Tethers Affect Spheno-occipital Synchondrosis Morphology via Vitamin-D Receptor-mediated Signaling

Disruptions to spheno-occipital synchondrosis (SOS) growth and morphology can lead to craniofacial defects, yet three-dimensional SOS morphology during endochondral ossification and possible mechanisms that regulate these morphological changes are not well characterized. Bone bridges (tethers) have been found in growth plates, and may influence tissue morphology and stability, but have yet to be investigated in the SOS. Objectives: This study was conducted to determine if tethers are present in the SOS, how SOS morphology and tether accumulation develop with age, and how disrupting SOS endochondral ossification by knocking out the vitamin-D receptor (VDR), affects this development. Methods: SOSs of wild-type male C57BL/6 mice from 2 to 15 weeks of age were isolated from microCT scans. VDR+/+ and VDR-/- mice were fed regular and high Ca, P, lactose, and vitamin-D₃ diets (rescue diet) until 10 weeks of age at which time their SOSs were scanned (n=6 for each group). Results: SOS size decreased rapidly over the first 8 weeks then remained constant. Tether accumulation had a sinusoidal increase with the most rapid growth occurring between week 6 and 10. Knocking out VDR significantly increased SOS size but nearly eliminated all tethers. Rescue diet restored SOS volume but did not statistically restore SOS thickness or tether phenotype as overall tether accumulation was ~30-40% compared to wild-type. Tether number/mm, volume percentage, and width were linearly correlated with animal weight (R²>0.70) in different age wild-type animals. Tether parameters were logarithmically related to SOS thickness in both different aged wild-type mice and in VDR-/- mice on different diets at week 10 (R²>0.84). Conclusion: Similar relationships between tether accumulation and SOS thickness in both different age wild-type mice and in VDR-/- mice on different diets at week 10 suggest that tethers influence SOS morphology. Additionally, these results suggest that lα,25-dihydroxy vitamin-D₃ regulates tether formation.