MOUSE MOLAR MORPHOGENESIS REVISITED BY 3-DIMENSIONAL RECONSTRUCTION .1. ANALYSIS OF INITIAL-STAGES OF THE FIRST UPPER MOLAR DEVELOPMENT REVEALED 2 TRANSIENT BUDS

Early stages of tooth development in the maxillary cheek region in the mouse were investigated by combined analysis of histological sections , computer assisted 3D reconstructions and morphometry. In ED 12.5 emb ryos, 3D reconstructions revealed an accessory epithelial bud (R1) and a large bud (R2), which appeared as a single bud-shaped epithelium in frontal sections. This developmentally most advanced dental epitheliu m in the mouse embryonic maxilla until ED 13.5, generally considered a s the bud of the first molar, regressed during later development. Mean while the bud and cap of the first upper molar originated more posteri orly, from ED 13.5. The regression of R1 and R2 was associated with ep ithelial apoptosis. Apoptotic cells and bodies were apparent on sectio ns in the R1 epithelium from ED 12.5. The R2 epithelium maintained the large bud-shaped appearance on sections, representing the largest par t of the dental epithelium in the maxillary cheek region until ED 14.0 ; apoptoses were detected there as late as from ED 13.5. During regres sion, the R2 rudiment was transformed into the medial and lateral epit helial ridges. posteriorly in continuity with the arising cap of the f irst molar. The reduced R1 epithelium seemed to contribute to the medi al ridge. These results should be taken into consideration in the inte rpretation of early odontogenesis in the upper jaw in the mouse. The i nteresting problem of the identification of tooth homology of the rudi ments should be elucidated by further comparative morphological and pa leontological investigations.