Coordinated tether formation in anatomically distinct mice growth centers is dependent on a functional vitamin D receptor and is tightly linked to three-dimensional tissue morphology

Bone bridges linking the epiphysis and metaphysis termed "tethers" have been found in the femoral growth plates of C57Bl/6 mice and are disrupted when the vitamin D receptor (VDR) is ablated. It is unknown if tethers are found in other growth centers, if they are regulated in a comparable manner, or if they have a functional role in skeletal development or stability. To address this, distal femoral growth plates (GPs) and spheno-occipital synchondroses (SOSs) of wild-type C57Bl/6 mice from 2 to 15weeks of age were analyzed using μCT scans. The GPs and SOSs of VDR+/+ and VDR-/- mice fed regular or rescue diets to restore mineral homeostasis until 10weeks of age were also scanned. Tethers in GPs and SOSs both thickened and accumulated in number as these growth centers decreased in size. Ablating the VDR made GPs and SOSs rachitic and nearly eliminated tether formation. Rescue diets restored the volume of both growth centers but only partially restored growth center thickness and tether formation, suggesting that lα,25-dihydroxy vitamin D ₃ partially regulates tether formation in these growth centers via its receptor. In VDR+/+ mice 2-15weeks in age, growth center thickness was inversely correlated to animal weight whereas tether phenotype (tether volume/growth center volume, tether number/mm, tether width, tether spacing) was significantly related to animal weight. In both 2-15week old VDR+/+ and 10week old VDR+/+ and VDR-/- mice on normal and rescue diets, tether phenotype (tether number/mm, tether spacing) had strikingly similar relationships to growth center thickness. These results show that tethers are present in growth centers in different anatomic and undergo developmental changes in a comparable manner; in both sites, VDR-regulated tether formation is strongly linked to growth center morphology; and tether formation is associated with body weight, suggesting a role in maintaining growth plate stability during growth.