Meeting Abstract
The orientation of vascular canals in primary bone may reflect differences in growth rate and/or adaptation to biomechanical loads. Circularly-oriented canals (forming laminar bone) are hypothesized to reflect rapid growth rate or locomotion-induced torsional loading. The femur and tibiotarsus (tbt) of emu experience large shear strains (torsional loads) during locomotion that increase through ontogeny. Emu wings are very reduced and minimally loaded. Here, we test how growth rate and biomechanical loading affect bone laminarity in 5 elements (femur, tbt, humerus, ulna, and radius) from growing emu (2-60 wks). If laminarity reflects rapid growth rate, it should be most elevated at the growth spurt in all elements. If laminarity reflects biomechanical loading, it should increase with shear strains and be most abundant in the adult in hindlimbs only. Xylenol orange and calcein were injected into growing emu to fluorescently tag periosteal growth fronts. Transverse mid-shaft sections were prepared and imaged. To calculate daily growth rate, we divided the distance between fluorescent tags by days between injections. The proportion of circularly oriented canals (laminarity index) was measured. Overall, the growth rate exhibits a bell-shaped distribution with age with the growth spurt occurring at 5 wks. Bone laminarity decreases with increased growth rate in all elements. A positive relationship between laminarity and shear strain in the femur and tbt suggests that elevated laminarity is related to the relatively larger torsional loads placed on the hindlimb elements as the bird increases in age/mass. In conclusion, biomechanical loads seem to play a dominant role in the development of bone microstructure in the emu hindlimb.
