Morphogenesis of the lower incisor in the mouse from the bud to early bellstage

The development of the lower incisor in the mouse was investigated from his tological sections using computer-aided 3D reconstructions. At ED 13.0, the incisor was still at the bud sta ge. At ED 13.5, the initial cap was delim ited by a short cervical loop, the development of which proceeded on the la bial side, but was largely retarded on the medial side. This difference was maintained up to ED 15.0. From ED 16.0, the bell stage was achieved. Metap hases had a ubiquitous distribution both in the enamel organ and in the den tal papilla from the bud to early bell stage. Apoptosis gradually increased in the mesenchyme posteriorly to the labial cervical loop from ED 13.5 to 14.0 and then disappeared; this apoptosis was not related to the posterior growth of the incisor. From ED 13.5, a high apoptotic activity was observed in the stalk. A focal area of apoptosis was observed at ED 13.5 in the ena mel organ, approaching the epithelio-mesenchymal junction at the future tip of the incisor. There,the inner dental epithelium formed a bulbous protrus ion towards dental papilla, reminiscent of the secondary enamel knot of mou se molars. This epithelial protrusion was still maintained at the bell stag e. The ena mel knot in the incisor demonstrated specific features, differen t from those characterizing the enamel knot in the molar: the concentric ar rangement of epithelial cells was much less prominent and the occurrence of apoptosis was very transitory in the incisor at ED 13.5. The disappearance of the enamel knot despite a low apoptotic activity and the maintenance of the protrusion suggested a histological reorganization specific for rodent incisor.