MOUSE MOLAR MORPHOGENESIS REVISITED BY 3-DIMENSIONAL RECONSTRUCTION .3. SPATIAL-DISTRIBUTION OF MITOSES AND APOPTOSES UP TO BELL-STAGED FIRST LOWER MOLAR TEETH

Computer-assisted 3D reconstructions were used to follow the developme nt of the embryonic mouse first lower molar (M-1). At ED 12.5, the thi ckening of the oral epithelium, which was thought to correspond to the molar dental lamina, regressed in its anterior part as a result of ap optosis. Only the posterior part later gave rise to molars. The transi tion to the cap stage entailed medial and lateral extensions of the de ntal epithelium. The growth and histo-morphogenesis of the enamel orga n as well as cervical loop formation proceeded more rapidly in the ant erior part of the M, during the cap and early bell stages producing si gnificant morphological differences along the antero-posterior axis. A poptosis was temporarily intensive in the anterior part of the bud-and cap-shaped epithelium and thus pointed domains which do not participa te in the formation of the final M, enamel organ. In the well-formed c ap, apoptoses displayed maximum concentration in the enamel knot (EK). No increase in the number of metaphases could be detected in the vici nity of the EK. Mitoses were distributed throughout the epithelial com partment until cap stage and then mainly concentrated in the inner den tal epithelium at the early bell stage. At this later stage, either la teral 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 demon strated an increasing postero-anterior gradient.