The presence of rudimentary odontogenic structures in the mouse embryonic mandible requires reinterpretation of developmental control of first lower molar histomorphogenesis

In the mouse embryonic maxilla, rudimentary tooth primordia have been ident ified, which can be mistaken for the first upper molar. In order to determi ne whether such a situation might exist in the lower jaw as well, tooth dev elopment was investigated in the mouse mandibular cheek region during ED 12 .5-15.0. A combination of histology, morphometry and computer-aided 3D reco nstructions demonstrated the existence of rudimentary dental structures, wh ose gradual appearance and regression was associated with the segmental pro gress of odontogenesis along the mesio-distal axis of the jaw: 1) At ED 12. 5, the mesial segment (MS) was the most prominent part of the dental epithe lial invagination. It included an asymmetrically budding dental lamina. The MS, although generally mistaken for the lower first molar (M-1 primordium, regressed and did not finally participate in M-1 cap formation. 2) At ED 1 3.5, a wide dental bud (called segment R2) appeared distally to the MS. Alt hough the R2 segment transiently represented the predominant part of the de ntal epithelium at ED13.5, it participated only in the formation of the mes ial end of the M-1 cap. 3) The top of the R2 segment at ED13.5 was not the precursor of the enamel knot (EK), contrary to what has been assumed. 4) Th e central segment of the M-1 cap as well as the EK developed later and dist ally to the R2 segment. 5) Time-space specific apoptosis correlated with th e retardation in growth of the R2 segment as well as with strong regressive changes in the epithelium situated mesially to it. These highlight the nee d to reinterpret current molecular data on early M-1 development in the mou se in order to correlate the expression of signalling molecules with specif ic morphogenetic events in the appropriate antemolar or molar segments of t he embryonic mandible.