Mouse molar morphogenesis revisited by three-dimensional reconstruction. III. Spatial distribution of mitoses and apoptoses up to bell-staged first lower molar teeth.

Computer-assisted 3D reconstructions were used to follow the development of the embryonic mouse first lower molar (M1). At ED 12.5, the thickening of the oral epithelium, which was thought to correspond to the molar dental lamina, regressed in its anterior part as a result of apoptosis. Only the posterior part later gave rise to molars. The transition to the cap stage entailed medial and lateral extensions of the dental epithelium. The growth and histo-morphogenesis of the enamel organ as well as cervical loop formation proceeded more rapidly in the anterior part of the M1 during the cap and early bell stages producing significant morphological differences along the antero-posterior axis. Apoptosis was temporarily intensive in the anterior part of the bud- and cap-shaped epithelium and thus pointed domains which do not participate in the formation of the final M1 enamel organ. In the well-formed cap, apoptoses displayed maximum concentration in the enamel knot (EK). No increase in the number of metaphases could be detected in the vicinity of the EK. Mitoses were distributed throughout the epithelial compartment until cap stage and then mainly concentrated in the inner dental epithelium at the early bell stage. At this later stage, either lateral views or thick virtual sections performed in the reconstruction demonstrated a clear cut distribution of mitoses and apoptoses in the enamel organ. At the early bell stage, mitoses in the mesenchyme demonstrated an increasing postero-anterior gradient.