The Effect of Growth Rate and Biomechanical Loading on Bone Laminarity in the Emu Hindlimb

Abstract

The orientation of vascular canals in primary bone may reflect differences in growth rate and/or adaptation to biomechanical loads. Previous studies link specific canal orientations to bone growth rates, but results are contradictory. In particular, circularly‐oriented vascular canals (forming laminar bone) are hypothesized to reflect rapid growth rate or locomotion‐induced torsional loading. Previous work on the locomotion of the emu shows that the femur and tibiotarsus experience large shear strains (torsional loads) that increase through ontogeny. Here, we test how growth rate and biomechanical loading effect bone laminarity in the femur and tibiotarsus from growing emu (2 – 60 wk). If bone laminarity is purely an expression of growth rate, it should be most elevated at the growth spurt and decrease with age. Alternatively, if laminar bone reflects biomechanical accommodation, it should become more abundant with age. Transverse mid‐shaft histological sections from the femur and tibiotarsus were prepared and imaged. Birds had been intramuscularly injected with xylenol orange and then six days later with calcein. On fluorescent images, distance was measured between xylenol and calcein tags and divided by six days to give daily growth rate. Vascular canal orientation was quantified using laminarity index (proportion of circularly oriented canals). Overall, the growth rate of the femur and tibiotarsus each exhibits a bell‐shaped distribution with age. The growth spurt in which the rate is at a maximum (99 and 84 μm/d for the femur and tibiotarsus, respectively) occurs at 8 wks. Although in both elements laminarity is low to moderate (0.02–0.56), it increases with age. Elevated laminarity may develop as a response to the relatively larger shear strains placed on the femur and tibiotarsus 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.