Landmark‐based 3D Morphometrics of the Developing Human Dentition in the Middle Trimester

Human primary tooth formation begins around 6 weeks in utero and by the time a baby is born, all 20 teeth have mineralized and are embedded in the jaw bone. Our study is addressing the period during which tooth morphogenesis and jaw development are taking place, the middle trimester. The tooth‐bone relationship involves inductive interactions between the condensed mesenchyme of the dental follicle surrounding the tooth germs and the non‐dental mesenchyme of the jaws. As teeth pass through bud, cap and bell stages, intramembranous bone is being deposited by osteoblasts but the patterning of the bone in relation to the teeth is not well understood. From studies in mouse, the tooth follicle is thought to induce bone and in turn the bone is thought to restrict tooth organ growth. Mouse molars are completely encased in bone as soon as the crown is completed. However in humans the bone surrounding tooth organs is fenestrated. The buccal and occlusal surfaces of the first molar (m1) and canine (c) are not covered by bone whereas the incisors (lateral incisor – i2, central incisor – i1) are surrounded by bone on buccal, lingual, gingival, mesial and distal. Therefore we wondered whether crypt growth is isometric or allometric and whether there were differences between the tooth morphotypes. To answer this question, we scanned 25 intact fetal jaws with microCT from a collection of human specimens maintained at UBC. Scans were imported into Amira (Thermo‐Fisher) and used for segmentation or landmark placement. Landmarks were placed in a consistent manner on all surfaces of the bony tooth crypt (Fig. B,D) and then exported to MorphoJ for morphological analyses (Fig. A,C). In addition segmented tooth volumes and linear dimensions were measured in Amira. The principal component and canonical variate analysis revealed that there are significant shape differences in the crypts between 12–14 weeks and 17–19 weeks. The intermediate period 15–16 weeks was not significantly different than the early or later timeframes. These trends matched segmented volumetric data for the tooth germs and mandible. Shifts in specific landmarks particularly the buccal landmarks adjacent to m1 and c were observed between the early and late period which is consistent with the lack of constraint by bone on those surfaces (Fig. A). Allometric growth in the occlusal gingival direction was also observed with increased depth to the crypts for all teeth over the developmental period (Fig. C). Some of these changes are consistent with reconstruction data from others showing relatively greater bone resorption on the internal surfaces of the crypt. Our data suggests that the all the tooth germs are interacting with the bone that will later surround the roots. The prominent buccal bulges of m1, plus the formation of the mental foramen on the buccal side of the m1 crypt, may require spatially restricted tissue interactions Support or Funding Information Funded by the Faculty of Dentistry, UBC; Human ethics approval number H08‐0257 This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .

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