Dental Dynamics: A Fast New Tool for Quantifying Tooth and Jaw Biomechanics in 3D Slicer

Teeth reveal how organisms interact with their environment. Biologists have long looked at the diverse form and function of teeth to study the evolution of feeding, fighting, and development. The exponential rise in the quantity and accessibility of computed tomography (CT) data has enabled morphologists to study teeth at finer resolutions and larger macroevolutionary scales. Measuring tooth function is no easy task, in fact, much of our mechanical understanding is derived from dental shape. Categorical descriptors of tooth shape such as morphological homodonty and heterodonty, overlook nuances in function by reducing tooth diversity for comparative analysis. The functional homodonty method quantitatively assesses the functional diversity of whole dentitions from tooth shape. This method uses tooth surface area and position to calculate the transmission of stress and estimates a threshold for functionally homodont teeth through bootstrapping and clustering techniques. However, some vertebrates have hundreds or thousands of teeth and measuring the shape and function of every individual tooth can be a painstaking task. Here, we present Dental Dynamics, a module for 3D Slicer that allows for the fast and precise quantification of dentitions and jaws. The tool automates the calculation of several tooth traits classically used to describe form and function (i.e., aspect ratio, mechanical advantage, force, etc.). To demonstrate the usefulness of our module we used Dental Dynamics to quantify 780 teeth across 20 salamanders that exhibit diverse ecologies. We coupled these data with the functional homodonty method to investigate the hypothesis that arboreal Aneides salamanders have novel tooth functions. Dental Dynamics provides a new and fast way to measure teeth and increases the accessibility of the functional homodonty method. We hope Dental Dynamics will encourage further theoretical and methodological development for quantifying and studying teeth.

Amphibian teeth: current knowledge, unanswered questions, and some directions for future research

Elucidation of the mechanisms controlling early development and organogenesis is currently progressing in several model species and a new field of research, evolutionary developmental biology, which integrates developmental and comparative approaches, has emerged. Although the expression pattern of many genes during tooth development in mammals is known, data on other lineages are virtually non-existent. Comparison of tooth development, and particularly of gene expression (and function) during tooth morphogenesis and differentiation, in representative species of various vertebrate lineages is a prerequisite to understand what makes one tooth different from another. Amphibians appear to be good candidates for such research for several reasons: tooth structure is similar to that in mammals, teeth are renewed continuously during life (=polyphyodonty), some species are easy to breed in the laboratory, and a large amount of morphological data are already available on diverse aspects of tooth biology in various species. The aim of this review is to evaluate current knowledge on amphibian teeth, principally concerning tooth development and replacement (including resorption), and changes in morphology and structure during ontogeny and metamorphosis. Throughout this review we highlight important questions which remain to be answered and that could be addressed using comparative morphological studies and molecular techniques. We illustrate several aspects of amphibian tooth biology using data obtained for the caudate Pleurodeles waltl. This salamander has been used extensively in experimental embryology research during the past century and appears to be one of the most favourable amphibian species to use as a model in studies of tooth development.

Morphological variations in a tooth family through ontogeny inPleurodeles waltl (Lissamphibia, Caudata)

Most nonmammalian species replace their teeth continuously (so-called polyphyodonty), which allows morphological and structural modifications to occur during ontogeny. We have chosen Pleurodeles waltl, a salamander easy to rear in the laboratory, as a model species to establish the morphological foundations necessary for future molecular approaches aiming to understand not only molecular processes involved in tooth development and replacement, but also their changes, notably during metamorphosis, that might usefully inform studies of modifications of tooth morphology during evolution. In order to determine when (in which developmental stage) and how (progressively or suddenly) tooth modifications take place during ontogeny, we concentrated our observations on a single tooth family, located at position I, closest to the symphysis on the left lower jaw. We monitored the development and replacement of the six first teeth in a large growth series ranging from 10-day-old embryos (tooth I1) to adult specimens (tooth I6), using light (LM), scanning (SEM), and transmission electron (TEM) microscopy. A timetable of the developmental and functional period is provided for the six teeth, and tooth development is compared in larvae and young adults. In P. waltl the first functional tooth is not replaced when the second generation tooth forms, in contrast to what occurs for the later generation teeth, leading to the presence of two functional teeth in a single position during the first 2 months of life. Larval tooth I1 shows dramatically different features when compared to adult tooth I6: a dividing zone has appeared between the dentin cone and the pedicel; the pulp cavity has enlarged, allowing accommodation of large blood vessels; the odontoblasts are well organized along the dentin surface; tubules have appeared in the dentin; and teeth have become bicuspidate. Most of these modifications take place progressively from one tooth generation to the next, but the change from monocuspid to bicuspid tooth occurs during the tooth I3 to tooth I4 transition at metamorphosis.

