Microtomographic quantification of growth in dental development using formed teeth

Objectives: Teeth constitute an ideal model for studying morphogenesis; their development is based on epithelial-mesenchymal relations and involves numerous growth factors and processes. The aim of this study was to show the differential development of the second deciduous molar and the first permanent molar and how the timing of switch-on / switch-off mechanisms is reflected in the topography of the dentino-enamel-junction and extent of enamel apposition. Methods: Serial microCT imaging was used to differentiate between dentin and enamel and provide accurate three-dimensional reconstructions of developing lower human second deciduous molars and first permanent molars. In each cusp, growth and differentiation proceed apically so that location of the cusp tips and their bases reflects their order of initiation and coalescence, while cusp volume at the DEJ reflects cell division. Results: In the first permanent molar the dentin size proportion of the distal cusp to the other cusps is much smaller, but this is compensated for by extensive enamel apposition over the distal cusp. The pattern of cusp fusion is more staggered in the second deciduous molar than in the first permanent molar, in which all cusps fuse at a relatively similar time. We also show that in the first permanent molar, there is a reverse correlation between mesio-distal gradients in growth and enamel thickness. Conclusion: There is no correlation between duration of growth and the final size of the cusps. Our model enables us to pinpoint changes in developmental processes, from the initiation of mineralization to crown completion and provides a unique approach to identifying the timing in changes in growth trajectories. We are currently applying this approach to investigate phylogenetic relationships of fossil hominids and to identify the onset of growth insults resulting in dental anomalies.