Cap-to-bell stage molar tooth morphogenesis occurs through proliferation-independent sulcus sharpening and condensation-associated tension in the dental papilla.

The anatomy of molar teeth is important both functionally for chewing food and in evolutionary studies as a well-preserved species marker in the fossil record. Molar teeth begin to develop their characteristic biting-surface shape of cusps (peaks) and sulci (valleys) at the bell stage, when corresponding folds in the dental epithelium become apparent. Theories about the developmental mechanisms of cusp and sulcus morphogenesis have hitherto largely focused on the non-proliferating nature of the secondary enamel knots (EKs) at the cusp tips. EKs have been thought to direct cusp/sulcus formation by stimulating proliferative growth of the surrounding epithelium which, being confined within a capsule of condensed mesenchyme, bends by mechanical buckling. Here we show, using explant inhibition and cut-and-recoil experiments, that cap-to-bell morphogenesis is largely proliferation-independent (sulcus sharpening entirely so) and that tension in the mesenchyme of the dental papilla, immediately sub-adjacent to the cusps, rather than compression by the mesenchyme surrounding the whole structure, is what holds the structure in shape. Fine mapping of the degree of condensation shows that it is highest in the mesenchyme of the dental papilla and becomes progressively more focused to the cusp regions, consistent with a key role in cusp shaping. Together these findings overturn the prevailing models of molar morphogenesis, including both cusp and sulcus formation.