Asymmetry of sonic hedgehog expression is necessary for early incisor tooth morphogenesis

The dentition of mice comprises only two tooth types, molars and incisors. Despite similar epithelial-mesenchymal signalling interactions during development, their final morphologies differ dramatically. Both molars and incisors grow from a similarly symmetric epithelial organ bud starting at embryonic (E) day 12.5. By E13.5, they develop outgrowths known as cervical loops (CLs), which grow symmetrically in the molar but highly asymmetrically in the incisor. Prior work has shown epithelial Shh is required for the symmetric growth of molar CLs, but the cellular basis of incisor asymmetry is not known. Here, we report that differential proliferation in the epithelium and asymmetric distal Shh expression at E12.5 underlie asymmetric incisor morphogenesis. Inhibiting proliferation or Shh expression at this stage ex vivo resulted in a loss of asymmetry. Furthermore, symmetrizing the source of Shh at this stage by adding ectopic SHH protein ex vivo or expressing ectopic Shh in vivo similarly caused a loss of asymmetry. Together, these results demonstrate that spatial asymmetry of differential proliferation dependent on Shh expression plays a crucial role in the developing murine incisor.