Heat load and cortisone significantly slowed dentin formation. Each depression, however, was followed by a "catch-up" period. Incremental line distances also increased as a function of the distance from the dentoenamel junction. There was, however, no systematic relationship between apposition rates and incremental line distance; these distances did not deviate from the observed trends during periods of slowed apposition. Incremental lines may by interpreted, not as natural growth markers, but as structural phenomena which are a function of dentin geometry and tubule bending.

Taxonomic implications of primate dental tissues.

Histological studies of dental tissues will provide new and important insights into mammalian evolution. From studies of dental histology, hominoid evolution has become much clearer. Data as to the thickness of enamel and enamel prism patterns clearly provide quantitative support concerning theories of hominoid/hominid differentiation. The importance of comparative dental histology has only recently been realized and when extended to other mammalian orders, including both living and fossil forms, a more complete and clearer understanding of mammalian evolution will emerge. Caution must be emphasized, however, for these features are extremely complex structures and without a thorough knowledge of comparative dental histology and the applications of the SEM results may be misleading or incorrect.

Comparative enamel histology of primate teeth.

The use of nonhuman primates as experimental models has markedly increased during the past decade. However, very little is known about primate dental tissues, especially enamel. Comparative studies of the enamel from 25 primate species reveal a wide variation in (1) the thickness of enamel, (2) microstructure and ultrastructure, and (3) the species' capacity to respond to systemic stress. These data provide a baseline of information presently not available to the dental researcher.

Hominoid enamel prism patterns.

Analysis of enamel prism patterns in a selected series of extant hominoids reveals that pongids have a pattern distinctively different from that of Homo sapiens. The pattern for a Miocene hominoid, Ramapithecus, is very similar to that seen in Homo sapiens. The finding allows a new approach to the evaluation of isolated teeth.

Functional implications of primate enamel thickness.

Recent evolutionary interpretations of Hominoidea have postulated functional relationships between tooth form, diet and masticatory biomechanics. A major consideration is the durability of the tooth under certain dietary conditions. Teeth with low cusps and thicker enamel are able to withstand heavy mastication of abrasive food bolus for a longer period. When comparisons are made between species of higher primates the variables of tooth size, cusp morphology, and enamel thickness appear to be related but until now no systematic analysis has been made to determine the functional relevance of several dental dimensions. This study provides data gained from comparisons of dentition of nine species of primates. Histological sections were made of the post canine teeth and 21 dimensions were compared. The relevant dimensions identified serve to withstand dental wear. The distribution of thicker enamel corresponded to the observed wear planes. Humans had thicker enamel than pongids while the macaque had the thinnest. These preliminary results tend to support theories which explain low, thick, enameled cusps in hominids.