The dentition of european Speleomantes spp. (Urodela, Plethodontidae) with special regard to sexual dimorphism

Dentigerous bones and dentition of juveniles and adults of several European Speleomantes-species were investigated using scanning electron microscopy. In the juveniles examined, all dentigerous bones bore bicuspid teeth. The adult males showed the well-known dentitional sexual dimorphism, i.e. monocuspid largely conical, elongated and strongly curved premaxillary teeth. However, teeth of the anterior portion of the maxillae and some teeth of the dentary were also clearly monocuspid; the remaining teeth had differently shaped apices considered to be different grades of bicuspidity. Vomerine teeth were exclusively bicuspid. Monocuspid, but smaller teeth were also found on the premaxilla of an adult female. The conical monocuspid tooth in adults is a modified metamorphosed tooth. Monocuspids, at least on the premaxillae, may be constantly present in males and development of monocuspids probably depends on differential sensitivities of the tooth producing tissue for androgen stimuli.

Teeth and their sex-dependent dimorphic shape in three species of Costa Rican plethodontid salamanders (Amphibia: Urodela)

The shape of the teeth and their sex-dependent dimorphic expression in three species of Costa Rican plethodontids (Bolitoglossa subpalmata, Oedipina uniformis and Nototriton abscondens) were studied using light microscopy and scanning electron microscopy. The teeth of the vomerine tooth patches are about one third larger than the teeth of the jaws in B. subpalmata and O. uniformis, whereas all teeth of N. abscondens are of about uniform size. The occurrence of bicuspid tooth germs in the fetus proves that primary teeth are bicuspid in these directly developing plethodontids. Females possess only bicuspid teeth consisting of a pedicel and a crown, as is considered characteristic for urodeles after metamorphosis. Adult males possess conical monocuspid teeth on the premaxillary. These teeth--which are similar to the typical late larval tooth of salamanders presenting a larval stage--are about twice as big as the neighbouring bicuspid maxillary teeth. N. abscondens males possess some monocuspid teeth and teeth of aberrant shapes on the premaxillary and the maxillaries. A tendency to build more monocuspid teeth in the premaxillary region than in the maxillary region can be observed in this species. We suppose that different degrees of sensitivity to androgens in each section of the dental lamina of the upper jaw cause the secondary occurrence of conical monocuspid teeth predominantly on the premaxillary section.

JH, Miller BT. Variation in the feeding kinematics of mole salamanders (Ambystomatidae:Ambystoma)

High-speed cinematography was used to investigate the prey-capture kinematics of six species of mole salamanders (Ambystomatidae). We compared the feeding behavior of the subgenus Ambystoma (A. californiense and A. macrodactylum) with that of the subgenus Linguaelapsus (A. mabeei, A. texanum, A. annulatum, and A. cingulatum). Prey capture by all six species is characterized by a 3-part gape cycle (a period of rapid mouth opening prior to extraoral tongue protraction, followed by a period of relatively stable gape angle during extraoral tongue protraction and retraction, followed by a period of rapid mouth closure), a tongue-extension cycle (protraction and retraction), and anterior head–body displacement. Among the six species, two distinct modes of prey capture are evident: (1) the Ambystoma mode (A. californiense, A. macrodactylum, A. mabeei, and A. texanum), and (2) the Linguaelapsus mode (A. annulatum and A. cingulatum). Most differences in prey-capture kinematics between the two modes are primarily differences of degree rather than the addition or loss of unique behaviors, and include a general reduction in the gape angles and a general increase in the elapsed times associated with specific events in the Linguaelapsus mode. We hypothesize that these differences are primarily the result of a prolonged period of tongue protraction in the Linguaelapsus mode during which the glandular tongue pad is fitted to the prey. In addition to differing from each other, the gape profiles of the ambystomatid subgenera differ markedly from the 4-part gape profiles of plethodontids and salamandrids.