UC Riverside

Nutrient canals (or foramina) provide the major blood supply for long bones and serve as the entry point for the nutrient artery. External loading from movement and activity causes bones to be constantly remodeling, but whether this would lead to differences in the size or configuration of nutrient canals has not been studied. To investigate the phenotypic plasticity of nutrient canals, we studied a mouse model in which 4 replicate High Runner (HR) lines have been selectively bred for voluntary wheel running over the course of 57 generations. The selection criteria is the average number of wheel revolutions on days 5 & 6 of a 6-day period of wheel access as young adults (~6-8 weeks old). An additional 4 lines are bred without selection to serve as controls (C). For this study, 100 female mice (half HR, half C) were split into an active group housed with wheels and a sedentary group housed without wheels for 12 weeks starting at ~24 days of age. We tested for evolved differences in various nutrient canal traits between HR and C mice, as well as plastic changes resulting from chronic exercise, and the interaction between activity and linetype. We created three-dimensional (3D) models of the femoral nutrient canals. Relatively few differences were found between the nutrient canals of HR vs C mice, or between the active and sedentary groups. We did find an interaction between linetype and activity for the total number of nutrient canals per femur, in which wheel access increased the number of canals in C mice but decreased it in HR mice. We did not replicate the results of an earlier study, prior to the HR lines reaching selection limits for wheel running, which found that mice from the HR lines had significantly larger total canal cross-sectional areas as compared with those from C lines. This discrepancy is consistent studies of other skeletal traits that found differences between HR and C mice to be somewhat inconsistent across generations, including the loss of some apparent adaptations with continuing selection after reaching selection limits.