Synchrotron-based 3D Mapping of Molar Morphogenesis and Retromolar Space Creation

Objectives: The extent to which surrounding jaw tissue constrains the developing tooth organ including its onset time is unclear. Our objective was to compare retromolar space with molar onset and length. We hypothesized that first(M1), second(M2) and third(M3) molars would initiate only as retromolar space lengthened via jaw growth. We aimed to directly visualize in three-dimensions(3D) earliest molar onset to crown completion, and to measure molar, total jaw (TJL) and retromolar (RML) lengths.
Methods: At the Canadian Light Source synchrotron, we micro-computed-tomography(µCT) scanned (8.75µm voxel size) a cross-sectional series of PFA-fixed, silver-stained wild-type(C57BL/6J) mice aged embryonic-day(E) 10 to postnatal-day(P) 32 (n≤3 specimens/stage). Using Amira software (FEI), we visualized these µCT scan-sets in 3D and, using sagittal, coronal and transverse planes, measured upper and lower molar crypt and crown mesio-distal lengths, TJL and RML. Measurement data were analyzed in Excel (Microsoft).
Results: From E14 to P32, in upper and lower jaws, for every stage/age there was space distal to the last-initiated molar. TJL rapidly grew from E12-E18 (spanning M1, M2 onset), slowed from P0/birth-P18 (spanning M3 onset), spiked at P21, and stabilized from P23-P32. Upper/lower molars followed a trend of M1>M2>M3, with lower molars closer to M1>M2≥M3. Until P3, RML≈M1 length in lower and upper jaws. After P6, lower RML>M1 length, while upper RML≤M3 length.
Conclusions: Space immediately distal to the last-initiated molar was available throughout the period of M1-M3 onset and development. Spatial relationships were comparable between upper and lower molars/jaws; however, upper RML was shorter by ~20%. Jaw space does not appear to constrain molar onset; rather our results support that conditions within the dental lamina likely exert greater influence on onset time.