A computerized model for reconstruction of dental ontogeny: A new tool for studying evolutionary trends in the dentition

Abstract

All hominid molars show the same sequence of cusp initiation, but differ in the later stages of development. The topography of the dentin-enamel junction (DEJ) represents the outcome of differential growth between cusps. Since the cusps grow in an orderly sequence from tip to base (defined by the plane of coalescence with adjacent cusps), quantification of cusp volume and relationships can be used to reconstruct successive stages in development and their contribution to the morphometry of the crown surface. Their volume and spatial relationships at the DEJ enable us to partition cell proliferation in relation to cusp initiation, while quantification of the amount and distrib ution of enamel overlying the DEJ provides the necessary discrimination between cell proliferation and cell function, expressed in enamel matrix apposition. We have developed a three-dimensional computerized model of a lower molar tooth that enables us to identify and quantify the different stages of tooth development defined above. The model is based on serial micro-computed tomography (microCT) images of human teeth that provide accurate quantification of the outer and inner enamel and dentin boundaries of individual cusps. We have used this model to compare morphogenesis in the lower second deciduous molar and lower first permanent molar. Spatial relationships of the cusps, expressed by the topography of the DEJ showed that shape differences were established in the early stages of morphogenesis by differential proliferation within the developing tooth germ and that cusp size and proportions were modified at the crown surface by enamel apposition. Reduction of the hypoconulid in the permanent molar shown at the DEJ was largely masked by the exceptional thickness of enamel on this cusp. We propose that this model provides a novel contribution to the identification of ontogenetic trajectories and their contribution to evolutionary trends in tooth size, shape and enamel thickness.